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Columbia University Lectures

THE DOCTRINE OF EVOLUTION

THE HEWITT LECTURES

1906-1907

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COLUMBIA UNIVERSITY LECTURES

THE DOCTRINE OF EVOLUTION

ITS BASIS AND ITS SCOPE

BY

HENRY EDWARD CRAMPTON, PH.D.

PROFESSOR OF ZOÖLOGY, COLUMBIA UNIVERSITY

New York

COLUMBIA UNIVERSITY PRESS

1916

All rights reserved

COPYRIGHT, 1911,

By THE COLUMBIA UNIVERSITY PRESS

Set up and electrotyped. Published June, 1911.
Reprinted December, 1912; September, 1916.

Norwood Press
J.S. Cushing Co.—Berwick & Smith Co.
Norwood, Mass., U.S.A.

PREFACE

The present volume consists of a series of eight addresses delivered as the Hewitt Lectures of Columbia University at Cooper Union in New York City during the months of February and March, 1907. The purpose of these lectures was to describe in concise outline the Doctrine of Evolution, its basis in the facts of natural history, and its wide and universal scope. They fall naturally into two groups. Those of the first part deal with matters of definition, with the essential characteristics of living things, and, at greater length, with the evidences of organic evolution. The lectures of the second group take up the various aspects of human evolution as a special instance of the general organic process. In this latter part of the series, the subject of physical evolution is first considered, and this is followed by an analysis of human mental evolution; the chapter on social evolution extends the fundamental principles to a field which is not usually considered by biologists, and its purpose is to demonstrate the efficiency of the genetic method in this department as in all others; finally, the principles are extended to what is called "the higher human life," the realm, namely, of ethical, religious, and theological ideas and ideals.

Naturally, so broad a survey of knowledge could not include any extensive array of specific details in any one of its divisions; it was possible only to set forth some of the more striking and significant facts which would demonstrate the nature and meaning of that department from which they were selected. The illustrations were usually made concrete through the use of photographs, which must naturally be lacking in the present volume. In preparing the addresses for publication, the verbal form of each evening's discussion has been somewhat changed, but there has been no substantial alteration of the subjects actually discussed.

The choice of materials and the mode of their presentations were determined by the general purpose of the whole course. The audiences were made up almost exclusively of mature persons of cultivated minds, but who were on the whole quite unfamiliar with the technical facts of natural history. It was necessary to disregard most of the problematical elements of the doctrine so as to bring out only the basic and thoroughly substantiated principles of evolution. The course was, in a word, a simple message to the unscientific; and while it may seem at first that the discussions of the latter chapters lead to somewhat insecure positions, it should be remembered that their purpose was to bring forward the proof that even the so-called higher elements of human life are subject to classification and analysis, like the facts of the lower organic world.

It may seem that the biologist is straying beyond his subject when he undertakes to extend the principles of organic evolution to those possessions of mankind that seem to be unique. The task was undertaken in the Hewitt Lectures because the writer holds the deeply grounded conviction that evolution has been continuous throughout, and that the study of lower organic forms where laws reveal themselves in more fundamental simplicity must lead the investigator to employ and apply those laws in the study of the highest natural phenomena that can be found. Another motive was equally strong. Too frequently men of science are accused of restricting the application of their results to their own particular fields of inquiry. As individuals they use their knowledge for the development of world conceptions, which they are usually reluctant to display before the world. It is because I believe that the accusation is often only too well merited that I have endeavored to show as well as circumstances permit how universal is the scope of the doctrine based upon the facts of biology, and how supreme are its practical and dynamic values.

It remains only to state that the present volume contains nothing new, either in fact or in principle; the particular form and mode of presenting the evolutionary history of nature may be considered as the author's personal contribution to the subject. Nothing has been stated that has not the sanction of high authority as well as of the writer's own conviction; but it will be clear that the believers in the truth of the analysis as made in the later chapters may become progressively fewer, as the various aspects of human life and of human nature are severally treated. Nevertheless, I believe that this volume presents a consistent reasonable view that will not be essentially different from the conceptions of all men of science who believe in evolution.

CONTENTS

CHAPTER PAGE

I. EVOLUTION. THE LIVING ORGANISM AND ITS NATURAL HISTORY 1

II. THE STRUCTURE AND DEVELOPMENT OF ANIMALS AS EVIDENCE OF EVOLUTION 35

III. THE EVIDENCE OF FOSSIL REMAINS 73

IV. EVOLUTION AS A NATURAL PROCESS 106

V. THE PHYSICAL EVOLUTION OF THE HUMAN SPECIES AND OF HUMAN RACES 150

VI. THE MENTAL EVOLUTION OF MAN 197

VII. SOCIAL EVOLUTION AS A BIOLOGICAL PROCESS 241

VIII. EVOLUTION AND THE HIGHER HUMAN LIFE 278

INDEX 313

I

EVOLUTION. THE LIVING ORGANISM AND ITS NATURAL HISTORY

The Doctrine of Evolution is a body of principles and facts concerning the present condition and past history of the living and lifeless things that make up the universe. It teaches that natural processes have gone on in the earlier ages of the world as they do to-day, and that natural forces have ordered the production of all things about which we know.

It is difficult to find the right words with which to begin the discussion of so vast a subject. As a general statement the doctrine is perhaps the simplest formula of natural science, although the facts and processes which it summarizes are the most complex that the human intellect can contemplate. Nothing in natural history seems to be surer than evolution, and yet the final solution of evolutionary problems defies the most subtle skill of the trained analyst of nature's order. No single human mind can contain all the facts of a single small department of natural science, nor can one mind comprehend fully the relations of all the various departments of knowledge, but nevertheless evolution seems to describe the history of all facts and their relations throughout the entire field of knowledge. Were it possible for a man to live a hundred years, he could only begin the exploration of the vast domains of science, and were his life prolonged indefinitely, his task would remain forever unaccomplished, for progress in any direction would bring him inevitably to newer and still unexplored regions of thought.

Therefore it would seem that we are attempting an impossible task when we undertake in the brief time before us the study of this universal principle and its fundamental concepts and applications. But are the difficulties insuperable? Truly our efforts would be foredoomed to failure were it not that the materials of knowledge are grouped in classes and departments which may be illustrated by a few representative data. And it is also true that every one has thought more or less widely and deeply about human nature, about the living world to which we belong, and about the circumstances that control our own lives and those of our fellow creatures. Many times we withdraw from the world of strenuous endeavor to think about the "meaning of things," and upon the "why" and "wherefore" of existence itself. Every one possesses already a fund of information that can be directly utilized during the coming discussions; for if evolution is true as a universal principle, then it is as natural and everyday a matter as nature and existence themselves, and its materials must include the facts of daily life and observation.

Although the doctrine of evolution was stated in very nearly its present form more than a century ago, much misunderstanding still exists as to its exact meaning and nature and value; and it is one of the primary objects of these discussions to do away with certain current errors of judgment about it. It is often supposed to be a remote and recondite subject, intelligible only to the technical expert in knowledge, and apart from the everyday world of life. It is more often conceived as a metaphysical and philosophical system, something antagonistic to the deep-rooted religious instincts and the theological beliefs of mankind. Truly all the facts of knowledge are the materials of science, but science is not metaphysics or philosophy or belief, even though the student who employs scientific method is inevitably brought to consider problems belonging to these diverse fields of thought. A study of nervous mechanism and organic structure leads to the philosophical problem of the freedom of the will; questions as to the evolution of mind and the way mind and matter are related force the investigator to consider the problem of immortality. But these and similar subjects in the field of extra-science are beyond its sphere for the very good reason that scientific method, which we are to define shortly, cannot be employed for their solution. Evolution is a science; it is a description of nature's order, and its materials are facts only. In method and content it is the very science of sciences, describing all and holding true throughout each one.

The overwhelming importance of knowing about natural laws and universal principles is not often realized. What have we to do with evolution and science? Are we not too busy with the ordering of our immediate affairs to concern ourselves with such remote matters? So it may appear to many, who think that the study of life and its origin, and of the vital facts about plants and animals may be interesting and may possess a certain intellectual value, but nothing more. The investigation of man and of men and of human life is regarded by the majority as a mere cultural exercise which has no further result than the recording of present facts and past histories; but it is far otherwise. Science and evolution must deal with mere details about the world at large, and with human ideals and with life and conduct; and while their purpose is to describe how nature works now and how it has progressed in the past, their fullest value is realized in the sure guidance they provide for our lives. This cannot be clear until we reach the later portions of our subject, but even at the outset we must recognize that knowledge of the great rules of nature's game, in which we must play our parts, is the most valuable intellectual possession we can obtain. If man and his place in nature, his mind and social obligations, become intelligible, if right and wrong, good and evil, and duty come to have more definite and assignable values through an understanding of the results of science, then life may be fuller and richer, better and more effective, in direct proportion to this understanding of the harmony of the universe.

And so we must approach the study of the several divisions of our subject in this frame of mind. We must meet many difficulties, of which the chief one is perhaps our own human nature. For we as men are involved, and it is hard indeed to take an impersonal point of view,—to put aside all thoughts of the consequences to us of evolution, if it is true. Yet emotion and purely human interest are disturbing elements in intellectual development which hamper the efforts of reason to form assured conceptions. We must disregard for the time those insistent questions as to higher human nature, even though we must inevitably consider them at the last. Indeed, all the human problems must be put aside until we have prepared the way for their study by learning what evolution means, what a living organism is, and how sure is the evidence of organic transformation. When we know what nature is like and what natural processes are, then we may take up the questions of supreme and deep concern about our own human lives.

* * * * *

Human curiosity has ever demanded answers to questions about the world and its make-up. The primitive savage was concerned primarily with the everyday work of seeking food and building huts and carrying on warfare, and yet even he found time to classify the objects of his world and to construct some theory about the powers that made them. His attainments may seem crude and childish to-day, but they were the beginnings of classified knowledge, which advanced or stood still as men found more or less time for observation and thought. Freed from the strife of primeval and medieval life, more and more observers and thinkers have enlarged the boundaries and developed the territory of the known. The history of human thought itself demonstrates an evolution which began with the savages' vague interpretation of the "what" and the "why" of the universe, and culminates in the science of to-day.

What, now, is a science? To many people the word denotes something cold and unfeeling and rigid, or something that is somehow apart from daily life and antagonistic to freedom of thought. But this is far from being true. Karl Pearson defines science as organized knowledge, and Huxley calls it organized common sense. These definitions mean the same thing. They mean that in order to know anything that deserves confidence, in order to obtain a real result, it is necessary in the first place to establish the reality of facts and to discriminate between the true, the not so sure, the merely possible, and the false. Having accurate and verified data, scientific method then proceeds to classify them, and this is the organizing of knowledge. The final process involves a summary of the facts and their relations by some simple expression or formula. A good illustration of a scientific principle is the natural law of gravitation. It states simply that two bodies of matter attract one another directly in proportion to their mass, and inversely in proportion to the square of the distance between them. In this concise rule are described the relations which have been actually determined for masses of varying sizes and at different distances apart,—for snowflakes falling to the earth, for the avalanche on the mountain slope, and for the planets of the solar system, moving in celestial coördination.

Such a principle as the law of gravitation, like evolution, is true if the basic facts are true, if they are reasonably related, and if the conclusion is drawn reasonably from them. It is true for all persons who possess normal minds, and this is why Huxley speaks of science as "common sense,"—that is, something which is a reasonable and sensible part of the mental make-up of thinking persons that they can hold in common. The form and method of science are fully set forth by these definitions, and the purpose also is clearly revealed. For the results of investigation are not merely formulæ which summarize experience as so much "conceptual shorthand," as Karl Pearson puts it, but they must serve also to describe what will probably be the orderly workings of nature as future experience unfolds. Human endeavor based upon a knowledge of scientific principles must be far more reliable than where it is guided by mere intuition or unreasoned belief, which may or may not harmonize with the everyday world laws. Just as the law of gravitation based upon past experience provides the bridge builder and the architect with a statement of conditions to be met, so we shall find that the principles of evolution demonstrate the best means of meeting the circumstances of life.

Evolution has developed, like all sciences, as the method we have described has been employed. Alchemy became chemistry when the so-called facts of the medievalist were scrutinized and the false were discarded. Astrology was reorganized into astronomy when real facts about the planets and stars were separated from the belief that human lives were influenced by the heavenly bodies. Likewise the science of life has undergone far-reaching changes in coming down to its present form. All the principles of these sciences are complete only in so far as they sum up in the best way the whole range of facts that they describe. They cannot be final until all that can be known is known,—until the end of all knowledge and of time. It is because he feels so sure of what has been gained that the man of science seems to the unscientific to claim finality for his results. He himself is the first to point out that dogmatism is unjustified when its assertions are not so thoroughly grounded in reasonable fact as to render their contrary unthinkable. He seeks only for truth, realizing that new discoveries must oblige him to amend his statement of the laws of nature with every decade. But the great bulk of knowledge concerning life and living forms is so sure that science asserts, with a decision often mistaken for dogmatism, that evolution is a real natural process.

* * * * *

The conception of evolution in its turn now demands a definite description. How are we to regard the material things of the earth? Are they permanent and unchanged since the beginning of time, unchanging and unchangeable at the present? We do not need Herbert Spencer's elaborate demonstration that this is unthinkable, for we all know from daily experience that things do change and that nothing is immutable. Did things have a finite beginning, and have they been "made" by some supernatural force or forces, personified or impersonal, different from those agencies which we may see in operation at the present time? So says the doctrine of special creation. Finally, we may ask if things have changed as they now change under the influence of what we call the natural laws of the present, and which if they operated in the past would bring the world and all that is therein to be just what we find now. This is the teaching of the doctrine of evolution. It is a simple brief statement of natural order. And because it has followed the method of common sense, science asserts that changes have taken place, that they are now taking place, and furthermore that it is unnecessary to appeal to other than everyday processes for an explanation of the present order of things.

Wherever we look we see evidence of nature's change; every rain that falls washes the earth from the hills and mountains into the valleys and into the streams to be transported somewhere else; every wind that blows produces its small or greater effect upon the face of the earth; the beating of the ocean's waves upon the shore, the sweep of the great tides,—these, too, have their transforming power. The geologists tell us that such natural forces have remodeled and recast the various areas of the earth and that they account for the present structure of its surface. These men of science and the astronomers and the physicists tell us that in some early age the world was not a solid globe, with continents and oceans on its surface, as now; that it was so very hot as to be semi-fluid or semi-solid in consistency. They tell us that before this time it was still more fluid, and even a mass of fiery vapors. The earth's molten bulk was part of a mass which was still more vast, and which included portions which have since condensed to form the other bodies of the solar system,—Mars and Jupiter and Venus and the rest,—while the sun remains as the still fiery central core of the former nebulous materials, which have undergone a natural history of change to become the solar system. The whole sweep of events included in this long history is called cosmic evolution; it is the greater and more inclusive process comprising all the transformations which can be observed now and which have occurred in the past.

At a certain time in the earth's history, after the hard outer crust had been formed, it became possible for living materials to arise and for simple primitive creatures to exist. Thus began the process of organic evolution—the natural history of living things—with which we are concerned in this and later addresses. Organic evolution is thus a part of the greater cosmic process. As such it does not deal with the origin of life, but it begins with life, and concerns itself with the evolution of living things. And while the investigator is inevitably brought to consider the fundamental question as to the way the first life began, as a student of organic forms he takes life for granted and studies only the relationships and characteristics of animals and plants, and their origins.

But even as a preliminary definition, the statement that organic evolution means natural change does not satisfy us. We need a fuller statement of what it is and what it involves, and I think that it would be best to begin, not with the human being in which we are so directly interested, nor even with one of the lower creatures, but with something, as an analogy, which will make it possible for us to understand immediately what is meant by the evolution of a man, or of a horse, or of an oak tree. The first steam locomotive that we know about, like that of Stephenson, was a crude mechanism with a primitive boiler and steam-chest and drive-wheels, and as a whole it had but a low degree of efficiency measured by our modern standard; but as time went on inventive genius changed one little part after another until greater and greater efficiency was obtained, and at the present time we find many varied products of locomotive evolution. The great freight locomotive of the transcontinental lines, the swift engine of the express trains, the little coughing switch engine of the railroad yards, and the now extinct type that used to run so recently on the elevated railroads, are all in a true sense the descendants of a common ancestor, namely the locomotive of Stephenson. Each one has evolved by transformations of its various parts, and in its evolution it has become adapted or fitted to peculiar circumstances. We do not expect the freight locomotive with its eight or ten powerful drive-wheels to carry the light loads of suburban traffic, nor do we expect to see a little switch engine attempt to draw "the Twentieth Century Limited" to Chicago. In the evolution, then, of modern locomotives, differences have come about, even though the common ancestor is one single type; and these differences have an adaptive value to certain specific conditions. A second illustration will be useful. Fulton's steamboat of just a century ago was in a certain true sense the ancestor of the "Lusitania," with its deep keel and screw propellers, of the side-wheel steamship for river and harbor traffic like the "Priscilla," of the stern-wheel flat-bottom boats of the Mississippi, and of the battleship, and the tug boat. As in the first instance, we know that each modern type has developed through the accumulation of changes, which changes are likewise adjustments to different conditions. The diversity of modern types of steamships may be attributed therefore to adaptation.

The several kinds are no more interchangeable than are the different forms of locomotives that we have mentioned. The flat-bottom boat of the Mississippi would not venture to cross the Atlantic Ocean in winter, nor would the "Lusitania" attempt to plow a way up the shallow mud-banked Mississippi. These products of mechanical development are not efficient unless they run under the circumstances which have controlled their construction, unless they are fitted or adapted to the conditions under which they must operate.

Evolution, then, means descent with adaptive modification. We must examine the various kinds of living creatures everywhere to see if they, like the machines, exhibit in their make-up similar elements which indicate their common ancestry in an earlier age, and if we can interpret their differences as the results of modifications which fit them to occupy different place in nature.

Two objections to the employment of these analogies will present themselves at once. The definition may be all very well as far as the machines are concerned, but, it may be asked, should a living thing like a horse or a dog be compared with the steamship or the locomotive? Can we look upon the living thing as a mechanism in the proper sense of the word? A second objection will be that human invention and ingenuity have controlled the evolution of the steamship and engine by the perfection of newer and more efficient parts. It is certainly true that organic evolution cannot be controlled in the same way by men, and that science has not yet found out what all the factors are. And yet we are going to learn in a later discussion that nature's method of transforming organisms in the course of evolution is strikingly similar to the human process of trial and error which has brought the diverse modern mechanisms to their present conditions of efficiency. This matter, however, must remain for the time just as it stands. The first objection, namely, that an organism ought not to be viewed as a machine, is one that we must meet immediately, because it is necessary at the very outset to gain a clear idea of the essentially mechanical nature of living things and of their relations to the conditions under which they live. It is only when we have such a clear understanding that we can profitably pursue the further inquiries into the evidence of evolution. Our first real task, therefore, is an inquiry into certain fundamental questions about life and living things, upon which we shall build as we proceed.

* * * * *

All living things possess three general properties which seem to be unique; these are a peculiar chemical constitution, the power of repairing themselves as their tissues wear out, and the ability to grow and multiply. The third property is so familiar that we fail to see how sharply it distinguishes the creatures of the organic world. To realize this we have only to imagine how strange it would seem if locomotives and steamships detached small portions of themselves which could grow into the full forms of the parent mechanisms. Equally distinctive is the marvelous natural power which enables an animal to re-build its tissues as they are continually used up in the processes of living; for no man-made, self-sustaining mechanism has ever been perfected. The property of chemical composition is believed by science to be the basis of the second and the third; but this matter of chemical constitution must take its proper place in the series of structural characters, which we shall discuss further on as we develop the conception of organic mechanism.

Whatever definition we may employ for a machine or an engine, we cannot exclude the living organism from its scope. As a "device for transforming and utilizing energy" the living organism differs not at all from any "dead" machine, however complex or simple. The greatest lesson of physiological science is that the operations of the different parts of the living thing, as well as of the whole organism itself, are mechanical; that is, they are the same under similar circumstances. The living creature secures fresh supplies of matter and energy from the environment outside of itself; these provide the fuel and power for the performance of the various tasks demanded of an efficient living thing, and they are the sources upon which the organism draws when it rebuilds its wasted tissues and replenishes its energies. The vital tasks of all organisms must be considered in due course, but at first it is necessary to justify our analogies by analyzing the structural characteristics of animals and plants, just as we might study locomotives in a mechanical museum before we should see how they work upon the rails.

Among the familiar facts which science reveals in a new light are the peculiarly definite qualities of living things as regards size and form. There is no general agreement in these matters among the things of the inorganic world. Water is water, whether it is a drop or the Pacific Ocean; stone is stone, whether it is a pebble, a granite block, or a solid peak of the Rocky Mountains. It is true that there is a considerable range in size between the microscopic bacterium at one extreme and the elephant or whale at the other, but this is far less extensive than in the case of lifeless things like water and stone. In physical respects, water may be a fluid, or a gas in the form of steam, or a solid, as a crystal of snow or a block of ice. But the essential materials of living things agree throughout the entire range of plant and animal forms in having a jellylike consistency.

But by far the most striking and important characteristic of living things is their definite and restricted chemical composition. Out of the eighty and more chemical elements known to science, the essential substance of living creatures is formed by only six to twelve. These are the simple and obvious characteristics of living things which are denoted by the word "organic." Everyone has a general idea of what this expression signifies, but it is important to realize that it means, in exact scientific terms,—constituted in definite and peculiar ways.

The living thing, then, possesses a definite constitution, which is a mechanical characteristic, while furthermore it is related to its surroundings in a hard and fast way. Just as locomotives are different in structure so that they may operate successfully under different conditions, so the definite characteristics of living things are exactly what they should be in order that organisms may be adjusted or fitted into the places in nature which they occupy. This universal relation to the environment is called adaptation. It is only too obvious when our attention is directed to it, but it is something which may have escaped our notice because it is so natural and universal. The trunk of a tree bears the limbs and branches and leaves above the ground, while the roots run out into the surrounding soil from the foot of the trunk; they do not grow up into the air. An animal walks upon its legs, the wings of a bird are just where they should be in order that they may be useful as organs of flight. And these mechanical adjustments in the case of living creatures occur for the same reason as in mechanisms like the steamship, which has the propeller at its hinder end and not elsewhere, and which bears its masts erect instead of in any other way.

The next step in the analysis of organisms reveals the same wonderful though familiar characteristics. The living organism is composed of parts which are called organs, and these differ from one another in structural and functional respects. Each of them performs a special task which the others do not, and each differentiated organ does its part to make the whole creature an efficient mechanism. The leg of the frog is an organ of locomotion, the heart is a device for pumping blood, the stomach accomplishes digestion, while the brain and nerves keep the parts working in harmony and also provide for the proper relation of the whole creature to its environment. So rigidly are these organs specialized in structure and in function that they cannot replace one another, any more than the drive wheels of the locomotive could replace the smokestack, or the boiler be interchanged with either of these. All of the organs are thus fitted or adjusted to a particular place in the body where they may most efficiently perform their duties. Each organ therefore occupies a particular place in an organic environment, so to speak. Thus the principle of adaptation holds true for the organs which constitute an organism, as well as for organisms themselves in their relations to their surroundings.

The various organs of living things are grouped so as to form the several organic systems. There are eight of these, and each performs a group of related tasks which are necessary for complete life. The alimentary system concerns itself with three things: it gets food into the body, or ingests; it transforms the insoluble foods by the intricate chemical processes of digestion; and it absorbs or takes into itself the transformed food substances, which are then passed on to the other parts of the body. It is hardly necessary to point out that the ingestive structures for taking food and preparing it mechanically lie at and near the mouth, while the digesting parts, like the stomach, come next, because chemical transformation is the next thing to be done; while finally the absorbing portions of the tract, or the intestines, come last. The second group of organs, like gills and lungs, supplies the oxygen, which is as necessary for life as food itself; this respiratory system also provides for the passage from the body of certain of the waste gases, like carbonic acid gas and water vapor. The excretory system of kidneys and similar structures collects the ash-waste produced by the burning tissues, and discharges this from the whole mechanism, like the ash hoist of a steamship. The circulatory system, made up of smaller and larger vessels, with or without a heart, transports and propels the blood through the body, carrying the absorbed foods, the supplies of oxygen, and the waste substances of various kinds. All of these four systems are concerned with "commissary" problems, so to speak, which every individual must solve for and by itself.

Another group of systems is concerned with wider relations of the individual and its activities. For example, the motor system accomplishes the movements of the various organs within the body, and it also enables the organism to move about; thus it provides for motion and locomotion. Systems of support, comprising bones or shells, occur in many animals where the other organs are soft or weak. Perhaps the most interesting of the individual systems of relation is the nervous system. The strands of its nerve fibers and its groups of cells keep the various organs of the body properly coördinated, whereas in the second place, through the sensitive structures at the surface of the body, they receive the impressions from the outside world and so enable the organism to relate itself properly to its environment. The last organic system differs from the other seven in that the performance of its task is of far less importance to the individual than it is to the race as a whole. It is the reproductive system, with a function that must be always biologically supreme. We can very readily see why this must be so; it is because nature has no place for a species which permits the performance of any individual function to gain ascendency over the necessary task of perpetuating the kind. Nature does not tolerate race suicide.

All organisms must perform these eight functions in one way or another. The bacterium, the simplest animal, the lowest plant, the higher plants and animals,—all of these have a biological problem to solve which comprises eight terms or parts, no more and no less. This is surely an astonishing agreement when we consider the varied forms of living creatures. And perhaps when we see that this is true we may understand why adaptation is a characteristic of all organisms, for they all have similar biological problems to solve, and their lives must necessarily be adjusted in somewhat similar ways to their surroundings.

Carrying the analysis of organic structure one step further, it is found that the various organisms are themselves complex, being composed of tissues. A frog's leg as an organ of locomotion is composed of the protecting skin on the outside, the muscles, blood vessels, and nerves below, and in the center the bony supports of the whole limb. Like the organs, these tissues are differentiated, structurally and functionally, and they also are so placed and related as to exhibit the kind of mechanical adjustment which we call adaptation. The tissues, then, in their relations to the organs are like the organs in their relations to the whole creature, i.e. adapted to specific situations where they may most satisfactorily perform their tasks.

Finally, in the last analysis, all organisms and organs and tissues can be resolved into elements which are called cells. They are not little hollow cases, it is true, although for historical reasons we employ a word that implies such a condition. They are unitary masses of living matter with a peculiar central body or nucleus, and every tissue of every living thing is composed of them.

The cells of bone differ from those of cartilage mainly in the different consistency of the substances secreted by the cells to lie between them; skin cells are soft-walled masses lying close together; even blood is a tissue, although it is fluid and its cells are the corpuscles which float freely in a liquid serum. Thus an organism proves to be a complex mechanism composed of cells as structural units, just as a building is ultimately a collection of bricks and girders and bolts, related to one another in definite ways.

Our analysis reveals the living creature in an entirely new light, not only as a machinelike structure whose parts are marvelously formed and coordinated in material respects, but also as one whose activities or workings are ultimately cellular in origin. Structure and function are inseparable, and if an animal or a plant is an aggregate of cells, then its whole varied life must be the sum total of the lives of its constituent cells. Should these units be subtracted from an animal, one by one, there would be no material organism left when the last cells had been disassociated, and there would be no organic activity remaining when the last individual cell-life was destroyed. All the various things we do in the performance of our daily tasks are done by the combined action of our muscle and nerve and other tissue cells; our life is all of their lives, and nothing more. The cell, then, is the physiological or functional unit, as truly as it is the material element of the organic world. Being combined with countless others, specialized in various ways, relations are established which are like those exhibited by the human beings constituting a nation. In this case the life of the community consists of the activities of the diverse human units that make it up. The farmer, the manufacturer, the soldier, clerk, and artisan do not all work in the same way; they undertake one or another of the economic tasks which they may be best fitted by circumstances to perform. Their differentiation and division of labor are identical with the diversity in structure and in function as well, exhibited by the cells of a living creature. We might speak of the several states as so many organs of our own nation; the commercial or farming or manufacturing communities of a state would be like the tissues forming an organ, made up ultimately of human units, which, like cells, are engaged in similar activities. As the individual human lives and the activities of differentiated economic groups constitute the life of a nation and national existence, so cell-lives make the living of an organism, and the expressions "division of labor" and "differentiation" come to have a biological meaning and application.

* * * * *

The cell, then, is in all respects the very unit of the organic world. Not only is it the ultimate structural element of all the more familiar animals and plants that we know, as the foregoing analysis demonstrates, but, in the second place, the microscope reveals simple little organisms, like Amoeba, the yeast plant and bacteria, which consist throughout their lives of just one cell and nothing more. Still more wonderful is the fact that the larger complex organisms actually begin existence as single cells. In three ways, therefore,—the analytic, the comparative, and the developmental,—the cell proves to be the "organic individual of the first order." As the ultimate biological unit, its essential nature must possess a profound interest, for in its substance resides the secret of life.

This wonderful physical basis of life is called protoplasm. It contains three kinds of chemical compounds known as the proteins, carbohydrates, and hydrocarbons. Proteins are invariably present in living cells, and are made up of carbon, hydrogen, nitrogen, sulphur, and usually a little phosphorus. The elements are also combined in a very complex chemical way. For example, the substance called hæmoglobin is the protein which exists in the red blood cells and which causes those cells to appear light red or yellow when seen singly. Its chemical formula states the precise number of atoms which enter into the constitution of a single molecule as: C_{600}H_{960}N_{154}FeO_{179}. This is truly a marvelously complex substance when compared with the materials of the inorganic world, like water, for example, which has the formula H_{2}O. And just as the peculiar properties of H_{2}O are given to it by the properties of the hydrogen and the oxygen which combine to form it, just so, the scientist believes, the marvelous properties of protein are due to the assemblage of the properties of the carbon and hydrogen and other elements which enter into its composition.

It would be interesting to see how each one of these elements contributes some particular characteristic to the whole compound. The carbon atom, for example, is prone to combine with other atoms in definite varied ways, and the high degree of complexity which the protein molecule possesses may depend in greater part upon the combining power of its carbon elements. The nitrogen atom makes the protein an extremely volatile compound, so that the latter burns readily in the tissue cells; and the hydrogen and oxygen bring their specific characteristics to the total molecule. And furthermore, it is evident that the great complexity of this constituent, protein, gives to protoplasm its power of doing work, or, in a word, its power of living. In constructing it, much energy has been absorbed and stored up as potential energy, and so, like the stored-up energy in a watch spring or in gunpowder, this may be converted, under proper conditions, into the kinetic energy and the work of actual operation. On account of its peculiar and complex nature, it possesses great capacity for burning or oxidization, thus serving as a source of vital power. It burns in the living tissue just as coal oxidizes in the boiler of an engine; its atoms fly apart and unite with oxygen so as to satisfy their chemical affinities for this substance. If we could only see what happens to the protein molecule when it undergoes oxidization, we would witness a violent explosion, like that of a mass of gunpowder. And the astonishing fact is that this process is actually the same for the living molecule, for exploding gunpowder, and for the fuel which burns in the locomotive boiler. Does this mean that the essential process of what we call life is a chemical one? So it would seem on the basis of this fact alone, but a conclusion must be deferred until we reach a later point.

The second kind of substance which we find in protoplasm is the carbohydrate. A typical member of this group is common sugar, C_{6}H_{12}O_{6}; another sugar has the formula C_{12}H_{22}O_{11}. Starch is again a typical carbohydrate, and its formula is C_{6}H_{10}O_{5}, or some multiple of this. One sees at a glance that these substances agree in having twice as many hydrogen atoms as there are oxygen atoms, the same proportion that the hydrogen bears to the oxygen in the compound water,—a characteristic which makes it easy to remember the general constitution of carbohydrate as compared with the protein. The substances of this second class are obviously much less complex, both as regards the different kinds of atoms and in respect to the numbers of each kind that enter into the formation of a single molecule. Therefore the carbohydrates do not possess so much power or energy as the protein molecule; in short, they are not such good fuels for the living mechanism.

Finally, we find almost always in protoplasm other substances composed of carbon and hydrogen and oxygen which are called hydrocarbons, distinguished from carbohydrates by the fact that the number of oxygen atoms is less than half the number of hydrogen atoms. These substances are the fats and oils of various kinds, less powerful sources of energy than the proteins, but they contain more potential energy than the carbohydrates because they are more oxidizable.

Besides the characteristic substances of these three classes, protoplasm contains certain other chemical compounds, like the various salts of sodium, chlorine, magnesium and potassium, and a few others, which bring the list of chemical elements to the number twelve. We have already noted how strikingly small and restricted is the list of elements composing living matter as compared with the long array of eighty-odd different kinds of chemical atoms existing in the world as a whole.

But an astonishing result is reached through the brief analysis we have just made. It is this: we do not find peculiar kinds of atoms which occur exclusively in living matter; the materials are exactly the same as those of the outer world. In short, the elements of both the organic and inorganic divisions of the universe prove to be the same. Carbon is carbon, whether it is part of the substance of a living brain cell, or black inert coal, or the glistening diamond, or an incandescent part of the fiery sun. Hydrogen is the same, whether it be a constituent of the ocean, of the air, or of the living muscle fiber. And so it is with all of the other elements of the living mechanism. This starts us upon a line of thought which leads to a significant conclusion, namely, that a living thing which seems so distinct and permanent is after all only a temporary aggregate of elements which come to it from the not-living world; existing for a time in peculiar combinations which render life possible, they pass incessantly away from the living thing and return to the inorganic world. Every breath we draw sends out particles which were at one time living portions of ourselves; every movement we make involves the destruction of living muscle cells, whose protoplasm breaks down into the ash and gas and fluid wastes which eventually return to the world of dead things. A tree loses its living leaves with each recurring season, and the antlers of the stag are lost annually, to be replaced anew. Indeed the major part of some organisms is itself actually dead. The bones and hair and nails of such an animal as a cat are almost entirely lifeless, even though they are integral and necessary portions of the organism as a whole. They are constructed by living protoplasm which has died in their making. Thus without going beyond the boundaries of the individual body, these substances have passed from the sphere of life, and are dead. The apparent gap on the other side between the lifeless and living world is equally imaginary, for our living substance is continually replenished and rebuilt from the elements of our dead foods. So, as Huxley says, a living organism is like a flame or a whirlpool, which is an ever changing though seemingly constant individuality. We look at a gas flame, and we see in the flame itself those particles of gas which have come through the pipe to be agitated violently in the higher temperature of the flame as they are oxidized or burnt. These particles immediately pass off as carbonic acid gas and water vapor which are no longer parts of the flame. A fountain is continually replenished by the water which is not-fountain, but which becomes for the time a part of the graceful jet, falling out and away as it leaves the fountain itself. Just so a living organism is an ever changing, ever renewed, and ever destroyed mass of little particles—the atoms of the inorganic world which combine and come to life for a time, but which return inevitably to the world of lifeless things. This is one of the most fundamental facts of biology. The independence of a living thing like a human being or a crustacean is a product of the imagination. How can we be independent of the environment when we are interlocked in so many ways with inorganic nature? Our very substance with its energies has been wrested from the environment; and as we, like all other living things, must replenish our tissues as we wear out in the very act of living, we cannot cease to maintain the closest possible relations with the environment without surrendering our existence in the battle of life.

From the foregoing discussion, it will be evident, I am sure, that there is ample justification for the biological dictum that a living individual is a mechanism. Not only is the organism composed always of cell units grouped mechanically in tissues and organs and organic systems; not only are the operations which make up its life constant and regular under similar conditions; not only is the whole creature mechanically connected with the inorganic world; but above all the whole activity of a biological individual is concerned necessarily and again mechanically with the acquisition of materials endowed with energy, which materials and energy are mechanically transformed into living matter and its life. Even though an organism is so much more complex than a locomotive, and so plastic, nevertheless, in so far as both are mechanisms, the conception of the evolution of the former may be much more readily understood through a knowledge of the historical transformation of the latter.

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What, now, is life? To most people "life seems to be something which enters into a combination of carbon and hydrogen and the other elements, and makes this complex substance, the protoplasm, perform its various activities." Nearly every one finds it difficult to regard life and vitality as anything but actuating principles that exist apart from the materials into which they enter, and which they seem to make alive. According to this general conception, "life is something like an engineer who climbs into the cab of the locomotive and pulls the levers which make it go," as health might supposedly be regarded as something that does not inhere in well-being, but gets into the body to alter it. But is this conception really justified by the facts of animal structure and physiology? Let us recall the steps of our analysis. The living organism is a collection of differentiated parts, the organs; the life of an organism is a series of activities of the several organic systems and organs. If we could take away one organ after another, there would be nothing left after the last part had been subtracted. In a similar manner, the activities of organs prove to be the combined activities of the tissue-cells, and again the truth of this statement will be clear when we imagine the result of taking away one cell after another from organisms like the frog or tree. When the last cell had been withdrawn, there would be nothing left of the frog's structure, and there would be no element of the frog's life. It is true that the particular way the tissue-cells are combined is of primary importance, but it is none the less true that the life of a cell is the kind of element out of which the life of even the most complex organism is built. And we have seen that the essential substance of a cell is a complex chemical compound we call protoplasm, whose elements are identical with chemical substances outside the living world. Is there any ground for supposing that the properties of protoplasm are due to any other causes than those which may be found in the chemical and physical constitution of protoplasm? In brief, is life physics and chemistry? Nowadays the majority of biologists believe that it is. Just as the properties of water are contributed by the elements hydrogen and oxygen which unite to form it, just so the marvelous properties of protoplasm are regarded as the inevitable derivatives of the combined properties of the various chemical elements which constitute protoplasm. Biologists have known for more than a century, since the work of Lavoisier and Laplace in 1780, that the fundamental process of the living mechanism is oxidation, and that this process is the same, as they said, for the burning candle and the guinea pig. Beginning with Woehler, in 1828, scores of students of physiological chemistry have duplicated the chemical processes of living matter, which were regarded as so peculiar to the living organism that they seemed to be due to the operation of a non-mechanical and vital cause. The investigator mentioned was the first to construct artificially from inorganic substances the nitrogen-containing ash product of the living organism called urea. Now hundreds of so-called organic compounds have been made synthetically and their number is added to week after week. Therefore, the biologist who finds that a physical and chemical analysis of some vital processes is possible, and that the analysis is being extended with astonishing rapidity, finds himself unable to regard protoplasmic activity as anything different in kind or category from the processes of physics and chemistry which go on in the world of dead things.

It is true that even at the present time some biologists are reluctant to accept the thoroughgoing mechanical interpretation of organic phenomena, partly because these are so complex that their ultimate constituents cannot be discerned, but more often on account of the apparently purposeful nature of biological processes. Some, indeed, have gone so far as to postulate something like consciousness which controls and directs the formation of protoplasm, and the exercise of its distinctive properties in the way of growth, reproduction, and embryonic development into the adapted adult. But the fact remains that wherever analysis has been possible the constituent elements of an organic process prove to be physical and chemical. Protoplasm differs from inorganic materials only in its complexity and in the properties which seem to owe their existence to this complexity. As Huxley points out, it is no more justifiable to postulate the existence of a vitalistic principle in protoplasm than it would be to set up an "aquosity" to account for the properties of water, or a "saltness" for the qualities of a certain combination of sodium and chlorine. We may not know how the elements produce the properties of the compound, but we do know that such properties are the invariable products of their respective constituents in combination. As far as the evidence goes, it tells strongly and invariably in favor of the mechanistic interpretation.

Under the present limitations, it is impossible to give this subject the further discussion it deserves. It is not our purpose to review the origin of life in times past, and the origin of living matter from inorganic constituents, though the subject is one of the most important in the field of cosmic evolution. We must begin with the living organism; and how the first one arose must be of less importance to us than the knowledge of its mechanical constitution and of its mechanical operation. Of far greater value is the realization that a living creature is not an independent thing, but that, on the contrary, it must hold the closest possible relations with the world of materials and energies constituting its environment. We must again insist upon the importance of that mechanical adjustment to the conditions of life which is the universal characteristic of plants and animals. It is the history of these creatures and the origin of their adapted conditions that we are called upon to study. We must scrutinize the nature of to-day to see if we can find evidence that evolution is true, and if we can discern the forces which, acting upon the living mechanism as man has dealt with machines, might bring the various species of the present day to their modern forms.

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We have now learned that evolution means a common ancestry of living forms that have come to differ in the course of time; our common reason has shown us also that organisms are in a true sense complicated chemical mechanisms adapted to meet the conditions under which they must operate. We come now to the evidences offered by the organic world that evolution is true and that natural forces control its workings. Clearly the examination of the matter of fact is independent of the question of method. For just as the chemist may experiment with various substances to see if they will dissolve in water and not in alcohol before it is necessary or desirable for him to take up the further studies of the laws of solution, so reasonable grounds must be found for regarding evolution as true before passing to its method of accomplishment. And in the following discussions, the animals will be used almost exclusively, not because the study of plants fails to discover the same relations and principles, but because the better known animal series is more varied and extensive, and above all for the reason that the human organism arrays itself as the highest term of the animal series.

In the complete scheme adopted by most naturalists, five categories include the evidences bearing upon the fact of evolution. These are Classification; Comparative Anatomy, or Morphology; Comparative Development, or _Embryology; Palæontology, which comprises the facts provided by fossil relics of animals and plants of earlier geological ages; and Geographical Distribution. Each of these divisions includes a descriptive and analytical series of facts, whose characteristics are "explained" or summarized in the form of the general principles of the respective divisions. Such principles, taken singly and collectively, constitute the evidences of evolution.

The particular nature of any one of these categories, evolved in the development of science practically in the order stated, depends upon the special quality of an animal which it selects for comparison and organization in connection with other similar facts, and also in its own mode of viewing its facts. One and the same organism may present materials for two, three, or even all five of these divisions, for they are by no means mutually exclusive. For example, a common cat possesses certain definite characteristics which give it a particular place when animals more or less like it are grouped or classified according to their degrees of resemblance and difference, in small genera of very similar forms, in larger tribes or orders of similar genera, and in more and more inclusive groups of these lesser divisions, such as the classes and phyla, or main branches of the animal tree. The common cat and its relatives are even earlier to be regarded as anatomical subjects, and their thorough analysis belongs to comparative anatomy,—a name which explains itself. The purpose of this department of natural history is to explore the entire range of animal forms and animal structures, and to determine the degree of resemblance and difference exhibited by the general characters of entire organisms and by the special qualities of their several systems of organs. It provides the data from which classification selects those which indicate mutual affinities with greatest precision and surety. But its materials are all the facts of animal structure, and because each and every known organism can be and must be studied, the investigator engaged in formulating the evidence of evolution has at his disposal all the data referring to the entire realm of animals. The data of embryology are likewise coextensive with the territory of the animal world, for we do not know of any form which does not change in the course of its life history. An adult cat is the product of a kitten which is itself the result of a long series of changes from earlier and simpler conditions. In so far as it deals with structures in the making, embryology is a study of anatomy, but as it is concerned primarily with all of the plastic remodeling which animals undergo during the production of their final forms, it is an independent study. Nevertheless we shall learn how intimate are the relations of these two divisions of zoölogy and how the evolutionary teachings of each body of fact support and supplement those of the other.

Palæontology searches everywhere among the deposits of earlier ages for links to be fitted into their proper sequence of time, from which it constructs the chain of diverse types leading down to the species of the present. A cat of to-day is therefore viewed in an entirely different connection, as the last term in a consecutive series of species. Forming alliances with geology, and even with physics and chemistry, this department of zoölogy endeavors to reconstruct the past from what it learns to-day about organisms and the conditions under which they live. Finally the observations that cats of various kinds do not occur everywhere in the world, but only in certain more or less restricted localities, belong to the subject of geographical distribution, and illustrate its nature.

Our task is to learn the teachings of these several divisions by recalling and putting together what we know already about the commonest animals, or noting what can be observed in a visit to a zoölogical garden and aquarium. On account of the present limitations of time, the subject of classification will be combined with comparative anatomy; embryology will be taken up together with these subjects; palæontology will be the main subject of the next discussion, which will include also a brief statement of the meaning of distribution. Then we will be prepared to study nature to see how evolution works.

II

THE STRUCTURE AND DEVELOPMENT OF ANIMALS AS EVIDENCE OF EVOLUTION

In order to become acquainted with the way the structures of animals provide evidences of evolution, it is by no means necessary to review the entire range of their forms, because research has discovered that the principles of relationship are universal among animals, and that any group of examples will demonstrate what is taught by comparative anatomy as a whole. The commonest creatures may serve us best in order that we may come to view evolution as a process that involves each and every living thing that we know, and not as something which belongs only to the remote and unknown past.

Let us begin with the common cat and the group of carnivora or flesh-eating animals to which it belongs. As we pass along the streets of the city, we will see many cats which differ in some details, though they resemble one another closely. While they vary somewhat in form, the range in this quality is not so noticeable as in the matter of color; some of them will be gray, some maltese, while others will be yellowish or black, and they will differ in the striped or spotted character of their coloration. We readily classify them all as "cats" in spite of their differences, because they are alike in so many ways that we have learned to associate as the distinguishing characteristics of these animals, and to label—"cat." The animals which we might see in a walk of several blocks may reasonably be regarded as offspring of the same pair of ancestors of a few years back, even though they are dissimilar. We all know that the kittens of one and the same litter vary: no two of them are ever exactly alike in color or disposition or voice or size, nor is any one identical with either of its parents, although it may be necessary to employ exact means of measuring them in order to demonstrate their variation. The fact of difference, then, is surely not inconsistent with even the closest ties of blood, and we do not need to go beyond the scope of daily observation to find that this is true in nature wherever we look.

Should we extend our observations so as to include the cats of Boston and Philadelphia and San Francisco, the animals would probably vary over a wider range, but they would be so similar to New York cats in their make-up that we would have no difficulty in regarding them and all the others of the United States as the descendants of a single pairs of ancestors, perhaps brought over in the "Mayflower." But why does this view seem justified? Because experience has taught us that the living things which resemble each other most closely are those which are most intimately bound by ties of blood and common heritage. It is "natural" for relatives to resemble one another more than persons not related, and for brothers and sisters to be more alike than cousins. Science does not refer to something outside everyday observation when it states that the possession by two animals of a great body of similar characters beneath their minor differences is an indication of their common ancestry.

Thus at the very outset our simple illustration establishes the most fundamental principle of comparative anatomy. Let us see how it works further. The Manx cat possesses an abbreviated tail, although in other respects it is practically the same as the familiar long-tailed form; the Angora and the Persian differ in having long hair. All of these animals are so much alike in so many respects, and so closely resemble the wild cats, that it is not unreasonable to regard them all as the descendants of the same original wild ancestors, and as the varying products of lines which branched out from the same stock in different directions and at different times. It is, in a word, their "cat-ness" which demonstrates their relationships. But common sense need not stop here. Guided by the facts of anatomical similarity, it convinces us that the dun-colored lion and puma, the striped tiger and the spotted leopard are simply cats of a larger growth whose remoter ancestry is one with that of the previously cited forms. Not until we explore and compare their several systems do we see how thoroughgoing is their uniformity in structural plan. And because reason justifies the view regarding the origin of domestic cats from wild ancestors, the evolution of all the various members of the cat tribe must be acknowledged. These animals exhibit a fundamental likeness, which, to employ a musical analogy, is the "theme" of "cat-ness," and they are so many variations of this theme.

The members of another tribe of the familiar carnivora display in their own way the same kind of evidences of relationship. The varieties of domesticated dogs differ far more widely among themselves than do common cats, yet their community of ancestry is demonstrated not only by structural resemblances, but also by the striking fact that forms as diverse as the greyhound and the fox terrier can be crossed. Here again there are wild forms, like the wolf and fox and jackal, so like the domesticated members of the dog tribe that we cannot fail to recognize a common "dog-ness" and its significance as evidence of the relationship in ancestry of all these animals.

Extending our survey so as to include the other tribes of flesh-eaters, identical principles come to light. One is compelled to regard the polar and grizzly bears as obvious blood relatives of the brown bear, and even of the raccoon of our own territory. Instead of walking upon their toes like cats and dogs, these animals plant their feet flat upon the ground; and they agree in many other details of structure that place them together, but somewhat apart from the other tribes. The many kinds of seals and walruses and sea elephants form still another group displaying similar bodily characters, but differing more widely from the "cat theme" in these differences. They are all true carnivora, but in the course of their evolution they have progressively changed so as to be adapted to life in the water where they find their prey. The bones of the limbs are the same in number and arrangement as in the cat's limb, but the seal's anterior appendage or "arm" has altered in numerous ways so as to become an efficient flexible paddle, while the hind limbs have shifted posteriorly, very much as screw propellers have evolved in the history of steam vessels. How the members of the seal tribe have changed in their descent from purely terrestrial ancestors is partly explained by such intermediate animals as the otter. This form is adapted by its slender body and partly webbed feet to a semi-aquatic life; it seems to have halted at a point beyond which all of the seals have passed in their evolution.

Each one of these tribes by itself provides conclusive evidence of evolution, for it is most reasonable to regard the "theme" in every case as a product of common inheritance, while the variations of any theme are best understood as the results of adaptive changes in various directions. But the examples have disclosed a larger relation and a principle of wider scope, as indeed the assignment of all these tribes to the single natural group of the carnivora implies. These tribes are put together because comparative anatomy finds that the common characters of all cats are fundamentally like those of all dogs and bears and seals, and in these common qualities the carnivora differ from all other mammalia. Does this mean that the branches which bear respectively the various members of the several tribes are outgrowths of a single limb of the evolving animal tree? Science does not hesitate to give an affirmative answer, because, as in the case of the similar but varying domestic cats, no other explanation of tribal resemblance in structure seems so reasonable and natural.

So far the examples have been taken from one order of the highest class of backboned animals, called mammalia. When our survey is extended to other divisions of this class, additional laws of organic relationship are discovered. If in a series of evolving generations the line of modification proceeding from a terrestrial animal like a cat to semi-aquatic and marine types substantially like an otter and a seal should be carried further, it will inevitably lead to forms possessing characters such as those displayed by whales and the related porpoises, dolphins, and narwhals of the order cetacea. In their make-up all of these animals clearly possess the general characteristics of mammals, and they constitute collectively another limb which has sprung from the same stock as the carnivora, although at an earlier time. This we believe because of their plan of body and because their peculiar organization fits them even more perfectly than the seals for aquatic existence that is their only possible mode of life. In the case of the whales the bony framework of the fore limb is again like that of the cat's leg, although the whole structure is a flexible finlike paddle. The hind limb has disappeared as an efficient organ, but the significant fact is that small rudiments of hind limbs are present just where corresponding structures are placed in the seal. These vestiges cannot be reasonably accounted for, unless they are the degenerate hinder limbs of a remote four-footed ancestor. Furthermore the unborn whale possesses a complete coat of hair, which is afterwards replaced by blubber; but hair is a thatchlike coat to shed rain, as the way the hairs lie on a terrestrial mammal indicates. We are therefore forced to conclude that whales have originated from four-footed animals walking about on land, because no opposed explanation gives so reasonable an interpretation of the observed facts.

Another group of familiar animals materially reinforces the results already established. After what has been said, it will not be difficult to perceive the meaning of the resemblances among mice of the house and field, and of rats and rabbits and squirrels. All of them possess heavy curved gnawing teeth, or incisors, and lack the flesh-tearing or canine teeth. They agree in many other respects which distinguish them as a separate natural order of the mammals called the rodentia. Again we find a highly aberrant form in the flying squirrel, which leads toward an order with another plan of body. This animal is a true rodent, which lengthens its leap from branch to branch by means of a fold of skin stretching between its fore and its hind limbs. It is an animated aeroplane, and it shows in part how bats have originated. The wing of a bat is an elastic membrane stretching not only between the two legs of one side, but also between the greatly lengthened "fingers" of the fore limb. But the bones of arm, wrist, and fingers are almost precisely the same in number and relation as in walking forms. The fact that this peculiar wing adheres to a plan belonging to the anterior legs of walking or climbing types has no reasonable explanation save that of evolution.

The well-known group of hoofed animals, including horses and cattle, is also valuable for our present purposes, as well as in a later connection when the evidence of fossils is described. The elephant possesses five toes armed with well-developed nails or hoofs. A tapir has four or three toes, and it would seem that its ancestor had had five toes, of which one or two had been lost. A rhinoceros possesses three toes, and its foot is constructed internally like the elephant's with the outer elements absent. The horse comes last with one large toe and hoof, but on either side of the main bones of this digit are vestiges of what must have been toes in its ancestors. Among the even-toed forms the hippopotamus has four which reach the ground, with a vestige of a fifth, so this animal has apparently descended from a typical mammal with the full number along a different line from that taken by the odd-toed forms. A pig has a cloven hoof, made up of what we may call the third and fourth members of a series of five digits, but the second and fifth fingers and toes are present, though they are withdrawn from the ground so as to be no longer functional; this animal seems to have proceeded further along the same line taken by the hippopotamus. A deer, with still smaller rudiments at the sides of its double foot, leads in the comparative series to the camel with a cloven hoof devoid of any such relics.

We must pass with only brief mention the lower orders of mammalia, like the insect-eating forms to which armadillos and ant-bears belong. Of greater interest are the pouched mammals like the kangaroo and opossums, which live almost exclusively in the Australian realm. The kangaroo is endowed with a head somewhat like that of a goat, and well-developed hind legs that enable it to make leaps of astonishing length. Some of its relatives, such as the bandicoot, are like rats, or like bears, as in the case of the wombat. The Tasmanian wolf is another true marsupial, even though divergent adaptation has brought it to resemble the carnivora of the dog tribe in general appearance and in special structures like the teeth. Finally at the very bottom of the mammalian scale are two small forms living in the Australian faunal region. The duckbill or Ornithorhynchus is the better known animal, with its close fur, webbed feet, and flattened ducklike beak, while its only other near relative, the Echidna, is somewhat similar to the spiny hedgehog in external appearance. A unique peculiarity of these two forms is that they produce eggs much like those of reptiles and birds, and this fact, together with others of a structural nature, brings the whole group of mammals near to the lower classes of the Vertebrata.

Looking back on the several orders of mammals, it will be seen that the last mentioned are much less differentiated or specialized in their general organization. Above the level of the egg-layers and the pouched mammals, the higher orders branch out in different directions and reach up to various levels of the scale of animal organization.

The foregoing structural evidences of organic transformation in the past histories of cats and seals and whales insistently recall the analogies of the locomotive and the ship employed at the outset. All these animals, like the mechanical examples, have come to differ in their derivation from the same original parents, and their lines of descent have diverged so as to fit the products of evolutionary modification to diverse circumstances. Even the vestigial organs of animals have their counterparts in the machines. The cowcatcher was a large and important structure in the early days of railroading, but it has become relatively useless with the decrease of grade crossings and the construction of more complete lines of fence. The structure still persists, sometimes in a greatly reduced form. Even more obvious is the change of structure in the case of masts of vessels, which originally bore the sails for propelling the ship. When steam engines were employed to give motive power, masts did not disappear. They now provide the derrick supports of trading steamers; in battleships their function is changed to that of fighting tops and signal yards. Even the poles carried by canal boats to bear windmills must be regarded as the reduced vestiges of masts originally constructed to carry sails; and their adaptive evolution, like that of countless structures in animals, has been accomplished by degeneration.

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The birds are another class of backboned animals which exhibit identical principles of relationship. A heron has long legs and wide-spreading toes, which keep its body out of the water as it stalks about the marshes where it seeks its food; its bill is a long slender pincers. Compare it with an eagle; the latter has a short and heavily hooked beak to tear flesh, while its stout legs bear strongly curved talons to hold its struggling prey. Swimming birds like the swan and duck and loon possess feet which are constructed in general like those of the former examples, but they are webbed and shortened to serve as paddles. In the penguin we find a counterpart of the seal among mammals; its feathers are much reduced and its fore limbs are no longer wings enabling the animal to fly, but they are paddles which it uses when it swims in pursuit of fish. Finally the ostrich and wingless bird of New Zealand—the Apteryx—have wings that are useless vestiges, which, in the latter case, are hidden under the brushlike feathers covering the body. It is unnecessary to add more examples, for even these few illustrations establish exactly the same principles of relationship and evidences of evolution that are to be found in the series of mammalia.

Reptiles also are grouped, like the mammals and birds, as variations about a central theme. An ordinary lizard is perhaps the nearest in form to the remote ancestor from which all have sprung. Some lizards are long and very slender, with all four limbs of greatly reduced size. Others, which are still true lizards, have lost the hind limbs, or even all the legs, as in the "blind worms" of England. One step more, and an animal which has progressed further along a similar line of descent would be a snake. Just as whales as a group are derivable from forms which resemble types belonging to another order, so snakes as an order are to be regarded as more radically altered derivatives of some four-footed lizardlike creature. Alligators are very much like lizards in general form, and their order is a diverging branch from the same limb. Finally the evolution of turtles from the same ancestors is intelligible if we begin with a short stout animal like the so-called "horned toad" of Arizona, and proceed to the soft-shelled tortoise of the Mississippi River system; the establishment of a bony armor completes the evolution of the familiar and more characteristic turtle.

Frogs and salamanders constitute another lower class, called the amphibia, whose members are gilled during the earlier stages of development. An adult frog is essentially a salamander without a tail and with highly developed hinder limbs. The salamanders differ as regards the number of fishlike gill clefts that they all possess in their young stages, but which disappear entirely or in part during later life. In comparison with the lizard as a typical reptile, a salamander is more primitive in all of its inner organic systems, while in its nearly continuous body, with head and tail gradually merging into the trunk, it also displays a somewhat simpler form of body.

The fishes are the lowest among the common vertebrates, and they offer an abundance of independent testimony as to the truth of the principles of comparative anatomy. The common shark is perhaps the most fundamental form, with a hull-like body undivided into head, trunk, and tail, and from it have originated such peculiar variations as the hammerhead and skate. Among fishes with true bones, a cod or trout is the most typical in general features. Without ceasing to be true bony fishes, the trunk-fish and cow-fish are adapted by their peculiar characters of spine and armor plate to repel many enemies. The puff fish can take in a great amount of water, when disturbed, so as to become too large to be swallowed by some of its foes, illustrating another adaptive modification for self-defense. The wonderful colors and color patterns of the tropical fish of the reef, or of the open water forms like the mouse-fish of the Sargossa Sea, often render them more or less completely hidden from the foraging enemy. A flounder looks like a fish which was originally symmetrical, but which had come to lie flat on its side upon the bottom, whereupon the eye underneath had left its original place to appear on the upper surface. The difficult and unusual conditions of deep-sea existence have been met by fishes in two ways; some forms possess luminous frilled and weedlike fins, which lure their prey to within easy reach of their jaws, while others have enormous eyes, so as to make use of all possible rays of light in their pursuit of food organisms. But all of these diverse forms are true fishes, possessing a common heritage of structure which demonstrates their unity of origin.

The brief review of backboned animals has shown how comprehensive are the principles of relationship. The families and tribes of each order, such as the carnivora, are like branches arising from a single limb; the orders in their turn exhibit common qualities of structure which mean that they have grown from the same antecedents, while even the larger divisions or classes of mammals, birds, reptiles, amphibia, and fishes, possess a deep underlying theme whose dominant motif is the backbone, which proves their ultimate unity in ancestry. The greater and lesser branches have reached different levels, for the fish is clearly simpler in its make-up than the highly specialized bird. But the great fact is that structural evidences demonstrating the reality of genealogical affinities are displayed by the entire series of vertebrates; although they differ much or little in many or fewer respects they have one and the same ground-plan.

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The lower animals devoid of backbones, and therefore called invertebrates, are not so well-known except to the student of comparative anatomy, because they are not so often met with, and because they are usually very small or microscopic; but in many respects their importance to the evolutionist surpasses that of the vertebrates. Their structural plans are far more varied, and they range more widely from higher and relatively complicated organisms to the unitary one-celled animals. A knowledge of some of them is essential for our present purpose, which is to learn how sure is the basis for the principles of relationship and how complete is the structural evidence of evolution.

Worms are represented in the minds of most people by the common earthworm or sandworm. The body in either case is made up of a series of segments or joints which agree closely throughout the animal in external appearance and in internal constitution. A section of the digestive tract, a pair of nerve centers, two funnel-like tubes for excretion, and similar blood vessels occur in each portion.

Precisely similar features are displayed by the crustacea, which seem to be so different. Every one is familiar with the appearance of lobsters and crabs. Even in these animals the body is composed of segments, but these are not like one another, nor are they freely movable throughout the body. Five are fused in all crustacea to make a head; in lower members of the order the eight succeeding segments are free, but in the lobster they are joined together and united with the head. The hinder part of this animal is a long abdomen whose segments remain more primitive and independent. But in a crab, the whole plan has been modified by the shortening and broadening of the head-thorax, and by the reduction of the abdomen, which is also turned under the anterior part of the body. The internal organic systems are constructed upon a worm plan with modifications. Nearly every one of the segments bears one pair of appendages, which can be referred by their forked nature to the two-parted, oarlike flaps of sandworms, but the appendages of crustacea have departed from their prototypes in functional respects and in details of structure. They are variously feelers, jaws, legs, pincers, and swimming paddles, evolved to serve different purposes, just as the limbs of the vertebrates we have described have become variously arms, wings, flippers and paddles in apes, bats, seals, and whales.

Butterflies, beetles, bees, and grasshoppers seem at first sight to be entirely different, even though they agree in being more or less segmented. But all of them have heads with four pairs of appendages of the same essential plan, middle thoracic regions of three segments more or less united, bearing three pairs of legs and usually two pairs of wings, while the hinder part is a freely jointed abdomen without real limbs. In these respects the countless varieties of insects agree so that they also like crustacea of various kinds seem to have been derived from wormlike animals with more simply segmented bodies. Indeed spiders and scorpions and their relatives of the group arachnida prove for similar reasons to be derivatives of the same original stock, and own cousins of the insects.

In nearly every one of the invertebrate branches we find representatives which interest us chiefly because they appear to have reached their present condition by retrograde evolution. Barnacles are really crustacea, but they have lost their eyes as well as some other structures that are most useful in animals with a free existence, because they have adopted a fixed mode of life, which has also brought about the loss of the original freely jointed character of the body. A tapeworm as an example of internal parasites is an extremely degenerate form which lacks a digestive tract, because this is superfluous in an animal which lives bathed in the nutrient fluids of its host. Comparing it in other respects with other low wormlike creatures, it appears to be a relative of peculiar simple worms with complete organization and independence of life. All these degenerate forms enlarge our conception of adaptation by adding the essential point that progress is not always the result of evolution. Indeed we have learned this in the case of vestigial and rudimentary structures of higher forms like whales, and now we find that entire animals may degenerate as a result of changes no less adaptive than progressive modifications.

Passing by other invertebrate groups made up of species arranged like higher animals in smaller and larger branches according to their degree of fundamental similarity, we arrive at a place in the scale occupied by two-layer animals without the highly developed and clearly differentiated organic systems of the forms above. The fresh-water animal Hydra exemplifies the creatures of this level, where also we find sea-anemones and the soft polyps which form corals and coral reefs by their combined skeletons. Hydra is an animal to which we must return again and again as we study one or another aspect of organic evolution. In general form it is a hollow cylinder closed at one end, by which it attaches itself, while at the upper end, surrounded by a group of tentacles, is the mouth which leads to the central cavity. The wall of this simple body is composed of two layers of cells, between which there is a gelatinous layer rarely invaded by cells. The inner layer lines the central space into which food organisms are thrust by the tentacles, and it is concerned primarily with digestion. The outer layer comprises cells for protection and sensation primarily. Cells of both layers have muscular prolongations which by their operation enable the whole animal to change its form and to move from one place to another.

It may seem that such an animal is totally unlike any of the higher and more complex types. In certain respects, however, it is identical with the other forms inasmuch as it performs all of the eight biological tasks demanded by nature. It is also similar in so far as its inner layer, like the innermost sheet of cells in higher forms, is concerned with problems of taking and preparing food, while the protective outer layer resembles in function the outermost covering of all animals higher in the scale. Beyond these a still more fundamental agreement is found in its cellular composition.

At the lower end of the animal scale are organisms which consist of one cell and nothing more. Amoeba, to which we must refer again and again, is an example of this group which possesses an overwhelming importance to the comparative student because the origins of all the characteristics of animals higher in the scale are to be found within it. Amoeba itself is a naked mass of protoplasm, about 1/100 of an inch in diameter, enclosing a nucleus. Its form is not constant during activity, for fingerlike processes called pseudopodia are pushed out tentatively in many directions to be followed as circumstances direct by the materials of the whole cell body. Other protozoa differ in possessing constant forms, or in having constant vibratile processes, or shells of some kind, while in still other cases like individuals combine to make colonies which are more or less definite and permanent. Here at the very foot of the organic scale are found animals which seem to be entirely different from those above. Upon examination they, like Hydra, prove to be the same as regards the number and kind of functions they perform, but in structural regards their evolutionary relation to all higher animals is indicated solely by the fact that they are cells composed of protoplasm. Nevertheless the principle which states that resemblance means consanguinity still holds true, for cellular constitution is a unique possession of things of the living world,—something which demonstrates the common origin of all living things just as truly as the "cat-ness" of our first series of examples reveals for a smaller group the significance of likeness and the nature of the basic law of comparative anatomy.

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Employing a figure of speech, we have climbed down the animal tree from the higher regions where the mammals belong. Having reached the very foot of the trunk we are in a position to review and summarize the evidences which we have discovered all about us as we have descended. The various examples we have mentioned and the groups to which they belong clearly occupy different places in the scale which begins with the protozoa and extends upward to the most complicated and differentiated animals. Hydra takes its place above the protozoa for obvious structural reasons; worms belong to a still higher zone, surpassed by the more complex jointed animals like crustacea and insects. Far above these are the vertebrates, among which we have already demonstrated the occurrence of different grades of organization, from the fish up to the higher amphibia and reptiles, and beyond in two directions to the diverging birds and mammals. The basic characteristics of every group in a high position may be traced back to some one or another of the divisions at a lower level, so that the general sequence of the structural levels from low to high becomes intelligible as the order of their evolution.

To my mind the rudimentary and vestigial structures of animals are in themselves proof positive of a natural history of change. The few illustrations can be reinforced by countless examples offered by every group of living animals. If such structures have not evolved naturally by degenerating from more efficient counterparts in ancestors of earlier times, and if they have been specially created, they are utterly meaningless and their very existence is unreasonable. If common sense is to be employed, they demonstrate evolution.

Everywhere throughout the whole series animals place themselves in a treelike arrangement, for in their respective levels they occur like leaves at the ends of the lines of descent which have led up to them and which are comparable to the branches and limbs arising from the trunk of a tree. Thus the major and minor divisions of animals do not follow in the order of the rungs of a ladder, even though they must be assigned to different levels according to the complexity of their construction. The summary given above, namely, that the occurrence of lower and higher levels reveals an order of evolution, is amplified and not contradicted by the statement that the species of animals are group in a treelike arrangement. It is the task of the evolutionist, provided with all the facts of comparative anatomy and dealing only with the various species as separate leaves, so to speak, to reconstruct the now invisible but not unreal twigs and branches and limbs of the animal tree, and to show how they have diverged at one time or another as they have grown and spread to produce the species of the present day. This he may do in so far as he may find sufficient materials to enable him to employ the methods of comparative anatomy and the great natural principle established by this method—that essential likeness means consanguinity.

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No evidence of evolution could be more significant and interesting than the results provided by the comparative study of development. In the first place it is an obvious fact that every living thing changes in the course of its life-history, and if as an adult it occupies a high place in the animal scale, its embryological transformation is more elaborate and intricate than in the case of a lower form. Every one knows that organisms do develop, and yet I believe that few appreciate the tremendous significance of the mere fact that this is true, while still fewer are aware that the peculiar and characteristic early stages through which an animal passes in becoming an adult are even more striking than the fact of development itself. We shall learn something of these earlier conditions in the development of some of our most familiar animals, but at the outset nothing can be more important than an appreciation of the first great lesson of this department of natural history—namely that organic transformation is real and natural. We do not need to employ the methods of formal logic to know that in growing up a human infant undergoes the changes of childhood and adolescence, that kittens become cats, and that an oak tree is produced by an acorn, for we know these things directly by observing them. It is natural for development to take place under normal conditions, and if it does not, then something has interfered with nature. Inasmuch as "growing up" is accomplished by the alteration of an organic mechanism with one structure into an individual with a changed plan of body, it is in essence the actual process of evolution which the comparative study of grown animals of to-day demonstrates in the way we have learned. The study of animal structure discovers the process of evolution because the most reasonable interpretation of the similarities and minor differences exhibited everywhere by the various groups of animals is that descent with adaptive and divergent modification has taken place; the result is reached by inference, it is true, but by scientific and logical inference. With development it is otherwise. No reasoning is necessary to tell us that organic transformation is a real and a natural process. We see it everywhere about us and we ourselves have come to be what we are by a natural history of change. Can we consistently deny that it is possible for a species to alter in the long course of time when a few brief weeks are sufficient for the new-laid egg of the fowl to develop into a fledgling? Many indeed strain at the gnat of the longer process in the past when without hesitation they recognize the real and obvious fact of individual development in a brief period.

I have said that development is a "natural" process. We employ this word for the familiar and everyday occurrence or thing; it does not imply that everything is known about the object or phenomenon, because science knows that complete and final knowledge is impossible. We say that it is natural for rain to fall to the earth, and we speak of the law of gravitation according to which this takes place as a natural principle, but it may not have occurred to many to inquire what makes rain fall and why do masses of matter everywhere behave toward one another in the consistent manner described by the law in question. Sunshine is natural, but we do not know why light travels as it does from the sun to the earth, and this is another question which, like the inquiry into the ultimate cause of the familiar and natural phenomenon of gravitation, has not yet been answered. But it is still regarded as natural for the rain to fall and for the sun to shine. In the same way does science view development, denoting it natural because it is an ordinary everyday matter. And we are under no more obligation to postulate supernatural control for the changing forms in the life-history of a chick or a cat than we need to assume that gravitation and the radiation of light demand immediate supernatural direction. The embryology of no form is fully understood or described or explained, but no intelligent person would be willing to assert that because complete knowledge is lacking, it is unnatural for organic transformation to take place during growth. Whatever may be the ultimate origin and nature of the directing powers behind gravitation and development and other phenomena, we have no concern with such matters because they cannot be handled by scientific methods and one belief about them is on the same plane with any other. Our task is to deal with the everyday phenomena of life and the production of living species.

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It is not necessary to go far afield to find an animal which will introduce us to the general principles of embryology. In the present instance as in the case of comparative anatomy almost any form will disclose the meaning of development, for animate nature is uniform and consistent in its methods of operation throughout its wide range. We shall begin with the familiar frog which every one knows is a product of a tadpole; passing on to the chick we will learn more facts that will enable us to formulate the main principle of comparative embryology in definite terms; we will then be prepared to extend our survey so as to include somewhat less familiar facts and animals that are even more significant than the first illustrations.

If we should visit a woodland pond in early spring, we would find somewhere among the leaves and sticks in the water large masses of a clear jellylike consistency enclosing hundreds of little black spheres about an eighth of an inch in diameter. These are the egg masses and eggs of a common frog. Watching them day by day we see the small one-celled egg spheres divide into more and more numerous portions which are the daughter-cells, destined to form by their products the many varied tissues and organs of the developing larva and adult frog. After three or four days the egg changes from its globular form into an oval or elliptical mass, and from one end of this a small knob projects to become a flattened waving tail a few days later. On the sides of the larger anterior portion shallow grooves make their appearance and soon break through from the throat or pharynx to the exterior as gill-slits. Shortly afterwards the little embryo wriggles out of its encasing coat of jelly, develops a mouth, and begins its independent existence as a small tadpole, with eyes, nasal and auditory organs, and all other parts that are necessary for a free life. Thus the one-celled egg has transformed into something that it was not at first, and in doing this it has proved the possibility and the reality of organic reconstruction.

The tadpole breathes by means of its gills, and it is at first entirely devoid of the lungs which the adult frog possesses and uses. When we speak of the larval respiratory organs as gills we imply that they are like the organs of a fish which have the same name; they are truly like those of fishes, for the blood-vessels which go to them are essentially the same as in the lower types and they are supported by simple skeletal rods like the gill-bars of the fish. In a word, they are the same things.

The animal feeds and grows during the months of its first summer, and hibernates the following winter; with the warmth of spring it revives and proceeds further along the course of its development. Near the base of the tail two minute legs grow out from the hinder part of the body, and while these are enlarging two front legs make their appearance a little behind the gills. The tadpole now rises more frequently to the surface where it takes small mouthfuls of air. Meanwhile great changes are effected inside the body where the various systems of fishlike organs become remodeled into amphibian structures. A sac is formed from the wall of the esophagus, and this enlarges and divides to form the two simple lungs. The legs increase in size, the tail dwindles more and more, the gills close up, and soon the animal hops out on land as a complete young frog. From this time on it breathes by means of its lungs instead of gills, even though it returns to the water to escape its foes, to seek its prey, and to hibernate in the mud of the lake bed during the winter months.

All these changes are familiar and natural, but until science places them and similar facts in their proper relations their significance is lost to us. The tadpole is essentially a fish in its general structure and mode of life, even though its heritage is such that it can develop into a higher animal. When it does become a frog it proves beyond a doubt that there is no impassable barrier between fishes and amphibia. Our earlier comparison of the structures of these two classes of vertebrates led to the conclusion that the latter had evolved from antecedents like the former, and had thus followed them upon the earth; now that sequence seems to have some connection with the method by which a tadpole, obviously not a fish but nevertheless actually fishlike, changes into a frog, a member of a higher class of vertebrates. This method is employed by developing frogs apparently because it follows the ancestral order of events, and because, so to speak, the only way a frog knows how to become a frog is to develop from an egg first into a fishlike tadpole and then to alter itself as its ancestors did during their evolution in the past. We begin to see, then, that in addition to the impressive fact of development itself, the mode of organic transformation is far more conclusive evidence of evolution, because it reveals an order of events which parallels the order established by comparative anatomy as the evolutionary sequence.

However it is well to review some of the changes by which a chick comes into existence before attempting to comprehend fully the fundamental principle of development that the tadpole's history discloses to us. The egg of a common fowl is certainly not a chick. Within the calcareous shell are two delicate membranes that enclose the white or albumen; within this, swung by two thickened cords of the albumen, is the yellow yolk ball enclosed by a proper membrane of its own. In the earliest condition, even before the albumen and the shell are added and before the egg is laid, on one side of the yolk-mass there is a tiny protoplasmic spot which is at first a single cell and nothing more. The hen's egg is relatively enormous, but nevertheless, like that of the frog, it starts upon its course of development as a single unitary biological element—a cell. During the earliest subsequent hours the first cell divides again and again to form a small disk upon the surface of the yolk. Soon the cells along the middle line of this small sheet become rearranged to make an obvious streak or band, and about this line a simple tube is constructed which is destined to become the future brain and spinal cord. The whole disk continues to enlarge by further division of its constituent elements so that it encloses more and more of the yolk mass, but the little chick itself is made out of the cells along the central line of the original plate, from which it folds at the sides and in front and behind so as to lie somewhat above and apart from the flatter enclosing cell layers which partly surround the yolk.

At the sides of the primitive nerve-tube small blocks of cells arise to develop into primitive muscles and other structures. As nourishment is brought to the embryo from the surrounding layers enclosing the nutrient yolk, one system after another takes its shape and builds its several parts into organs which can be recognized as elementary structures of a chick. Among the more interesting ones are small clefts or slits formed in the side walls of the rudimentary throat or pharynx. Blood-vessels go forward from the simple heart to run up through the intervening bars exactly as in the tadpole and the fish. In brief, the young chick possesses a series of gill-slits, for these structures are the same in essential plan and relations as the clefts of tadpoles and fishes. Does this mean that even birds have descended from gill-breathing ancestors? Science answers in the affirmative, because evolution gives the only reasonable explanation of such facts as these. The case seems different from that of the frog, because gills are used by the tadpole, but gill-slits and gill-bars can have no conceivable value for the chick as organs concerned with the purification of the blood. None the less, if the transition from a gilled tadpole to the adult with lungs means an evolution of amphibia from fishlike ancestors, then the change of a chick embryo with gill-clefts into the fledgling without them is most reasonably interpreted as proof that birds as well as amphibia have had ancestors as simple as fishes.

As development progresses four small pads make their appearance; two of these lie on either side of the body back of the head and the other two arise near the posterior end. They are far from being wings and legs, but as day follows day they become molded into somewhat similar limbs, as much alike in general plan as the four legs of a lizard; subsequently the ones at the front change into real wings and the hinder ones become legs. Meanwhile the internal organs slowly transform from fishlike structures into things that display the characteristics of reptilian counterparts, and only later do they become truly avian. Last of all the finishing touches are made, and the whole creature becomes a particular kind of a bird which picks its way out of the shell and shifts for itself as a chick.

Only a few of the countless details have been mentioned which demonstrate the resemblance of the successive stages first to fishes, and later to amphibia and reptiles. We have a wide choice of materials, but even the foregoing brief list of illustrations shows that the order in which the stages follow is the one which comparative anatomy independently proves to be the order of the evolution of fishes, amphibia, reptiles, and birds. Why, now, should it be necessary for a developing bird to follow this order? The answer has been found in the immense array of embryological facts that investigators have verified and classified, that all tell the same story. It is, that birds have arisen by evolution from ancestors which were really as simple as the members of these lower classes. It seems then that the only way a bird of to-day can become itself is to traverse the path along which its progenitors had progressed in evolution. Stating its conclusions precisely, science formulates the principle in the following words: individual development is a brief résumé of the history of the species in past times, or, more technically, ontogeny recapitulates phylogeny. To be sure, the full history is not reviewed in detail, for the chick embryo does not actually swim in water and breathe by means of gills. Only a condensed account of evolution of its kind is presented by an embryo during its development; as Huxley and Haeckel have put it, whole lines and paragraphs and even pages are left out; many false passages of a later date are inserted as the result of peculiar larval and embryonic needs and adjustments. But in its major statements and as a general outline, the account is a trustworthy natural document submitted as evidence that higher species of to-day have evolved from ancestors which must have been like some of the present lower animals.

Coming now to the mammalia, it might seem that we have reached forms so highly developed that they would not exhibit the same kind of developmental history, but would have their own mode of growing up. This is not so, for like the adult fish, the larval tadpole, and the embryo chick, an embryo of a cat or a man is at one time constructed with a series of gill-clefts and with blood-vessels and skeletal supports of fishlike nature that are everywhere associated with gills. The embryos of wildcats and dogs, rabbits and rats, pigs, deer, and sheep, and of all other mammalia, possess similar structures. Thus they all pass through a stage which is found also in the development of reptiles, birds, and amphibia,—a stage which corresponds to the fish throughout its life. Unless these facts mean that the great classes of vertebrates have originated together from the same or closely similar ancestors, they are unintelligible; for we cannot see why a cat or a chick should have to be essentially fishlike at any time unless this is so. Comparative anatomy states as we have learned that the amphibia as a class have evolved from and have out-developed the fishes, that reptiles have progressed still higher, and that birds and mammals have originated from reptilian ancestors along roads that have diverged beyond the immediate parent class. Because the members of each class have to pass along the same path trodden by their many varied ancestors, although at express speed, as it were, the similarity of the earliest stages in their development is explained, for during these periods they are traversing a path over which their ancestors passed together.

The places where the developing embryos depart from the common mode show where the several divisions took leave of one another in their evolution,—a point that comes out with great clearness when the facts of mammalian development are broadly compared. The embryos of carnivora and rodents and hoofed animals are alike in their earlier development, and their agreement means a community of origin. At a certain point the cat and dog depart from the common mode, but they remain alike up to a far later stage than the one in which they are similar to the embryos of rats and sheep. The rat and squirrel and rabbit, on their part, remain together until long after they take leave of the carnivora and ungulates; while the sheep and cattle and pigs have their own branch line, which they follow in company after leaving the embryos of the other orders. The reasons for these facts seem to be that the members of the three orders exemplified have evolved from the same stock, which accounts for their embryonic similarity for a long time after they collectively come to differ from amphibia and reptiles, while the members in each order became differentiated only later, wherefore their embryonic paths coincide for a longer period. Thus the degree of adult resemblance which indicates the closeness of relationship corresponds with the degree of embryonic agreement; that is, the cat and dog are much alike and their modes of development are essentially the same to the latest stages, while the cat and horse agree only during the earliest and middle stages, and their lines diverge before those of the cat and dog on the one hand, or those of the horse and pig on the other.

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Like the fundamental principle of comparative anatomy in its sphere, the Law of Recapitulation, formulated as a summary description of the foregoing and similar facts, is one that holds true throughout the entire range of embryology and for every division of the animal series, however large or small. We have discussed its broader application, and now we may take up some of the more or less special cases mentioned in the earlier section of the present chapter, to see how it may work in detail.

The flounder was noted as a variant of the fish theme which seemed to be a descendant of a symmetrical ancestor because its structural plan was like that of other bony fishes. If this be true, and if in its development a flounder must review its mode of evolution as a species, the young fish ought to be symmetrical; and it actually is. The grotesque skate and hammerhead shark were demonstrated to be derivatives of a simpler type of shark; their embryos are practically indistinguishable from those of ordinary dogfish and sharks.

Among the jointed animals a wealth of interesting material is found by the embryologist. All crabs seemed to be modified lobsterlike creatures; to confirm this interpretation, based solely upon details of adult structure, young crabs pass through a stage when to all intents and purposes they are counterparts of lobsters. Even the twisted hermit crab, which has a soft-skinned hinder part coiled to fit the curve of the snail shell used as a protection, is symmetrical and lobster-like when it is a larva.

Among the insects many examples occur that are already familiar to every one. The egg of a common house-fly hatches into a larva called a maggot; in this condition the body destined to become the vastly different fly is composed of soft-skinned segments very much alike and also similar to the joints of a worm. Comparative anatomy demonstrates that the fly and all other insects have arisen from wormlike ancestors, whose originally similar segments later differentiated in various ways to become the diverse segments of adult insects; the embryonic history of flies of to-day corroborates these assertions, in so far as every individual fly actually does become a wormlike larva before it changes into the final and complete adult insect. The other kinds of insects are equally striking in their life-histories. All beetles, such as the potato bug and June bug, develop from grubs which, like the maggots of flies, are similar to worms in numerous respects. Butterflies and moths pass through a caterpillar stage having even more striking resemblances to worms. All the larvæ of insects are therefore like one another, and like worms also, in certain fundamental characters of internal and external structure; so the conclusion that the whole group of insects has arisen by evolution from more primitive ancestors resembling the worms of to-day is based upon mutually explanatory details of comparative anatomy and embryology.

* * * * *

Let us now turn back to some of the earlier pages of the embryological record which we passed over in order that we might translate the later portions dealing with more familiar and intelligible structures like gills. Before the egg of the frog becomes an elliptical mass of cells, it is at one time a double-walled sac enclosing a central cavity; in this stage it is called a gastrula. Tracing back the mode of its formation, we find that it is produced from a hollow sphere of fewer cells that are essentially alike; this stage also is so important that the special term blastula is applied to it. Still earlier, there are fewer cells—128 or thereabouts, 64, 32, 16, 8, 4, 2, and 1. In other words, the starting point in the development of the frog is a single biological unit; this divides and its products redivide to constitute the many-celled blastula and the double-walled gastrula. All the other animals we have mentioned begin like the frog, as eggs which are single cells and nothing more; they too pass on to become blastulæ and gastrulæ, similar to those of the frog in all essential respects, particularly as regards the nature of the organs produced by each of the two primary layers, and the mode of their formation. Does the occurrence of blastulæ and gastrulæ and one-celled beginnings mean that the higher animals composed of numerous and much differentiated cells have evolved in company from two-layered saccular ancestors which were themselves the descendants of spherical colonies of like cells, and ultimately of one-celled animals?

Comparative anatomy has asserted that this is so, as we have already learned, for it finds that adult animals array themselves at different levels of a scale beginning at the bottom with the protozoa, continuing on to the two-layered animals like Hydra and jellyfish and sea-anemones, and then extending upwards to the region of the more complicated invertebrates and vertebrates. It was difficult perhaps to believe that these successive grades of organic structure indicated an order of evolution, because it seemed impossible that an animal so simple as a protozoan could produce offspring with the complex organization of a frog or a cat, even in long ages. But development delivers its evidence relating to this matter with telling and impressive force. How can we doubt the possibility of an evolution of higher animals from ancestors as simple as Hydra and Amoeba when a frog and a cat, like all other complicated organisms, begin individual existence as single cells, and pass through gastrula stages? If we deny it, we contradict the evidence of our senses, for the development is actually accomplished by the transformation of a single cell into a double-walled sac, and of this into different and more intricate organic mechanisms. The process can take place, for it does take place. Not until the investigator becomes familiar with a wide range of diverse animals and the peculiar qualities of their similar early stages, can he estimate the tremendous weight of the facts of comparative embryology. Were the statement iterated and reiterated on every page and in every paragraph, there would be no undue emphasis put upon the astounding fact that the apparently impassable gap between a one-celled animal like Amoeba and a mammal like a cat is actually compassed during the development of the last-named organisms from single cells. The occurrence of gill-slits in the embryos of lizards, birds, and mammals now seems a small thing when compared with the correspondences disclosed by the earliest stages of development. But in spite of their complexity, all the changes of "growing up" are explained and understood by the simple formula that the mode of individual development owes its nature primarily to the hereditary influence of earlier ancestors back to the original animals which were protozoa.

* * * * *

Embryology as a distinct division of zoölogy has grown out of studies of classification and comparative anatomy. Its beginnings may be found in medieval natural history, for as far back as 1651 Harvey had pointed out that all living things originate from somewhat similar germs, the terse dictum being "Ex ovo omnia." By the end of the eighteenth century many had turned to the study of developing organisms, though their views by no means agreed as to the way an adult was related to the egg. Some, like Bonnet, held that the germ was a minute and complete replica of its parent, which simply unfolded and enlarged like a bud to produce a similar organism. Even if this were true, little would be gained, for it would still remain unknown how the germinal miniature originated to be just what it was conceived and assumed to be. Wolff was the originator of the view that is now practically universal among naturalists, namely, that development is a real process of transformation from simpler to more complex conditions.

The subject of comparative embryology grew rapidly during the nineteenth century as the field of comparative anatomy became better known, and when naturalists became interested in animals, not only as specific types, but also as the finished products of an intricate series of transformations. When life-histories were more closely compared, the meaning of the resemblances between early stages of diverse adult organisms was read by the same method which in comparative anatomy finds that consanguinity is expressed by resemblance. The great law of recapitulation, stated in one form by Von Baer and more definitely by Haeckel in the terms employed in the foregoing sections, was for a time too freely used and too rigidly applied by naturalists whose enthusiasm clouded their judgment. A strong reaction set in during the latter part of the nineteenth century, when attention was directed to the anachronisms of the embryonic record and to the alterations that are the results of larval or embryonic adaptation as short cuts in development. Nevertheless, it is not seriously questioned, I believe, that the main facts of a single life-history owe their nature to the past evolution of the species to which a given animal belongs.

Nowadays the problems in this well-organized department are concerned not only with more accurate accounts of the development of animals, but also with the mechanics of development, with the relative value of external and internal influences, and above all with the physical basis of inheritance. It is clear that the factors that direct the development of a wood frog's egg so that it becomes a wood-frog and not a tree-toad must lie in the egg itself, as derivatives from the two parent organisms. Weismann and his followers have proved that a peculiar substance in the nuclei of the egg and its daughter-products contains the essential factors of development, whatever these may be. Experiments dealing with the phenomena of heredity in pure and mixed breeds have largely confirmed Weismann's doctrine, and they have prepared the way for a deeper investigation of the marvelous process of biological inheritance.

However much he may be interested in the details of embryological science, the general student of natural history is more concerned with the bearing of its primary laws upon the great problem of evolution. In the foregoing brief review of the fundamental facts and principles of this subject, the purpose has been to show how the phenomena of development are viewed by men of science, and how they take their place in the doctrine of organic evolution. And it has also been made plain that comparative anatomy and comparative embryology support and supplement one another in countless ways and places, although each in itself is a complete demonstration that evolution is a real and a natural process.

III

THE EVIDENCE OF FOSSIL REMAINS

Few natural objects appeal to the interest and imagination of the student with more force than the fragments of animals and plants released from the rocks where they have been entombed for ages. Our lives are so brief that it is impossible for us to comprehend the full duration of the slow process which constructed the burial shrouds of these creatures of long ago. We try to picture the earth and its inhabitants as they were when lizards were the highest forms of animals, and we wonder how life was lived in the dense forests of the coal age. Science can never learn all about the ancient history of the earth and of the organisms of bygone times; yet it has been able to accomplish much through its endeavors to reconstruct the past, for its method is one by which sure results can always be obtained whenever there are definite facts with which it can work. In our present study of evolution we reach the point when we must examine the testimony of the rocks, and the results and methods of that department of knowledge called palæontology, which is concerned with fossils and their interpretation.

The word "palæontology" means literally the "science of living things of long ago." It deals directly with the remains of animals and plants found as fossils, and it interprets them through its knowledge of the way modern animals are constructed and of the changes the earth's crust has undergone. A skull-like object may be found in a coal field and may come into the hands of the palæontologist: from his acquaintance with the head skeletons of recent types he will be able to assign the extinct creature which possessed the skull to a definite place in the animal scale and to understand its nearer or wider affinities with other animals of later times and of earlier epochs. In doing these things palæontology employs the methods of comparative anatomy with which we have now become familiar. In the performance of its other tasks, however, palæontology must work independently. It is necessary to know when a fossilized animal lived, not that its time need be measured by an absolute number of a few thousands or millions of years antedating our own era, for that is impossible. But the important thing is to know its relative age, and whether it preceded or followed other similar animals of its own group or of different divisions. The rocks themselves must be understood, how they have been formed and how they are related in mineralogical nature and in historical succession. Palæontology also deals with a number of subjects that are not in themselves biological, such as the combination of circumstances necessary for the adequate preservation of fossil relics. In so far as it is concerned with physical matters, as contrasted with strictly biological data, it is one with geology. Indeed, the investigators in these two departments must always work side by side and render mutual assistance to one another in countless ways, for each division needs the results of the other in order to accomplish its own distinct purposes. It must be evident to every one that it is impossible to understand the meaning of fossils and the place of the testimony of the rocks in the doctrine of evolution without knowing much about the geological history of the earth and the influences at work in the past. For these reasons palæontology differs somewhat from the other divisions of zoölogy where direct observation gives the materials for arrangement and study; in this case the individual data, that is, the fossil fragments themselves, can be made available only through a knowledge of their exact situations, of the reasons for their occurrence in particular places in the rock series and of the way rocks themselves are constructed and worked over by natural agencies. Our task is therefore twofold: certain physical matters of a geological nature must first be investigated before the biological facts can be described.

No doubt most people feel justified in believing that the whole doctrine of evolution must stand or fall according to the cogency of the palæontological evidences. Plain common sense says that the owners of shelly or bony fragments found in the deeply-laid strata of the earth must have lived countless years ago, and if the evolutionist asserts that primitive organic forms of ancient times have produced changed descendants of later times, it would seem that fossil evidence would be supremely and overwhelmingly important. It is true, of course, that this evidence is peculiarly significant, because in some ways it is more direct than that of the other categories already outlined. But it must not be forgotten that the doctrine is already securely founded upon the basic principles of anatomy and embryology. Science must treat the data of this category by different methods and must view them in different ways. Therefore we are interested in palæontology because of the way it tells the story of evolution in its own words, and because we are justified in expecting that its account should include a description of some such order of events as that revealed by the developing embryos of modern organisms and that demonstrated by the comparative anatomy of the varied species of adult animals.

It is true that palæontology gives direct testimony about the evolutionary succession of animals in geologic time. But we now know that embryology is even more direct in its proof that organic transformation is natural and real; while at the same time there is a completeness in the full series of developmental stages connecting the one-celled egg with the adult creature that must be forever lacking in the case of the fossil sequence of species. If paragraphs and pages are missing from the brief embryonic recapitulation, whole chapters and volumes of the fossil series have been lost for all time. The investigators whose task it has been to decipher the story of the earth's evolution have had to meet numerous and exasperating difficulties which do not confront the embryologist and anatomist who study living materials. Nevertheless the library of palæontological documents is one which has been founded for over a century, and it has grown fast during recent decades, so that consistent accounts may now be read of the great changes in organic life as the earth has altered and grown older. And in all this record, there is not a single line or word of fact that contradicts evolution. What definite evidence there is tells uniformly in favor of the doctrine, for it is possible, in the first place, to work out the order of succession of many of the great groups of animals, and this order is found to be the same as that established by the other bodies of evidence. Secondly, some fossil groups are astonishingly complete, so that the ancient history of a form like the horse can be written with something approaching fullness. Finally, the remains of certain animals have been found so situated in geological ways, and so constructed anatomically, that the zoölogist is justified in denoting them "missing links," because they seem to have been intermediate between groups that have diverged so widely during recent epochs as to render their common ancestry scarcely credible.

With these general results in mind, we must now become acquainted with such subjects as the interpretation of fossils, the causes for the incompleteness of the series, the conditions for fossilization, the forces of geological nature, and other matters that make the fossils themselves intelligible as scientific evidence.

* * * * *

Many views have been entertained regarding the actual nature of the relics of antiquity exhumed from the rocks or exposed upon the surface by the wear and tear of natural agencies. In earliest times such things were variously considered as curious freaks of geological formation, as sports of nature, or as the remains of the slain left upon the battle-ground of mythical Titans. Some of the Greeks supposed that fossils were parts of animals formed in the bowels of the earth by a process of spontaneous generation, which had died before they could make their way to the surface. They were sometimes described as the bones of creatures stranded upon the dry land by tidal waves, or by some such catastrophe as the traditional flood of the scriptures. In medieval times, and even in our own day, some people who have been opposed to the acceptance of any portion of the doctrine of evolution have actually defended the view that the things called fossils were never the shells or bones of animals living in bygone times, but that they only simulate such things and have been created as such together with the layers of rock from which they may have been taken. If we employed the same arguments in dealing with the broken fragments of vases and jewelry taken from the Egyptian tombs or from the buried ruins of Pompeii, we would have to believe that such pieces were created as fragments and that they were never portions of complete objects, just because no one alive to-day has ever seen the perfect vessel or bracelet fashioned so long ago. Common sense directs us to discard such a fantastic interpretation in favor of the view that fossils are what they seem to be—simply relics of creatures that lived when the earth was younger.

Until this common sense view was adopted there was no science of palæontology. Cuvier was the first great naturalist to devote particular attention to the mainly unrelated and unverified facts that had been discovered before his time. He was truly the originator of this branch of zoölogy, for he brought together the observations of earlier men and extended his own studies widely and surely, emphasizing particularly the necessity for noting carefully the geological situation of a fossil in rocks of an older or later period of formation. His great result was the demonstration that many groups of animals existed in earlier ages that seem to have no descendants of the same nature to-day, and also that many or most of our modern groups are not represented in the earliest formed sedimentary rocks, although these recent forms possess hard parts which would surely be present somewhere in these levels if the animals actually existed in those times. But the meaning of these facts escaped Cuvier's mind. He was a believer in special creation, like Linnæus and all but a few among his predecessors, and he explained the diversity of the faunas of different geological times in what seems to us a very simple and naïve way. In the beginning, he held, when the world was created, it was furnished with a complete set of animals and plants. Then some great upheaval of nature occurred which overwhelmed and destroyed all living creatures. The Creator then, in Cuvier's view, proceeded to construct a new series of animals and plants, which were not identical with those of the former time, but were created according to the same general working plans or architectural schemes employed before. Another cataclysm was supposed to have occurred, which destroyed the second series of organisms and laid a new covering of rocks over the earth's surface for a subsequent period of relative quiet; and so the process was continued. By this account, Cuvier endeavored to reconcile the doctrine of supernatural creation and intervention with the obvious facts that organisms have differed at various times in the earth's history. Although he saw that animals of successive periods displayed similar structures, like the skeleton of vertebrates, which testified to some connection, Cuvier could not bring himself to believe that this connection was a genealogical one.

Mainly through the influence of the renowned English man of science, Charles Lyell, the students of the earth came to the conclusion that its manifold structures had developed by a slow and orderly process that was entirely natural; for they found no evidence of any sudden and drastic world-wide remodeling such as that postulated by the Cuvierian hypothesis of catastrophe. The battle waged for many years; but now naturalists believe that the forces, of nature, whose workings may be seen on all sides at the present time, have reconstructed the continents and ocean beds in the past in the same way that they work to-day. The long name of "uniformitarianism" is given to Lyell's doctrine, which has exerted an influence upon knowledge far outside the department of geology. Darwin tells us how much he himself was impressed by it, and how it led him to study the factors at work upon organic things to see if he could discern evidence of a biological uniformitarianism, according to which the past history of living things might be interpreted through an understanding of their present lives.

* * * * *

What, now, are the reasons why the palæontological evidence is not complete and why it cannot be? In the first place the seeker after fossil remains finds about three fifths of the earth's surface under water so that he cannot explore vast areas of the present ocean beds which were formerly dry land and the homes of now extinct animals. Thus the field of investigation is seriously restricted at the outset, but the naturalist finds his work still more limited, in so far as much of the dry land itself is not accessible. The perennial snows of the Arctic region render it impossible to make a thorough search in the frigid zone, and there are many portions of the temperate and torrid zones that are equally unapproachable for other reasons. But even where exploration is possible, the surface rocks are the only ones from which remains can be readily obtained, for the layers formed in earlier ages are buried so deeply that their contents must remain forever unknown in their entirety. Only a few scratches upon the earth's hard crust have been made here and there, so it is small wonder that the complete series of extinct organisms has not been produced by the palæontologist.

A brief survey of the varied groups of animals themselves is sufficient to bring to light many biological reasons which account for still more of the vacant spaces in the palæontological record. We would hardly expect to find remains of ancient microscopic animals like the protozoa, unless they possessed shells or other skeletal structures which in their aggregate might form masses like the chalk beds of Europe. Jellyfish and worms and naked mollusks are examples of the numerous orders of lower animals having no hard parts to be preserved, and so all or nearly all of the extinct species belonging to these groups can never be known. But when an animal like a clam dies its shell can resist the disintegrating effects of bacteria and other organic and inorganic agencies which destroy the soft parts, and when a form like a lobster or a crab, possessing a body protected by closely joined shell segments, falls to the bottom of the sea, the chances are that much of the animal's skeleton will be preserved. Thus it is that corals, crustacea, insects, mollusks, and a few other kinds of lower forms constitute the greater mass of invertebrate palæontological materials because of their supporting structures of one kind or another. Perhaps the skeletal remains of the vertebrates of the past provide the student of fossils with his best facts, on account of the resistant nature of the bones themselves, and because the backboned animals are relatively modern; then, too, the rocks in which their remains occur have not been so much altered by geological agencies, or buried so deeply under the strata formed later. Of course only the hardest kinds of shells would remain as such after their burial in materials destined to turn into rock; in the majority of cases, an entombed bone is infiltrated or replaced by various mineral substances so that in time little or nothing of the original thing would remain, though a mold or a cast would persist.

But even if an animal of the past possessed hard structures, it must have satisfied certain limited conditions to have its remains prove serviceable to students of to-day. A dead mammal must fall upon ground that has just the right consistency to receive it; if the soil is too soft, its several parts will be separated and scattered as readily as though it had fallen upon hard ground where it would be torn to pieces by carnivorous animals. The dead body must then be covered up by a blanket of silt or sand like that which would be deposited as the result of a freshet. If a skeleton is too greatly broken up or scattered, it may be difficult or even impossible for its discoverer to piece together the various fragments and assemble them in their original relations. Very few individuals have been so buried and preserved as to meet the conditions for the formation of an ideal fossil. To realize how little may be left of even the most abundant of higher organisms, we have only to recall that less than a century ago immense herds of bison and wild horses roamed the Western plains, but very few of their skulls or other bones remain to be enclosed and fossilized in future strata of rocks. When we appreciate all these difficulties, both geological and biological, we begin to see clearly why the ancient lines of descent cannot be known as we know the path and mode of embryonic transformation. The wonder is not that the palæontological record is incomplete, but that there is any coherent and decipherable record at all. Yet in view of the many and varied obstacles that must be surmounted by the investigator, and the adverse factors which reduce the available evidence, the rapidly growing body of palæontological facts is amply sufficient for the naturalist to use in formulating definite and conclusive principles of evolution.

* * * * *

For the purposes of palæontology, the most essential data of geology are those which indicate the relative ages of the strata that make up the hard outer crust of the earth, for only through them can the order of animal succession be ascertained. It does not matter exactly how old the earth may be. While it is possible to determine the approximate length of time required for the construction of sedimentary rocks like those which natural agencies are producing to-day, there are few definite facts to guide speculation as to the mode or duration of the process by which the first hard crystalline surface of the earth was formed. But palæontology does not care so much about the earliest geological happenings, for it is concerned with the manifold animal forms that arose and evolved after life appeared on the globe. Questions as to the way life arose, and as to the earliest transformations of the materials by which the earth was first formed are not within the scope of organic evolution, although they relate to intensely interesting problems for the student of the process of cosmic evolution.

According to the account now generally accepted, the original material of the earth seems to have been a semi-solid or semi-fluid mass formed by the condensation of the still more fluid or even gaseous nebula out of which all the planets of the solar system have been formed and of which the sun is the still fiery core. As soon as the earth had cooled sufficiently its substances crystallized and wrinkled to form the first mountains and ridges; between and among these were the basins which soon filled with the condensing waters to become the earliest lakes and oceans. The wear and tear of rains and snows and winds so worked upon the surfaces of the higher regions that sediments of a finer or coarser character like sand and mud and gravel were washed down into the lower levels. These sediments were afterwards converted into the first rocks of the so-called stratified or sedimentary series, as contrasted with the crystalline or plutonic rocks like the original mass of the earth and the kinds forced to the surface by volcanic eruptions. Later the earth wrinkled again in various ways and places so that new ridges and mountains were formed with new systems of lakes and oceans and rivers; and again the elements continued to erode and partially destroy the higher masses and to lay down new and later series of sedimentary rocks upon the old.

It seems scarcely credible that the apparently weak forces of nature like those we have mentioned are sufficiently powerful to work over the massive crust of the earth as geology says they have. Our attention is caught, as a rule, only by the greater things, like the earthquakes at San Francisco and Valparaiso, and the tidal waves and cyclones of the South Seas; but the results of these sporadic and local cataclysms are far less than the effects of the persistent everyday forces of erosion, each one of which seems so small and futile. When we look at the Rocky Mountains with their high and rugged peaks, it seems almost impossible that rain and frost and snow could ever break them up and wear them down so that they would become like the rounded hills of the Appalachian Mountain chain, yet this is what will happen unless nature's ways suddenly change to something which they are not now. A visitor to the Grand Cañon of the Colorado sees a magnificent chasm over a mile in depth and two hundred miles long which has actually been carved through layer after layer of solid rock by the rushing torrents of the river. Perhaps it is easier to estimate the geological effects of a river in such a case as Niagara. Here we find a deep gorge below the famous falls, which runs for twenty miles or so to open out into Lake Ontario. The water passing over the brim of the falls wears away the edge at a rate which varies somewhat according to the harder or softer consistency of the rocks, but which, since 1843, has averaged about 104 inches a year. Knowing this rate, the length of the gorge, and the character of the rocky walls already carved out, the length of time necessary for its production can be safely estimated. It is about 30,000 to 40,000 years, not a long period when the whole history of the earth is taken into account. A similar length of time is indicated for the recession of the Falls of St. Anthony, of the Mississippi River, an agreement that is of much interest, for it proves that the two rivers began to make their respective cuttings when the great ice-sheet receded to the north at the end of the Glacial epoch.

What has become of the masses washed away during the formation of these gorges? As gravel and mud and silt the detritus has been carried to the still waters of the lower levels, to be laid down and later solidified into sandstone and slate and shale. All over the continents these things are going on, and indefatigable forces are at work that slowly but surely shear from the surface almost immeasurable quantities of earth and rock to be transported far away. In some instances it is possible to find out just how much effect is produced in a given period of time, especially in the case of the great river systems. For example, the mass of the fine particles of mud and silt carried in a given quantity of the water of the Mississippi as it passes New Orleans can be accurately measured, and a satisfactory determination can also be made of the total amount of water carried by in a year. From these figures the amount of materials in suspension discharged into the Gulf of Mexico becomes known. It is sufficient to cover one square mile to the depth of 269 feet; in twenty years it is one cubic mile, or five cubic miles in a century. Turning now to the other aspect of this process, and the antecedent causes which produce these effects, it appears that the area of the Mississippi River basin is 1,147,000 square miles—about one third of the total area of the United States. Knowing this, and the annual waste from its surface, it is easy to demonstrate that it will take 6000 years to plane off an average of one foot of soil and rock from the whole of this immense area. Of course only an inch or a few inches will be taken from some regions where the ground is harder or rockier, or where little rain falls, while many feet will be washed away from other places. The waters of the Hoang-ho come from about 700,000 square miles of country, from which one foot of soil is washed away in 1464 years. The Ganges River, draining about 143,000 square miles, carries off a similar depth of eroded materials from its basin in 823 years! Should we add to the above figures those that specify the bulk of the chemical substances in solution carried by these waters, the total would be even greater. We know that in the case of the Thames River, calcareous substances to the amount of 10,000 tons a year are carried past London, and all this mineral has been dissolved by rain-water from the chalky cliffs and uplands of England, so that the land has become less by this amount. Thus we learn that vast alterations are being made in the structure of great continents by rain and rivers, as well as by glaciers and other geological agencies. And at the same time that old strata are undergoing destruction new ones are in process of construction at other places, where animal remains can be embedded and preserved as fossils. The forces at work seem weak, but they continue their operations through ages that are beyond our comprehension and they accomplish results of world-building magnitude.

Thus the whole process of geological construction is such that older exposed strata continually undergo disintegration, but this involves the destruction of any fossils that they might contain. The very forces that preserve the relics of extinct animals at one time undo their work at a later period. There are many other influences besides that destroy the regularity of rock layers or change their mineralogical characters by metamorphosis. It is easier to see how volcanic outbursts alter their neighboring territory. The intense subterranean heat and imprisoned steam melt the deeper substances of the earth's crust, so that these materials boil out, as it were, where the pressure is greatest, and where lines of fracture and lesser resistance can be found. Because so much detritus is annually added to the ocean floors—enough to raise the levels of the oceans by inches in a century—it is natural that greater pressures should be exerted in these areas than in the slowly thinning continental regions. These are some of the reasons why volcanoes arise almost invariably along the shores or from the floors of great ocean beds. The chain that extends from Alaska to Chili within the eastern shore of the Pacific Ocean, and the many hundreds of volcanoes of the Pacific Islands bring to the surface vast quantities of eruptive rocks which break up and overlie the sedimentary strata formed regularly in other ways and at other times. The volcanoes of the Java region alone have thrown out at least 100 cubic miles of lava, cinders, and ashes during the last 100 years—twenty times the bulk of the materials discharged into the Gulf of Mexico by the Mississippi River in the same period of time.

From these and similar facts, the naturalist finds how agencies of the present construct new rocks and alter the old; and so in the light of this knowledge, he proceeds with his task of analyzing the remote past, confident that the same natural forces have done the work of constructing the lower geological levels because these earlier products are similar to those being formed to-day. After learning this much, he must immediately undertake to arrange the strata according to their ages. This might seem a difficult or even an impossible task, but the rocks themselves provide him with sure guidance.

Wherever a river has graven its deep way through an area of hard rocks, as in the case of Niagara, the walls display on their cut surfaces a series of lines and planes showing that they are superimposed layers formed serially by deposits that have differed some or much at different times according to the circumstances controlling the erosion of their constituent particles. A layer of several feet in thickness may be composed of compact shale, while above it will be a zone of limestone, and again above this another layer of shale. Successive strata like these, where they are parallel and obviously undisturbed, are evidently arranged in the order of their formation and age. But by far the most impressive demonstration of the basic principle of geology employed for the determination of the relative ages of rocks is the mighty Cañon of the Colorado. As the traveler stands on the winding rim of this vast chasm, his eye ranges across 13 miles of space to the opposite walls, which stretch for scores of miles to the right and left; upon this serried face he will see zone after zone of yellow and red and gray rock arranged with mathematical precision and level in the same order as on the steep slopes beneath him. Plain common sense tells him that the great sheets of rock stretched continuously at one time between the now separate walls, and that the various strata of sandstone and limestone were deposited in successive ages from below upwards in the order of their exposure. When now he extends his explorations to another state like Utah or Wyoming, he may find some but not all of the series exhibited in the Grand Cañon, overlaid or underlaid by other strata which in their turn can be assigned to definite places in the sequence. By the same method, the geologist correlates and arranges the rocks not only of different parts of the same state, or of neighboring states, but even those of widely separated parts of North America and of different continents. But he learns that he must refrain from over-hasty conclusions, for he soon finds that the sedimentary rocks have not been constructed at the same rate in different places during one and the same epoch, and that rocks formed even at one period are not always identical in nature. But his guiding principle is sensible and reasonable, and by employing it with due caution he provides the palæontologist with the requisite knowledge for his special task, which is to arrange the extinct animals whose remains are found as fossils of various earth ages in the order of their succession in time.

CONDENSED TABLE OF PALAEONTOLOGICAL FACTS

__________________________________________________________________________
| | | |
YEARS | NUMBER OF | | | ORDER OF
NECESSARY FOR | FEET IN | GEOLOGICAL | GEOLOGICAL | APPEARANCE OF
FORMATION | THICKNESS | AGE | EPOCH | CHARACTERISTIC
| | | | GROUPS
______________|___________|______________|_______________|________________
| | | |
| | | | M B R A F I
| | | | a i e m i n b
| | | | m r p p s v r
| | Recent | | m d t h h e a
| | or | | a s i i e r t
| | Quaternary | | l l b s t e
| | | | s e i e s
| | | | s a -
______________|___________|______________|_______________|||||||____
| | | | | | | | | |
| | | Pleistocene | | | | | | |
| | Cenozoic | Pliocene | | | | | | |
5,000,000 | 25,000 | or | Miocene | | | | | | |
| | Tertiary | Oligocene | | | | | | |
| | | Eocene | | | | | | |
______________|___________|______________|_______________|||||||____
| | | | | | | | | |
| | Mesozoic | Cretaceous | | | | | | |
4,000,000 | 23,000 | or | Jurassic | | | | | | |
| | Secondary | Triassic | | | | | |
______________|___________|______________|_______________|_____|||_|____
| | | | | | | |
| | | Permian | | | | |
| | Palæozoic | Carboniferous | | | |
21,000,000 | 106,000 | or | Devonian | | |
| | Primary | Silurian | | |
| | | Cambrian | | |
______________|___________|______________|_______________|________________
| | | |
20,000,000 | 30,000 | Azoic | Archæn |
______________|___________|______________|_______________|________________

After what seems an unduly long preparation, we now come to the actual biological evidence of evolution provided by the results of this division of zoölogical science. But all of the foregoing is fundamentally part of this department of knowledge and it is absolutely essential for any one who desires to understand what the fossils themselves demonstrate.

The oldest sedimentary rocks are devoid of fossil remains and so they are called the Azoic or Archæan. They comprise about 30,000 feet of strata which seem to have required at least 20,000,000 years for their formation. This period is roughly two-fifths of the whole time necessary for the formation of all the sedimentary rocks, and this proportion holds true even if the entire period of years should be taken as 100,000,000 instead of 50,000,000 or less. The earth during this early age was slowly organizing in chemical and physical respects so that living matter could be and indeed was formed out of antecedent substances—but this process does not concern us here. The important fact is that the second major period, called the Palæozoic, or "age of ancient animals," saw the evolution of the lowest members of the series,—the invertebrates,—and the most primitive of the backboned animals, like fishes and amphibia. The rocks of this long age include about 106,000 feet of strata, demanding some 21,000,000 or 22,000,000 years for their deposition. Thus it is proved that the invertebrate animals were succeeded in time by the higher vertebrates, which is exactly what the evidences of the previous categories have shown. When we remember that the lower animals are devoid as a rule of skeletal structures that might be fossilized, and when we recall the fact that the strata of the palæozoic provided the materials out of which the upper layers were formed afterwards, we can understand why the ancient members of the invertebrate groups are not known as well as the later and higher forms like vertebrates. Yet all the fossils of these relatively unfamiliar creatures clearly prove that no complex animal appears upon a geological horizon until after some simple type belonging to a class from which it may have taken its origin; in brief, there are no anachronisms in the record, which always corresponds with the record written by comparative anatomy, wherever the facts enable a comparison to be made.

But the extinct animals of the third and fourth ages are more interesting to us, because there are more of them and because they are more like the well-known organisms of our present era. These two ages are called the Mesozoic or Secondary, and the Cenozoic or Tertiary. The former is so named because it was a transitional age of animals that are intermediate in a general way between the primitive forms of the preceding age and those of the next period; the latter name means the "recent-animal" age, when evolution produced not only the larger groups of our present animal series, but also many of the smaller branches of the genealogical tree like orders and families to which the species of to-day belong.

Confining our attention to the large vertebrate classes, the testimony of the rocks proves, as we have said, that fishes appeared first in what are called the Silurian and Devonian epochs, where they developed into a rich and varied array of types unequaled in modern times. At that period, they were the highest existing animals—the "lords of creation," as it were. To change the figure, their branch constituted the top of the animal tree of the time, but as other branches grew upwards to bear their twigs and leaves, as the counterparts of species, the species of the branch of fishes decreased in number and variety, as do the leaves of a lower part of a tree when higher limbs grow to overshadow them.

Following the fishes, the amphibia arose during the coal age or Carboniferous, usurping the proud position of the lower vertebrate class. The reptiles then appeared and gained ascendancy over the amphibia, to become in the Mesozoic age the highest and most varied of the existing vertebrates. At that time there were the great land dinosaurs with a length of 80 feet, like Brontosaurus; aquatic forms like Ichthyosaurus and Plesiosaurus, whose mode of evolution from terrestrial to swimming habits was like that of seals and penguins of far later eras. Flying reptiles also evolved, to set an example for the bats of the mammalian class, for both kinds of flying organisms converted their anterior limbs into wings, although in different ways.

During the Triassic and Jurassic periods of the Mesozoic age, the first birds and mammals appeared to follow out their diverging and independent lines of descent. Palæontology makes it possible to trace the origin and development of many of the different branches that grew out of the mammalian limb from different places and at different times during the Mesozoic and the following age, called the Cenozoic, or age of recent animals. It is unnecessary, however, for us to review more of the details: the main result is obvious; namely, that the appearance of the great classes of vertebrates is in the order of comparative anatomy and embryology. Not only, then, is the fact of evolution rendered trebly sure, but the general order of events is thrice and independently demonstrated to be one and the same. Surely we must see that no reasonable explanation other than evolution can be given for these basic facts and principles.

Turning now to the second division of palæontological evidence, we come to those groups where abundant materials make it possible to arrange the animals of successive epochs in series that may be remarkably complete. For the reasons specified, the backboned animals provide the richest arrays of these series, and such histories as those of horses and elephants have taken their places in zoölogical science as classics. But even among the invertebrates significant cases may be found. For example, in one restricted locality in Germany the shells of snails belonging to the genus Paludina have been found in superimposed strata in the order of their geological sequence. The ample material shows how the several species altered from age to age by the addition of knobs and ridges to the surface of the shell, until the fossils in the latest rocks are far different from their ancestors in the lowermost levels. Yet the intervening shells fill in the gaps in such a way as to show almost perfectly how the animals worked out their evolutionary history. This example illustrates the nature of many other known series of mollusks and of brachiopods, extending over longer intervals and connecting more widely separated ages like the Secondary and the present period.

Since the doctrine of evolution and its evidences began to occupy the thoughts of the intellectual world at large, no fossil forms have received more attention than the ancient members of the horse tribe. As we have learned, a modern horse is described by comparative anatomy as a one-toed descendant of remote five-toed ancestors. When the hoofed animals of modern times were reviewed as subjects for comparative anatomical study, the odd-toed forms arranged themselves in a series beginning with an animal like an elephant with the full number of five digits on each foot and ending at the opposite extreme with the horse. A reasonable interpretation of these facts was that the animals with fewer toes had evolved from ancestors with five digits, of which the outer ones had progressively disappeared during successive geological periods, while the middle one enlarged correspondingly. The facts provided by palæontology sustain this contention with absolutely independent testimony. Disregarding some problematical five-toed forms like Phenacodus, the first type of undoubted relationship to modern horses is Hyracotherium, a little animal about three feet long that lived during the Eocene period of the Cenozoic epoch. Its forefeet had four toes each, and its hinder limbs ended with three toes armed with small hoofs, but one of its relatives of the same time has a vestige of another digit on the hind foot. By the geological time mentioned, therefore, the earliest true horses had already lost some of the toes that their progenitors possessed. In the Miocene the extinct species, obviously descended from the Eocene forms, had lost more of their toes; still higher, that is, in the rocks formed during succeeding periods of time, the animals of this division are much larger and each of their feet has only three toes, of which the middle one is the largest while the ones on the sides are small and withdrawn from the ground so as to appear as useless vestiges. To produce modern horses and zebras from these nearer ancestors, few additional changes in the structure of the feet are necessary, for the lateral toes need only to become a little more reduced and the middle one to enlarge slightly to give the one-toed limb of modern types, with its splint-like vestiges still in evidence to show that the ancestor's foot comprised more of these terminal elements. Comparing the animals of successive periods, these and other skeletal structures demonstrate that the ancestry of each group of species is to be found in the animals of the preceding epoch, and that the whole history of horses is one of natural transformation,—in a word, of evolution.

No less interesting in their own way are the remains of other hoofed forms that lead down to the elephants of to-day and to the mammoth and mastodon of relatively recent geologic times. Common sense would lead to the conclusion that a form like a modern tapir was the prototype from which these creatures have arisen, and common sense would lead us to expect that if any fossils of the ancestors of the modern group of elephants occurred at all they would be like tapirs. Thus a fossil of much significance in this connection is Moeritherium, whose remains have been found in the rocks exposed in the Libyan desert, for this creature was practically a tapir, while at the same time its characters of muzzle and tusk mark it as very close to the ancestors of the larger woolly elephants of later geological times, when the trunk had grown considerably and the tusks had become greatly prolonged. Again the fossil sequence confirms the conclusions of comparative anatomy, regarding the mode by which certain modern animals have evolved.

The fossil deer of North America, as well as many other even-toed members of the group of mammalia possessing hoofs, provide the same kind of conclusive evidence. The feature of particular interest in the case of their horns, is a correspondence between the fossil sequence and the order of events in the life-history of existing species,—that is, between the results of palæontology and of embryology. Horns of the earliest known fossil deer have only two prongs; in the rocks above are remains of deer with additional prongs, and point after point is added as the ancient history of deer is traced upwards through the rocks to modern species. We know that the life-history of a modern species of animals reviews the ancestral record of the species, and what happens during the development of deer can be directly compared with the fossil series. It is a matter of common knowledge that the year-old stag has simple spikes as horns, and that these are shed to be replaced the following year by larger forked horns. Every year the horns are lost and new ones grow out, and become more and more elaborately branched as time goes on, thus giving a series of developmental stages that faithfully repeats the general order of fossil horns. Even Agassiz, who was a believer in special creation and an opponent of evolution, was constrained to point out many other instances, mainly among the invertebrata, where there was a like correspondence between the ontogeny of existing species and their phylogenetic history as revealed by the fossil remains of their ancestors.

* * * * *

In the last place, we must give more than a passing consideration to some of the extinct types of animals that occupy the position of "links" between groups now widely separated by their divergence in evolution from the same ancestors. Perhaps the most famous example is Archæopteryx found in a series of slates in Germany. This animal is at once a feathered, flying reptile, and a primitive bird with countless reptilian structures. Its short head possesses lizard-like jaws, all of which bear teeth; its wings comprise five clawed digits; its tail is composed of a long series of joints or vertebræ, bearing large feathers in pairs; its breastbone is flat and like a plate, thus resembling that of reptiles and differing markedly from the great keeled breastbone of modern flying birds, whose large muscles have necessitated the development of the keel for purposes of firm attachment. In brief, this animal was close to the point where reptiles and birds parted company in evolution, and although it was a primitive bird, it is in a true sense a "missing link" between reptiles and the group of modern birds. Other fossil forms like Hesperornis and Ichthyornis, whose remains occur in the strata of a later date, fill in the gap between Archæopteryx and the birds at the present time, for among other things they possess teeth which indicate their origin from forms like Archæopteryx, while in other respects they are far nearer the birds of later epochs. That these links are not unique is proved by numerous other examples known to science, such as those which connect amphibia and reptiles, ancient reptiles and primitive mammals, as well as those which come between the different orders of certain vertebrate classes.

In summarizing the foregoing facts, and the larger bodies of evidence that they exemplify, we learn how surely the testimony of the rocks establishes evolution in its own way, how it confirms the law of recapitulation demonstrated by comparative embryology, and how it proves that the greater and smaller divisions of animals have followed the identical order in their evolution that the comparative study of the present day animals has independently described.

* * * * *

The facts of geographical distribution constitute the fifth division of zoölogy, and an independent class of evidences proving the occurrence of evolution. This department of zoölogy assumed its rightful status only after the other divisions had attained considerable growth. Many naturalists before Darwin and Wallace and Wagner had noticed that animals and plants were by no means evenly distributed over the surface of the globe, but until the doctrine of evolution cleared their vision they did not see the meaning of these facts. As in the case of all the other departments of zoölogy the immediate data themselves are familiar, but because they are so obvious the mind does not look for their interpretation but accepts the facts at their face value. While the phenomena of distribution are no less fascinating to the naturalist, and no less effective in their demonstration of evolution, their comprehensive treatment would demand more space than the whole purpose of the present description of organic evolution would justify. Thus a brief outline only can be given of the salient principles of this subject in order that their bearing upon the problem of species may be indicated.

Even as children we learn many facts of animal distribution; every one knows that lions occur in Africa and not in America, that tigers live in Asia and Malaysia, that the jaguar is an inhabitant of the Brazilian forests, and that the American puma or mountain lion spreads from north to south and from east to west throughout the American continents. The occurrence of differing human races in widely separated localities is no less familiar and striking, for the red man in America, the Zulu in Africa, the Mongol and Malay in their own territories, display the same discontinuity in distribution that is characteristic of all other groups of animals and of plants as well. As our sphere of knowledge increases, we are impressed more and more forcibly by the diversity and unequal extent of the ranges occupied by the members of every one of the varied divisions of the organic world. Another fact which becomes significant only when science calls our attention to it is the absence from a land like Australia of higher mammals such as the rabbit of Europe. The hypothesis of special creation cannot explain this absence on the assumption that the rabbit is unsuited to the conditions obtaining in the country named, for when the species was introduced into Australia by man, it developed and spread with marvelous rapidity and destructive effect. It may seem impossible that facts like these could possess an evolutionary significance, but they are actual examples of the great mass of data brought together by the naturalists who have seen in them something to be interpreted, and who have sought and found an explanation in the formularies of science.

The general principles of distribution appear with greatest clearness when an examination is made of the animals and plants of isolated regions like islands. The Galapagos Islands constitute a group that has figured largely in the literature of the subject, partly because Darwin himself was so impressed by what he found there in the course of his famous voyage around the world in the "Beagle." They form a cluster on the Equator about six hundred miles west of the nearest point of the neighboring coast of South America. Although the lizards and birds that live in the group differ somewhat among themselves as one passes from island to island, on the whole they are most like the species of the corresponding classes inhabiting South America. Why should this be so? On the hypothesis of special creation there is no reason why they should not be more like the species of Africa or Australia than like those of the nearest body of the mainland. The explanation given by evolution is clear, simple, and reasonable. It is that the characteristic island forms are the descendants of immigrants which in greatest probability would be wanderers from the neighboring continent and not from far distant lands. Reaching the isolated area in question the natural factors of evolution would lead their offspring of later generations to vary from the original parental types, and so the peculiar Galapagos species would come into being. The fact that the organisms living on the various islands of this group differ somewhat in lesser details adds further justification for the evolutionary interpretation, because it is not probable that all the islands would be populated at the same time by similar stragglers from the mainland. The first settlers in one place would send out colonies to others, where independent evolution would result in the appearance of minor differences peculiar to the single island. In this manner science interprets the general agreement between the animals of the Azores Islands and the fauna of the northwestern part of Africa, the nearest body of land, from which it would be most natural for the ancestors of the island fauna to come.

The land-snails inhabiting the various groups of islands scattered throughout the vast extent of the Pacific Ocean provide the richest and most ideal material for the demonstration of the principles of geographical distribution. In the Hawaiian Islands snails of the family of Achatinellidæ occur in great abundance, and like the lizards of the Galapagos Islands different species occur on the different members of the group. Within the confines of one and the same island, they vary from valley to valley, and the correlation between their isolation in geographical respects and specific differences on the other hand, first pointed out by Gulick, makes this tribe of animals classical material. In Polynesia and Melanesia are found close relatives of the Achatinellidæ, namely, the Partulæ, which are thus in relative proximity to the Achatinellidæ and not on the other side of the world. Furthermore, the Partulæ are not alike in all of the groups of Polynesia where they occur; the species of the Society Islands are absolutely distinct from those of the Marquesas, Tonga, Samoan, and Solomon Islands, although they agree closely in the basic characters that justify their reference to a single genus. The geological evidence tells us that these islands were once the peaks of mountain ranges rising from a Pacific continent which has since subsided to such an extent that the mountain tops have become separate islands. Thus the resemblances between Hawaiian and Polynesian snails, and the closer similarities exhibited by the species of the various groups of Polynesia, are intelligible as the marks of a common ancestry in a widespread continental stock, while the observed differences show the extent of subsequent evolution along independent lines followed out after the isolation of the now separated islands. The principle may be worked out in even greater detail, for it appears that within the limits of one group diverse forms occupy different islands, evolved in different ways in their own neighborhoods; while in one and the same island, the populations of the different valleys show marked effects of divergence in later evolution, precisely as in the case of the classic Achatinellidæ of the Hawaiian Islands.

The broad and consistent principle underlying these and related facts is this: there is a general correspondence between the differences displayed by the organisms of two regions and the degree of isolation or proximity of these two areas. Thus the disconnected but neighboring areas of the Galapagos Islands and South America support species that resemble each other closely, for the reasons given before; long isolated areas like Australia and its surroundings possess peculiar creatures like the egg-laying mammals, and all of the pouched animals or marsupials with only one or two exceptions like our own American opossum,—a correlation between a geological and geographical discontinuity on the one hand and a peculiarity on the other that reinforces our confidence in the faunal evolutionary interpretation of the facts of distribution.

It is true that the various classes of animals do not always appear with coextensive ranges. The barriers between two groups of related species will not be the same in all cases. A range like the Rocky Mountains will keep fresh-water fish apart, while birds and mammals can get across somewhere at some time. All these things must be taken into account in analyzing the phenomena of distribution, and many factors must be given due attention; but in all cases the reasons for the particular state of affairs in geographical and biological respects possess an evolutionary significance.

Having then all the facts of animal natural history at his disposal, and the uniform principles in each body of fact that demonstrate evolution, it is small wonder that the evolutionist seems to dogmatize when he asserts that descent with adaptive and divergent modification is true for all species of living things. The case is complete as it stands to-day, while it is even more significant that every new discovery falls into line with what is already known, and takes its natural place in the all-inclusive doctrine of organic evolution. Because this explanation of the characteristics of the living world is more reasonable than any other, science teaches that it is true.

IV

EVOLUTION AS A NATURAL PROCESS

The purpose of the discussions up to this point has been to present the reasons drawn from the principal classes of zoölogical facts for believing that living things have transformed naturally to become what they now are. Even if it were possible to make an exhaustive analysis of all of the known phenomena of animal structure, development, and fossil succession, the complete bodies of knowledge could not make the evolutionary explanation more real and evident than it is shown to be by the simple facts and principles selected to constitute the foregoing outline. We have dealt solely with the evidences as to the fact of evolution; and now, having assured ourselves that it is worth while to so do, we may turn to the intelligible and reasonable evidence found by science which proves that the familiar and everyday "forces" of nature are competent to bring about evolution if they have operated in the past as they do to-day. Investigation has brought to light many of the subsidiary elements of the whole process, and these are so real and obvious that they are simply taken for granted without a suspicion on our part of their power until science directs our attention to them.

For one reason or another, those who take up this subject for the first time find it difficult to banish from their minds the idea that evolution, even if it ever took place, has been ended. They think it futile to expect that a scrutiny of to-day's order can possibly find influences powerful enough to have any share in the marvelous process of past evolution demonstrated by science. The naturalists of a century ago held a similar opinion regarding the earth, viewing it as an immutable and unchanged product of supernatural creation, until Lyell led them to see that the world is a plastic mass slowly altering in countless ways. It is no more true that living things have ceased to evolve than that mountains and rivers and glaciers are fixed in their final forms; they may seem everlasting and permanent only because a human life is so brief in comparison with their full histories. Like the development of a continent as science describes it, the origin of a new species by evolution, its rise, culmination, and final extinction may demand thousands of years; so that an onlooker who is himself only a conscious atom of the turbulent stream of evolving organic life does not live long enough to observe more than a small fraction of the whole process. Therefore living species seem unchanged and unchangeable until a conviction that evolution is true, and a knowledge of the method of science by which this conviction is borne upon one, guide the student onwards in the further search for the efficient causes of the process.

The biologist employs the identical methods used by the geologist in working out the past history of the earth's crust. The latter observes the forces at work to-day, and compares the new layers of rock now being formed with the strata of deeper levels; these are so much alike that he is led to regard the constructive influences of the past as identical with those he can now watch at work. Similarly the biologist must first learn, as we have done, the principles of animal construction and development, and of other classes of zoölogical facts, and then he must turn his attention from the dead object of laboratory analysis to the workings of organic machines. The way an organism lives its life in dynamic relations to the varied conditions of existence, as well as the mutual physiological relations of the manifold parts of a single organism, reveal certain definite natural forces at work. Therefore his next task is to compare the results accomplished by these factors in the brief time they may be seen in operation with the products of the whole process of organic evolution, to learn, like the geologist in his sphere, that the present-day natural forces are able to do what reason says they have done in the past.

When the subject of inquiry was the reality of evolution, it was perhaps surprising to find that even the most familiar animals like cats and frogs provided adequate data for science to use in formulating its principles. So it is with the matter of method; it is unnecessary to go beyond the observations of a day or a week of human life to find forces at work, as real and vital as animal existence and organic life themselves. This is true, because evolution is true, and because the lives of all creatures follow one consistent law. Our task is therefore much more simple than most people suppose it to be; let us look about us and classify what we may observe, increasing our knowledge from the wide array of equally natural facts supplied by the biologist.

The analogies of the steamship and the locomotive proved useful at many times during the discussion of the fact of evolution, and even in the present connection they will still be of service. The evolution of these dead machines has been brought about by man, who, as an element of their environment, has been their creator as well as the director of their historical transformations. The result of their changes has been greater efficiency and better adjustment or adaptation to certain requirements fixed by man himself. The whole process of improvement has been one, in brief, of trial and error; new inventions have often been worthless, and they have been relegated to the scrap-heap, while the better part has been finally incorporated in the type machine. In brief, then, the important elements in the evolution of these examples have been three; first, adaptation, second, the origination of new parts, and third, the retention of the better invention.

Are the creatures of the living world so constituted that biological equivalents of these three essential elements of mechanical evolution can be found? Are organisms adapted to the circumstances controlling their lives, and are they capable of changing naturally from generation to generation, and of transmitting their qualities to their offspring? These are definite questions that bring us face to face with the fundamental problems relating to the dynamics or workings of evolution. We need not ask for or expect to find complete answers, for we know that it is impossible to obtain them. But we may expect to accomplish our immediate object, which is to see that evolution is natural. Our attention must be concentrated upon the three biological subjects of adaptation, variation, and inheritance, and we must learn why science describes them as real organic phenomena and the results of natural causes.

* * * * *

At the very outset, when the general characteristics of living things were considered, much was said on the subject of adaptation as a universal phenomenon of nature. It was not contended that perfection is attained by any living mechanism, but it was held that no place exists in nature for an organism that is incapable of adjusting itself to the manifold conditions of life. A modus vivendi must be established and some satisfactory degree of adaptation must be attained, or else an animal or a species must perish. With this fundamental point as a basis, we look to nature for two kinds of natural processes or factors, first, those which may originate variations as primary factors,—the counterparts of human ingenuity and invention in the case of locomotive evolution,—and the secondary factors of a preservative nature which will perpetuate the more adaptive organic changes produced by the first influences; it is clear that the latter are no less essential for evolution than the first causes for the appearance of variations.

The term "variation" is employed for the natural phenomenon of being or becoming different. It is an obvious fact that no child is ever exactly like either of its parents or like any one of its earlier ancestors; while furthermore in no case does an individual resemble perfectly another of its own generation or family. This departure from the parental condition, and the lack of agreement with others even of its closest blood-relatives, are two familiar forms of variation. As a rule, the degree to which a given organism is said to vary in a given character is most conveniently measured by the difference between its actual condition and the general average of its species, even though there is no such thing as a specimen of average nature in all of its qualities. In brief, then, variation means the existence of some differences between an individual and its parents, its fraternity, and, in a wider sense, all others of its species.

Passing now to the causes of variation, all of the countless deviations of living things can be referred to three kinds of primary factors; namely, the environmental, functional, and congenital influences that work upon the organism in different ways and at different times during its life. We shall learn that the evolutionary values of these three classes are by no means equal, but we take a long step forward when we realize that among the things we see every day are facts demonstrating the reality of three kinds of natural powers quite able to change the characters of organic mechanisms.

The "environment" of an organism is everything outside the creature itself. In the case of an animal it therefore includes other members of its own kind, and other organisms which prey upon its species or which serve it as food, as well as the whole series of inorganic influences which first come to mind when the term is used. For example, the environment of a lion includes other lions which are either members of its own family, or else, if they live in the same region, they are its more or less active rivals and competitors. In the next place, other kinds of animals exist whose lives are intimately related to the lion's life, such as the antelopes or zebras that are preyed upon, and the human hunter to whom the lion itself may fall a victim. In addition, there are the contrasted influences of inorganic nature which demand certain adjustments of the lion's activities. Light and darkness, heat and cold, and other factors have their direct and larger or smaller effects upon the life of a lion, although these effects are less obvious in this instance than in the case of lower organisms.

The reality of variations due to the inorganic elements of the environment is everywhere evident. Those who have spent much time in the sun are aware that sunburn may result as a product of a factor of this class. The amount of sunlight falling upon a forest will filter through the tree-tops so as to cause some of the plants beneath to grow better than others, thus bringing about variations among individuals that may have sprung from the myriad seeds of a single parent plant. In times of prolonged drought, plants cannot grow at the rate which is usual and normal for their species, and so many variations in the way of inhibited development may arise.

Then there are the variations of a second class, more complex in nature than the direct effects of environment,—namely, the functional results of use and disuse. A blacksmith uses his arm muscles more constantly than do most other men, and his prolonged exercise leads to an increase of his muscular capacity. All of the several organic systems are capable of considerable development by judicious exercise, as every one knows. If the functional modifications through use were unreal, then the routine of the gymnasium and the schoolroom would leave the body and the mind as they were before. Furthermore, we are all familiar with the opposite effects of disuse. Paralysis of an arm results in the cessation of its growth. When a fall has injured the muscles and nerves of a child's limb, that structure may fail to keep pace with the growth of the other parts of the body as a result of its disuse. These are simple examples of a wide range of phenomena exhibited everywhere by animals and even by the human organism, demonstrating the plasticity of the organic mechanism and its modification by functional primary factors of variation.

But by far the greater number of variations seem to be due to the so-called congenital causes, which are sharply contrasted with the influences of the first and second classes. It is quite true that the influences of the third class cannot be surely and directly demonstrated like the others, but however remote and vague they themselves may appear to be, their effects are obvious and real, while at the same time their effects are to be clearly distinguished from the products of the other two kinds. Congenital factors reside in the physical heritage of an organism, and their results are often evident before an individual is subjected to environmental influences and before it begins to use its various organs. For example, it is a matter of common observation that a child with light hair and blue eyes may have dark-eyed and brown-haired parents. The fact of difference is a phenomenon of variation; the causes for this fact cannot be found in any other category than that comprising the hereditary and congenital influences of parent upon offspring. How the effect is produced by such causes is less important in the present connection than the natural fact of congenital variation. Science, however, has learned much about the causes in question, as we shall see at a later point.

Thus the first step which is necessary for an evolution and transformation of organic mechanisms proves to be entirely natural when we give only passing attention to certain obvious phenomena of life. The fact of "becoming different" cannot be questioned without indicting our powers of observation, and we must believe in it on account of its reality, even though the ultimate analysis of the way variations of different kinds are produced remains for the future.

Having learned that animals are able to change in various ways, the next question is whether variations can be transmitted to future generations through the operation of secondary factors. Long ago Buffon held that the direct effects of the environment are immediately heritable, although the mode of this inheritance was not described; it was simply assumed and taken for granted. Thus the darker color of the skin of tropical human races would be viewed by Buffon as the cumulative result of the sun's direct effects. Lamarck laid greater stress upon the indirect or functional variations due to the factors of use and disuse, and he also assumed as self-evident that such effects were transmissible as "acquired characters." This expression has a technical significance, for it refers to variations that are added during individual life to the whole group of hereditary qualities that make any animal a particular kind of organism. If evolution takes place at all, any new kind of organism originating from a different parental type must truly acquire its new characteristics, but few indeed of the variations appearing during the lifetime of an animal owe their origin to the functional and environmental influences, whose effects only deserve the name of "acquired characters" in the special biological sense.

In sharp contrast to Lamarckianism, so called,—although it did not originate in the mind of the noted man of science whose name it bears,—is the doctrine of natural selection, first proposed in its full form by Charles Darwin. This doctrine presents a wholly natural description of the method by which organisms evolve, putting all of the emphasis upon the congenital causes of variation, although the reality of other kinds of change is not questioned. But the contrast between Darwinism and the other descriptions of secondary factors can best be made after a somewhat detailed discussion of the former, which has gained the adherence of the majority of the naturalists of to-day. However, we must not pass on without pointing out that however much the explanations given by various men of science may differ, they all agree in expressly recognizing the complete naturalness of the secondary as well as of the primary factors of evolution.

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The doctrine of natural selection forms the best basis for the detailed discussion of the way evolution has come about in the past and how it is going on to-day. This is true because it was the first description of nature's program to carry conviction to the scientific world, and because its major elements have stood the test of time as no other doctrine has done. Much has been added to our knowledge of natural processes during post-Darwinian times, and new discoveries have supplemented and strengthened the original doctrine in numerous ways, although they have corrected certain of the minor details on the basis of fuller investigation.

At the outset it must be clearly understood that Darwin's doctrine is concerned primarily with the method and not with the evidences as to the actual fact of evolution. Most of those who are not familiar with the principles of science believe that Darwin discovered this process; but their opinion is not correct. The reality of natural change as a universal attribute of living things had been clearly demonstrated long before Darwin wrote the remarkable series of books whose influence has been felt outside the domains of biology and to the very confines of organized knowledge everywhere. The "Origin of Species" was published in 1859, and only the last of its fourteen chapters is devoted to a statement of the evidence that evolution is true. In this volume Darwin presented the results of more than twenty-five years of patient study of the phenomena of nature, utilizing the observations of wild life in many regions visited by him when he was the naturalist of the "Beagle" during its famous voyage around the world. He also considered at length the results of the breeder's work with domesticated animals, and he showed for the first time that the latter have an evolutionary significance. Because his logical assembly of wide series of facts in this and later volumes did so much to convince the intellectual world of the reasonableness of evolution, Darwin is usually and wrongly hailed as the founder of the doctrine. It is interesting to note in passing that Alfred Russel Wallace presented a precisely similar outline of nature's workings at about the same time as the statement by Darwin of his theory of natural selection. But Wallace himself has said that the greater credit belongs to the latter investigator who had worked out a more complete analysis on the basis of far more extensive observation and research.

The fundamental point from which the doctrine of natural selection proceeds is the fact that all creatures are more or less perfectly adapted to the circumstances which they must meet in carrying on their lives; this is the reason why so much has been said in earlier connections regarding the universal occurrence of organic adaptation. An animal is not an independent thing; its life is intertwined with the lives of countless other creatures, and its very living substance has been built up out of materials which with their endowments of energy have been wrested from the environment. Every animal, therefore is engaged in an unceasing struggle to gain fresh food and new energy, while at the same time it is involved in a many-sided conflict with hordes of lesser and greater foes. It must prevail over all of them, or it must surrender unconditionally and die. There is no compromise, for the vast totality we individualize as the environment is stern and unyielding, and it never relents for even a moment's truce.

To live, then, is to be adapted for successful warfare; and the question as to the mode of origin of species may be restated as an inquiry into the origin of the manifold adaptations by which species are enabled to meet the conditions of life. Why is adaptation a universal phenomenon of organic nature?

The answer to this query given by Darwinism may be stated so simply as to seem almost an absurdity. It is, that if there ever were any unadapted organisms, they have disappeared, leaving the world to their more efficient kin. Natural selection proves to be a continuous process of trial and error on a gigantic scale, for all of living nature is involved. Its elements are clear and real; indeed, they are so obvious when our attention is called to them that we wonder why their effects were not understood ages ago. These elements are (1) the universal occurrence of variation, (2) an excessive natural rate of multiplication, (3) the struggle for existence entailed by the foregoing, (4) the consequent elimination of the unfit and the survival of only those that are satisfactorily adapted, and (5) the inheritance of the congenital variations that make for success in the struggle for existence. It is true that these elements are by no means the ultimate causes of evolution, but their complexity does not lessen their validity and efficiency as the immediate factors of the process.

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Taking up the first proposition, we return to the subject of variation that has been discussed previously for the purpose of demonstrating its reality. The observations of every day are enough to convince us that no two living things are ever exactly alike in all respects. The reason is that the many details of organic structure are themselves variable, so that an entire organism cannot be similar to another either in material or in functional regards, while furthermore it would be impossible for an animal to be related to environmental circumstances in the same way as another member of its species unless it was possible for two things to occupy the same space at the same time! Individual differences in physical constitution are displayed by any litter of kittens, with identical parents; it needs only a careful examination to find the variations in the shape of the heads, the length of their tails, and in every other character. Sometimes the differences are less evident in physical qualities than in disposition and mental make-up, for such variations can be found among related kittens just as surely as among the children belonging to a single human family.

Not only do all organisms vary, but they seem to vary in somewhat similar ways. While modern investigations have thrown much light upon the relations between variations and their causes, of particular value in the case of the congenital phenomena, the greatest advance since Darwin's time consists in the demonstration by the naturalists who have employed the laborious methods of statistical analysis that the laws according to which differences occur are the same where-ever the facts have been examined. A single illustration will suffice to indicate the general nature of this result. If the men of a large assemblage should group themselves according to their different heights in inches, we would find that perhaps one half of them would agree in being between five feet eight inches and five feet nine inches tall. The next largest groups would be those just below and above this average class,—namely, the classes of five feet seven to eight inches and five feet nine to ten inches. Fewer individuals would be in the groups of five feet five to six inches and five feet ten to eleven inches, and still smaller numbers would constitute the more extreme groups on opposite sides of these. If the whole assemblage comprised a sufficient number of men, it would be found that a class with a given deviation from the average in one direction would contain about the same number of individuals as the class at the same distance from the average in the opposite direction. Taking into account the relative numbers in the several classes and the various degrees to which they depart from the average, the mathematician describes the whole phenomenon of variation in human stature by a concise formula which outlines the so-called "curve of error." From his study of a thousand men, he can tell how many there would be in the various classes if he had the measurements of ten thousand individuals, and how many there would be in the still more extreme classes of very short and very tall men which might not be represented among one thousand people.

It is not possible to explain why variation should follow this or any other mathematical law without entering into an unduly extensive discussion of the laws of error. The mathematicians themselves tell us in general terms that the observations they describe so simply by their formulæ follow as the result of so-called chance, by which they mean that the combined operation of numerous, diverse, and uncorrelated factors brings about this result, and not, of course, that there is such a thing as an uncaused event or phenomenon.

Whenever any extensive series of like organisms has been studied with reference to the variations of a particular character, the variations group themselves so as to be described by identical or similar curves of error. It is certainly significant that this is true for such diverse characters, cited at random from the lists of the literature, as the number of ray-flowers of white daisies, the number of ribs of beech leaves, and of the bands upon the capsules of poppies, for the shades of color of human eyes, for the number of spines on the backs of shrimps, and for the number of days that caterpillars feed before they turn into pupæ.

To summarize the foregoing facts, we have learned that variation is universal throughout the living world, and that the primary factors causing organic difference—the counterparts of human ingenuity in the case of dead mechanisms—are the natural influences of the environment, of organic physiological activity, and of congenital inheritance. These factors are accorded different values in the evolution of new species, as we may see more clearly at a later juncture, but the essential point here is that they are not unreal, although they may not as yet be described by science in final analytical terms.

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We come now to the second element of the whole process of evolution, namely, what we may call overproduction or excessive multiplication. Like variation and so many other phenomena of nature, this is so real and natural that it escapes our attention until science places it before us in a new light. The normal rate of reproduction in all species of animals is such that if it were unchecked, any kind of organism would cumber the earth or fill the sea in a relatively short time. That this is universally true is apparent from any illustration that might be selected. Let us take the case of a plant that lives for a single year, and that produces two seeds before it withers and dies; let us suppose that each of these seeds produces an adult plant which in its turn lives one year and forms two seeds. If this process should continue without any interference, the twentieth generation after as many years would consist of more than one million descendants of the original two-seeded annual plant, provided only that each individual of the intervening years should live a normal life and should multiply at the natural rate. But such a result as this is rendered impossible by the very nature which makes annual plants multiply in the way they do. Let us take the case of a pair of birds which produce four young in each of four seasons. Few would be prepared for the figures enumerating the offspring of a single pair of birds at the end of fifteen years, if again all individuals lived complete and normal lives: at the end of the time specified there would be more than two thousand millions of descendants. The English sparrow has been on this continent little more than fifty years; it has found the conditions in this country favorable because few natural enemies like those of its original home have been met, and as a consequence it has multiplied at an astounding rate so as to invade nearly all parts of North America, driving out many species of song birds before it. About twenty years ago David Starr Jordan wrote that if the English sparrow continued to multiply at the natural rate of that time, in twenty years more there would be one sparrow to every square inch of the state of Indiana; but of course nature has seen to it that this result has not come about. A single conger-eel may produce fifteen million eggs in a single season, and if this natural rate of increase were unchecked, the ocean would be filled solid with conger-eels in a few years. Sometimes a single tapeworm, parasitic in the human body, will produce three hundred million embryos; the fact that this animal is relatively rare diverts our attention from the alarming fertility of the species and the excessive rate of its natural increase. Perhaps the most amazing figures are those established by the students of bacteria and other micro-organisms. Many kinds of these primitive creatures are known where the descendants of a single individual will number sixteen to seventeen millions after twenty-four hours of development under ordinarily favorable conditions. Though a single rodlike individual taken as a starting-point may be less than one five-thousandth of an inch in length, under natural circumstances it multiplies at a rate which within five days would cause its descendants to fill all the oceans to the depth of one mile. This is a fact, not a conjecture; the size of one organism is known, and the rate of its natural increase is known, so that it is merely a matter of simple arithmetic to find out what the result would be in a given time.

Even in the case of those animals that reproduce more slowly, an overcrowding of the earth would follow in a very short time. Darwin wrote that even the slow-breeding human species had doubled in the preceding quarter century. An elephant normally lives to the age of one hundred years; it begins to breed at the age of thirty, and usually produces six young by the time it is ninety. Beginning with a single pair of elephants and assuming that each individual born should live a complete life, only eight hundred years would be requisite to produce nineteen million elephants; a century or two more and there would be no standing room for the latest generation of elephants. It is only too obvious that such a result is not realized in nature, but it is on account of other natural checks, and not because the natural rate of reproductive increase is anything but excessive.

The third element of the process of natural selection is the struggle for existence which is to a large extent the direct consequence of over-multiplication. Because nature brings more individuals into existence than it can support, every animal is involved in many-sided battles with countless foes, and the victory is sometimes with one and sometimes with another participant in the conflict. A survivor turns from one vanquished enemy only to find itself engaged in mortal combat with other attacking forces. Wherever we look, we find evidence of an unceasing struggle for life, and an apparently peaceful meadow or pond is often the scene of fierce battles and tragic death that escape our notice only because the contending armies are dumb.

A community of ants, often comprising more individuals than an entire European state, depends for its national existence upon its ability to prevail over other communities with which it may engage in sanguinary wars where the losses of a single battle may exceed those of Gettysburg. The developing conger-eels find a host of enemies which greatly deplete their numbers before they can grow even into infancy. An annual plant does not produce a million living offspring in twenty years because seeds do not always fall upon favorable soil, nor do they always receive the proper amount of sunlight and moisture, or escape the eye of birds and other seed-eating animals. These three illustrations bring out the fact that there are three classes of natural conditions which must be met by every living creature if it is to succeed in life. In detail, the struggle for existence is intra-specific, involving some form of competition or rivalry among the members of a single species; it is inter-specific, as a conflict is waged by every species with other kinds of living things; and finally it involves an adjustment of life to inorganic environmental influences. While it may seem unjustifiable to speak of heat and cold and sunlight as enemies, the direct effects produced by these forces are to be reckoned with no less certainty than the attacks of living foes.

The three divisions of the struggle for existence are so important not only in purely scientific respects, but also in connection with the analysis of human biology, that we may look a little further into their details, taking them up in the reverse order. Regarding the environmental influences, the way that unfavorable surroundings decimate the numbers of the plants of any one generation has already been noted, and it is typical of the vital situation everywhere. English sparrows are killed by prolonged cold and snow as surely as by the hawk. The pond in which bacteria and protozoa are living may dry up, and these organisms may be killed by the billion. Even the human species cannot be regarded as exempt from the necessity of carrying on this kind of natural strife, for scores and hundreds die every year from freezing and sunstroke and the thirsts of the desert. Unknown thousands perish at sea from storm and shipwreck, while the recorded casualties from earthquakes and volcanic eruptions and tidal waves have numbered nearly one hundred and fifty thousand in the past twenty-eight years. The effects of inorganic influences upon all forms of organic life must not be underestimated in view of such facts as these.

In the second place, the vital struggle includes the battles of every species with other kinds of living things whose interests are in opposition. The relations of protozoa and bacteria, conger-eels and other fish, English sparrows and hawks, plants and herbivorous animals, are typical examples of the universal conflict in which all organisms are involved in some way. Again it is only too evident that human beings must participate every day in some form of warfare with other species. In order that food may be provided for mankind the lives of countless wild organisms must be sacrificed in addition to the great numbers of domesticated animals reared by man only that they may be destroyed. The wolf and the wildcat and the panther have disappeared from many of our Eastern states where they formerly lived, while no longer do vast herds of bison and wild horses roam the Western prairies. Because one or another human interest was incompatible with the welfare of these animals they have been driven out by the stronger invaders.

That the victory does not always fall to the human contestant is tragically demonstrated by the effects of the incessant assaults upon man made by just one kind of living enemy,—the bacillus of tuberculosis. Every year more than one hundred and twenty-five thousand people of the United States die because they are unable to withstand its persistent attacks; five million Americans now living are doomed to death at the hands of these executioners, and the figures must be more than doubled to cover the casualties on the human side in the battles with the regiments of all the species of bacteria causing disease.

The competition between and among the individuals of one and the same species is the third part of the struggle for existence, and it is often unsurpassed in its ferocity. When two lion cubs of the same litter begin to shift for themselves, they must naturally compete in the same territory, and their contest is keener than that which involves either of them and a young lion born ten or fifteen miles away. The seeds of one parent plant falling in a restricted area will be engaged in a competitive struggle for existence that is much more intense than many other parts of nature's warfare. In brief, the intensity of the competition will be directly proportional to the similarity of two organisms in constitution and situation, and to the consequent similarity of vital welfare. The interests of the white man and the Indian ran counter to each other a few hundred years ago, and the more powerful colonists won. The assumption of the white man's burden too often demonstrates the natural effect of diversity of interest, and the domination of the stronger over the weaker. In any civilized community the manufacturer, farmer, financier, lawyer, and doctor must struggle to maintain themselves under the conditions of their total inorganic and social environments; and in so far as the object of each is to make a living for himself, they are competitors. But the contest becomes more absorbing when it involves broker and broker, lawyer and lawyer, financier and magnate, because in each case the contestants are striving for an identical need of success.

Although the severity of the conflict imposed by nature is somewhat modified in the case of social organisms, where community competes with community and nation with nation, no form of social organization has yet been developed where the individual contest carried on by the members of one community has been done away with. It is an inexorable law of nature that all living things must fight daily and hourly for their very lives, because so many are brought into the world with each new generation that there is not sufficient room for all. No organism can escape the struggle for existence except by an unconditional surrender that results in death. Everywhere we turn to examine the happenings of organic life we can find nothing but a wearisome warfare in which it is the ultimate and cruel lot of every contestant to admit defeat.

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What now are the results of variation, over-multiplication, and competition? Since some must die because nature cannot support all that she produces, since only a small proportion of those that enter upon life can find a foothold or successfully meet the hordes of their enemies, which will be the ones to survive? Surely those that have even the slightest advantage over their fellows will live when their companions perish. It is impossible that the result could be otherwise; it must follow inevitably from what has been described before. The whole process has its positive and its negative aspects: the survival of the fittest and the elimination of the unfit. Perhaps it would be more correct to say the more real element is the negative one, for those which are least capable of meeting their living foes and the decimating conditions of inorganic nature are the first to die, while the others will be able to prolong the struggle for a longer or shorter period before they too succumb. Thus the destruction of the unfit leaves the field to the better adapted, that is, to those that vary in such a way as to be completely or at least partially adapted to carry on an efficient life. In this way Darwinism explains the universal condition of organic adjustment, showing that it exists because there is no place in nature for the incompetent.

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Finally we come to the process of inheritance as viewed by Darwin, and its part in the production and perfection of new species. In every case, Darwin said, the efficiency or inefficiency of an animal depends upon its characteristics of an inherited or congenital nature. Variations in these qualities provide the array of more or less different individuals from which impersonal nature selects the better by throwing out first the inferior ones. An organism can certainly change in direct response to environmental influence or by the indirect results of use and disuse, but not unless it is so constituted by heredity as to be able to change adaptively. Therefore the final basis of success in life must be sought in the inherited constitutions of organic forms.

For the reason that the qualities which preserve an animal's existence are already congenital, they are already transmissible, as Darwin contended. Since his time much has been learned about the course of inheritance and its physical basis, and the new discoveries have confirmed the essential truth of Darwin's statement that the congenital characters only possess a real power in the evolution of species.

We must devote some time to the subject of inheritance at a later juncture, but before leaving the matter an additional point must be established here; the selective process deals immediately with congenital results, as the heritable characters that make for success or failure in life, but by doing this it really selects the group of congenital factors behind and antecedent to their effects. For example, an ape that survives because of its superior cunning, does so because it varies congenitally in an improved direction; and the factors that have made it superior are indirectly but no less certainly preserved through the survival of their results in the way of efficiency. Hereditary strains are thus the ultimate things selected through the organic constitutions that they determine and produce.

Natural selection, as the whole of this intricate process, is simply trial and error on a gigantic scale. Nature is such that thousands of varying individuals are produced in order that a mere handful or only one survivor may be chosen to bear the burden of carrying on the species for another generation. The effect of nature's process is judicial, as it were. We may liken the many and varied conditions of life to as many jurymen, before which every living thing must appear for judgment as to its fitness or lack of it. A unanimous verdict of complete or partial approval must be rendered, or an animal dies, for the failure to meet a single vital condition results in sure destruction. Of course, we cannot regard selection as involving anything like a primitive conscious choice. It is because we individualize all of the complex totality of the world as "Nature" with a capital N that so many people unconsciously come to think of it as a human-like personality. He who would go further and hold that all of nature is actually conscious and the dwelling-place of the supernatural ultimate, must beware of the logical results of such a view. What must we think of the ethical status of such a conscious power who causes countless millions of creatures to come into the world and ruthlessly compels them to battle with one another until a cruel and tragic death ends their existence?

But that is a metaphysical matter, with which we need not concern ourselves in this discussion; the important point is that among the everyday happenings of life are processes that are quite competent to account for the condition of adaptation exhibited by various animal forms. These processes are real and natural, not imaginative or artificial, and so they will remain even though it will become clear that much is still to be learned about the causes of variation and the course of biological inheritance. Darwin was the first to contend that natural selection is but a part of nature's method of accomplishing evolution. As such it is content to recognize variations and does not concern itself with the origin of modifications; it accepts the obvious fact that congenital variations are inherited, although it leaves the question as to how they are inherited for further examination. Because the doctrine of natural selection does not profess to answer all the questions propounded by scientific inquisitiveness, it must not be supposed that it fails in its immediate purpose of giving a natural explanation of how evolution may be partly accounted for.

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Before proceeding to the post-Darwinian investigations that have done so much to amplify the account of natural evolution, let us consider the contrasted explanation given by Lamarck and his followers. As we have stated earlier, Lamarckianism is the name given to the doctrine that modifications other than those due to congenital factors may enter into the heritage of a species, and may add themselves to those already combined as the peculiar characteristics of a particular species. Let us take the giraffe and its long neck as a concrete example. The great length of this part is obviously an adaptive character, enabling the animal to browse upon the softer leafy shoots of shrubs and trees. The vertebral column of the neck comprises just the same number of bones that are present in the short-necked relatives of this form, so that we are justified in accepting as a fact the evolution of the giraffe's long neck by the lengthening of each one of originally shorter vertebræ. The Lamarckian explanation of this fact would be that the earliest forms in the ancestry of the giraffe as such stretched their necks as they fed, and that this peculiar function with its correlated structural modification became habitual. The slight increase brought about by any single individual would be inherited and transmitted to the giraffes of the next generation; in other words, an individually acquired character would be inherited. The young giraffes of this next generation would then begin, not where their parents did, but from an advanced condition. Thus, by continued stretching of the neck and by continued transmission of the elongated condition, the great length of this part of the body in the modern giraffe would be attained.

The explanation of natural selection would be quite different. The Darwinian would say that all the young giraffes of any one generation would vary with respect to the length of the neck. Those with longer necks would have a slight advantage over their fellows in the extended sphere of their grazing territory. Being better nourished than the others, they would be stronger and so they would be more able to escape from their flesh-eating foes, like the lion. For the reason that their variation would be congenital and therefore already transmissible, their offspring would vary about the advanced condition, and further selection of the longer necked individuals would lead to the modern result.

The Lamarckian explanation encounters one grave difficulty which is not met by the second one, in so far as it demands some method by which a bodily change may be introduced into the stream of inheritance. So far, this difficulty has not been overcome, and the present verdict of science is that the transmission of characters acquired as the result of other than congenital factors is not proved. It would be unscientific to say that it cannot be proved in the future, but there are good a priori grounds for disbelief in the principle, while furthermore the results of experiments that have been undertaken to test its truth have been entirely negative. Rats and mice have had their tails cut off to see if this mutilation would have its effect upon their young, and though this has been done for more than one hundred successive generations the length of the tail has not been altered. Quite unconscious of the scientific problem, many human races have performed precisely similar experiments through centuries of time. In some classes of Chinese, the feet of young girls have been bound in such a way as to produce a small, malformed foot, but this has not resulted in any hereditary diminution in the size of the feet of Chinese females. Many other similar mutilations have been practised, as for example, the flattening of the skull of some North American Indians, but the deformity must be produced again with each recurring generation. One after another, the cases that were supposed to give positive evidence have been reinvestigated, with the result that has been stated above. It would seem, therefore, that heredity and congenital modification must play by far the greater part in the evolution of species.

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The doctrine of natural selection took form in the mind of Darwin mainly on account of three potent influences; these were, first, the geological doctrine of uniformitarianism proposed by Lyell, second, his own observations of wild life in many lands and his analysis of the breeder's results with domesticated animals, and third, the writings of Malthus dealing with overpopulation. As Darwin had read the works of Buffon, Lamarck, and Erasmus Darwin, his grandfather, who had written a famous treatise under the title of "Zoonomia," he was familiar with the evidences known in his student days tending to prove that organic evolution was a real natural process. Lyell's doctrine of uniform geological history made an early and deep impression upon his mind, and it led him to ask himself whether the efficient causes of past evolution might not be revealed by an analysis of the present workings of nature. As naturalist of the "Beagle" during its four years' cruise around the world, Darwin saw many new lands and observed varied circumstances under which the organisms of the tropics and other regions lived their lives. The fierce struggle for existence waged by the denizens of the jungle recalled to him the views of Malthus regarding overpopulation and its results. These and other influences led him to begin the remarkable series of note-books, from which it is interesting indeed to learn how the doctrine of natural selection began to assume a definite and permanent form in his mind, as year followed year, and evidence was added to evidence. And it is a valuable lesson to the student of science that for twenty-five years Darwin devoted all his time to the acquisition of facts before he gave his doctrine to the world in the famous "Origin of Species."

Darwin was particularly impressed by the way mankind has dealt with the various species of domesticated animals, and he was the first naturalist to point out the correspondence between the breeder's method of "artificial selection," and the world-wide process of natural selection. As every one knows, the breeder of race horses finds that colts vary much in their speed; discarding the slower animals, he uses only the swifter for breeding purposes, and so he perfects one type of horse. With other objects in view, the heavy draught horse, the spirited hackney, and the agile polo pony have been severally bred by exactly the same method. Among cattle many kinds occur, again the products of an artificial or human selection; hornless breeds have been originated, as well as others with wide-spreading or sharply curved horns; the Holstein has been bred for an abundant supply of milk as an object, while Jerseys and Alderneys excel in the rich quality of their milk. Various kinds of domesticated sheep and rabbits and cats also owe their existence to the employment of the selfsame method, unconsciously copied by man from nature; for men have found variations arising naturally among their domesticated animals, and they have simply substituted their practical purposes or their fancy for nature's criterion of adaptive fitness, preserving those that they wish to perfect and eliminating those unfitted to their requirements or ideas.

In the case of many of these and other examples, wild forms still occur which seem to be like the ancestral stock from which the domesticated forms have been produced. All the varied forms of dogs—from mastiff to toy-terrier, and from greyhound to dachshund and bulldog—find their prototypes in wild carnivora like the wolf and jackal. In Asia and Malaysia the jungle fowl still lives, while its domesticated descendants have altered under human direction to become the diverse strains of the barnyard, and even the peculiar Japanese product with tail feathers sometimes as long as twenty feet. That far-reaching changes can be brought about in a relatively short time is proved by the history of the game cock, which has nearly doubled in height since 1850, while at the same time its slender legs, long spurs, and other qualities have been perfected for the cruel sport for which it has been bred. Again, the wild rock pigeon seems to be the ancestral form from which the fantail and pouter and carrier-pigeon with their diverse characters have taken their origin.

It is true that some biologists have urged certain technical objections to the employment of domesticated animals and their history as analogies to the processes and results in wild nature. To my mind, however, artificial selection is truly a part of the whole process of natural selection. Man is but one element of the environment of tame forms, and his fancy or need is therefore one of the varied series of external criteria that must be met if survival is to be the result; failing this, elimination follows as surely as under the conditions of an area uninhabited or uninfluenced by mankind. Congenital variation is real, selection is real and the heredity of the more fit modification is equally real. Surely Darwin was right in contending that the facts of this class amplify the conception of natural selection developed on the basis of an analysis of wild life.

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Knowing the elements of the selective process, it is possible to analyze and to understand many significant phenomena of nature, and to gain a clearer conception of the results of the struggle for existence, especially when the human factor is involved. Let us see how much is revealed when the foregoing results are employed in a further study of some of nature's vital situations.

As a consequence of the many-sided struggle for existence, the interrelations of a series of species will approach a condition of equilibrium in an area where the natural circumstances remain relatively undisturbed for a long time. For example, among the field-mice of one generation, just as many individuals will survive as will be able to find food and to escape hereditary foes such as cats and snakes and owls. The number of owls, in their turn, will be determined by the number of available mice and other food organisms, as well as by the severity of the adverse circumstances that cause elimination of the less fit among the fledglings brought into the world. The vital chain of connections is sometimes astonishingly long and intricate. One remarkable illustration is given by Fiske, as an elaboration of an example cited by Darwin. He points out that the fine quality of the traditional roast beef of England is directly determined by the number of elderly spinsters in that country. The chain of circumstances is as follows: the quality of the clover fields, furnishing the best food for cattle, depends largely upon the visits to the clover-blossoms by wild bees, that accomplish the fertilization of the flowers by carrying pollen upon their bodies from one plant to another. Field-mice devour the young in the nests of these bees, so if there are few field-mice there will be many bees, and consequently better grazing for the cattle. The number of field-mice will vary according to the abundance of cats, and so the number of these domestic animals will exert an influence upon the whole foregoing chain of forms. But, as Fiske points out, cats are the favorite companions of elderly spinsters; therefore, if there are many of the latter, there will be more cats, fewer field-mice, more bees, richer clover fields, and finer cattle! Each link is real and the whole chain is a characteristic example of the countless ways that the natural destinies of living things are interrelated and intertwined.

The reality of such organic interrelationships is revealed with wonderful clearness in the numerous instances where some disturbing factor has altered one or another element of the balanced system. The invasion of the new world by Europeans has directly led to the partial or complete extinction of the tribes of Indians to whom the land formerly belonged; they have disappeared almost entirely from our state of New York, together with the bear and wolf and many other species of animals that formerly existed here. Wild horses and bison have also vanished before the advances of civilization and the alteration of their homes. Sometimes the extermination of one pest has resulted in an increase in the number of another through human interference with nature's equilibrium. In some of our Western states, a bounty was offered for the scalps of wolves, so as to lessen the number of these predatory foes of sheep. But when the wolves were diminished in number, their wild food-animals, the prairie dogs, found their lot much bettered, and they have multiplied so rapidly that in some places they have become even more destructive than the wolves.

One of the most remarkable illustrations is that of the rabbits introduced into Australia. This island continent was cut off from the surrounding lands long before the higher mammals evolved in far distant regions, so that the balance of nature was worked out without reference to animals like the rabbit. When the first of these were introduced they found a territory without natural enemies where everything was favorable. They promptly multiplied so rapidly that within a few years their descendants were numerous enough to eat up practically every green thing they could reach. Two decades ago, the single province of Queensland was forced to expend $85,000,000 in a vain effort to put down the rabbit plague. The remarkable statement has been made that in some places nature has taken a hand in causing a new type of rabbit to evolve. Finding the situation desperate, some of the animals have begun to develop into tree-climbing creatures. The animals exist in such numbers that the available food upon the ground is insufficient for all, and so some elimination results. But the young rabbits with longer claws, varying in this way on account of congenital factors, have an advantage over their fellows because they can climb some of the trees and so obtain food inaccessible to the others. If the facts are correctly reported, and if the process of selection on the basis of longer claws and the climbing habit is continued, the original type of animal is splitting up into a form that will remain the same and live upon the ground, and another that will be to all intents and purposes a counterpart of our familiar squirrel. All the evidence goes to show that squirrels have evolved from terrestrial rodents; if the data relating to Australian rabbits are correct, nature is again producing a squirrel-like animal by evolution in a region where the former natural situation has been interfered with by man.

The laws of biological inheritance have received close and deep study by numerous investigators of Darwinian and post-Darwinian times, because from the first it was clearly recognized that a complete description of nature's method of accomplishing evolution must show how species maintain the same general characteristics from generation to generation, and also how new qualities may be fixed in heredity as species transform in the course of time. Before our modern era in biology, the fact of inheritance was accepted as self-sufficient; now much is known that supplements and extends the incomplete account given by natural selection of the way evolution takes place.

It is not possible in the present brief outline to describe all the results of recent investigations, but some of them are too important to be passed over. Perhaps the most interesting one is that the laws of heredity seem to be the same for man and other kinds of living creatures, as proved by Galton and Pearson and many others who have dealt with such characters as human stature, human eye color, and an extensive series of the peculiarities of lower animals and even of plants.

The researches dealing with the physical basis of inheritance and its location in the organism have yielded the most striking and brilliant results. Darwin himself realized that the doctrine of natural selection was incomplete, as it accepted at its face value the inheritance of congenital racial qualities without attempting to describe the way an egg or any other germ bears them, and he endeavored to round out his doctrine of selection by adding the theory of pangenesis. According to this, every cell of every tissue and organ of the body produces minute particles called gemmules, which partake of the characters of the cells that produce them. The gemmules were supposed to be transported throughout the entire body, and to congregate in the germ-cells, which in a sense would be minute editions of the body which bears them, and would then be capable of producing the same kind of a body. If true, this view would lead to the acceptance of Lamarck's or even Buffon's doctrine, for changes induced in any organ by other than congenital factors could be impressed upon the germ-cell, and would then be transported together with the original specific characters to future generations. Darwin was indeed a good Lamarckian.

But the researches of post-Darwinians, and especially those of the students of cellular phenomena, have demonstrated that such a view has no real basis in fact. Many naturalists, like Naegeli and Wiesner, were convinced that there was a specific substance concerned with hereditary qualities as in a larger way protoplasm is the physical basis of life. It remained for Weismann to identify this theoretical substance with a specific part of the cell, namely, the deeply staining substance, or chromatin, contained in the nucleus of every cell. Bringing together the accumulating observations of the numerous cytologists of his time, and utilizing them for the development of his somewhat speculative theories, Weismann published in 1882 a volume called "The Germ Plasm," which is an immortal foundation for all later work on inheritance. The essential principles of the germ-plasm theory are somewhat as follows. The chromatin of the nucleus contains the determinants of hereditary qualities. In reproduction, the male sex-cell, which is scarcely more than a minute mass of chromatin provided with a thin coat of protoplasm and a motile organ, fuses with the egg, and the nuclei of the two cells unite to form a double body, which contains equal contributions of chromatin from the two parental organisms. This gives the physical basis for paternal inheritance as well as for maternal inheritance, and it shows why they may be of the same or equivalent degree. When, now, the egg divides, at the first and later cleavages, the chromatin masses or chromosomes contained in the double nucleus are split lengthwise and the twin portions separate to go into the nuclei of the daughter-cells. As the same process seems to hold for all the later divisions of the cleavage-cells whose products are destined to be the various tissue elements of the adult body, it follows that all tissue-cells would contain chromatin determinants derived equally from the male and female parents. As of course only the germ-cells of an adult organism pass on to form later generations, and as their content of chromatin is derived not from the sister organs of the body, but from the original fertilized egg, there is a direct stream of the germ plasm which flows continuously from the germ-cell to germ-cell through succeeding generations. It would seem, therefore, that the various organic systems are, so to speak, sister products in embryonic origin. The reproductive organs are not produced by the other parts of the body, but their cells are the direct descendants of the common starting-point namely, the egg. As the cells of the reproductive organs are the only ones that pass over and into the next and later generations, it will be evident, in the first place, that the germ plasm of their nuclei is the only essential substance that connects parent and offspring. This stream of germ plasm passes on in direct continuity through successive generations—from egg to the complete adult, including its own germ-cells, through these to the next adult, with its germ-cells, and so on and on as long as the species exists. It does not flow circuitously from egg to adult and then to new germ-cells, but it is direct and continuous, and apparently it cannot pick up any of the body-changes of an acquired nature. Now we see why individual acquisitions are not transmitted. The hereditary stream of germ plasm is already constituted before an animal uses its parts in adult life; we cannot see how alterations in the structure of mature body parts through use and adjustment to the environment can be introduced into it to become new qualities of the species.

It must be clear, I am sure, that this theory supplements natural selection, for it describes the physical basis of inheritance, it demonstrates the efficiency of congenital or germ-plasmal factors of variation in contrast with the Lamarckian factors, and finally in the way that in the view of Weismann it accounts for the origin of variations as the result of the commingling of two differing parental streams of germ plasm.

At first, for many reasons, Weismann's theories did not meet with general acceptance, but during recent years there has been a marked return to many of his positions, mainly as the result of further cytological discoveries, and of the formulation of Mendel's Law and of De Vries's mutation theory. The first-named law was propounded by Gregor Mendel on the basis of extensive experiments upon plants conducted during many years, 1860 and later, in the obscurity of his monastery garden at Altbrünn, in Austria. It was rescued from oblivion by De Vries, who found it buried in a mass of literature and brought it to light when he published his renowned Mutation Theory in 1901. Mendelian phenomena of inheritance, confirmed and extended by numerous workers with plants and animals, prove that in many cases portions of the streams of germ plasm that combine to form the hereditary content of organisms may retain their individuality during embryonic and later development, and that they may emerge in their original purity when the germ-cells destined to form a later generation undergo the preparatory processes of maturation. They demonstrate also the apparent chance nature of the phenomena of inheritance. To my mind the most striking and significant result in this field is the demonstration that a particular chromosome or chromatin mass determines a particular character of an adult organism, which is quite a different matter from the reference of all the hereditary characters to the chromatin as a whole. Wilson and others have brought forward convincing proof that the complex character of sex in insects actually resides in or is determined by particular and definite masses of this wonderful physical basis of inheritance.

Mendel's principles also account in the most remarkable way for many previously obscure phenomena, like reversion, or a case where a child resembles its grandparent more than it does either of its parents; such phenomena are due, so to speak, to the rise to the surface of a hidden stream of germ plasm that had flowed for one or many generations beneath its accompanying currents. I believe that the law is replacing more and more the laws of Galton and Pearson, formulated as statistical summaries of certain phenomena of human inheritance taken en masse. According to Galton's celebrated law of ancestral inheritance, the qualities of any organism are determined to the extent of a certain fraction by its two parents taken together as a "mid-parent," that a smaller definite fraction is contributed by the grandparents taken together as a mid-grandparent, and so on to earlier generations. But Mendel's Law has far greater definiteness, it explains more accurately the cases of alternative inheritance, and it may be shown to hold for blended and mosaic inheritance as well.

De Vries's new "mutation theory" is clearly not an alternative but a complementary theory to natural selection, the Weismannian and Mendelian theories. Like these last, it emphasizes the importance of the congenital hereditary qualities contained in the germ plasm, though unlike the Darwinian doctrine it shows that sometimes new forms may arise by sudden leaps and not necessarily by the slow and gradual accumulation of slight modifications or fluctuations. The mutants like any other variants must present themselves before the jury of environmental circumstances, which passes judgment upon their condition of adaptation, and they, too, must abide by the verdict that means life or death.

From what has been said of these post-Darwinian discoveries, the Lamarckian doctrine, which teaches that acquired non-congenital characters are transmitted, seems to be ruled out. I would not lead you to believe that the matter is settled. I would say only that the non-transmission of racial mutilations, negative breeding experiments upon mutilated rats and mice, the results of further study of supposedly transmitted immunity to poisons—that all these have led zoölogists to render the verdict of "not proved." The future may bring to light positive evidence, and cases like Brown-Séquard's guinea-pigs, and results like those of MacDougal with plants, and of Tower with beetles, may lead us to alter the opinion stated. But as it stands now most investigators hold that there are strong general grounds for disbelief in the principle, and also that it lacks experimental proof.

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The explanation of natural evolution given by Darwinism and the principles of Weismann, Mendel, and De Vries, still fails to solve the mystery completely, and appeal has been made to other agencies, even to teleology and to "unknown" and "unknowable" causes as well as to circumstantial factors. A combination of Lamarckian and Darwinian factors has been proposed by Osborn, Baldwin, and Lloyd Morgan, in the theory of organic selection. The theory of orthogenesis propounded by Naegeli and Eimer, now gaining much ground, holds that evolution takes place in direct lines of progressive modification, and is not the result of apparent chance. Of these and similar theories, all we can say is that if they are true, they are not so well substantiated as the ones we have reviewed at greater length.

The task of experimental zoölogy is to work more extensively and deeply upon inheritance and variation, combining the methods and results of cellular biology, biometrics, and experimental breeding. We may safely predict that great advances will be made during the next few years in analyzing the method of evolution; and that a few decades hence men will look back to the present time as a period of transition like the era of reawakened interest and renewed investigation that followed the appearance of the "Origin of Species." For the present, we can justly say "that evolution, so far as it is understood, is a real and natural process."

V

THE PHYSICAL EVOLUTION OF THE HUMAN SPECIES AND OF HUMAN RACES

The teachings of science that relate to the origin and history of the human species constitute for us the most important part of the whole doctrine of organic evolution and now, having completely outlined this doctrine as a general one, we are brought to the point where we must deal frankly and squarely with the insistent questions arising on all sides as to the way that mankind is involved in the vast mechanism of nature's order. These questions have been ignored heretofore, in order that the natural history of animals in general might be discussed without any interference on the part of purely human interest and concern. It now becomes our privilege, and our duty as well, to employ and apply the principles we have learned in order to understand more completely the origin of the human body as an organic type, the history of human races, the development of human faculty and of social institutions, and the evolution finally of even the highest elements of human life. These are scientific problems, and if we are to solve them we must employ the now familiar methods of science which only yield sure results.

We must not underestimate the many difficulties to be encountered, for the field before us is a vast territory of complex human life and of manifold human relations. Without prolonged exercise in scientific methods, it is impossible to view our own kind impersonally, as we do the creatures of lower nature. Furthermore it seems to many that an analysis of human life and biological history, even if it is possible, must alter or degrade mankind in some degree; this is no more true than that a knowledge of the principles of engineering according to which the Brooklyn Bridge has been constructed renders that structure any different or unsafe for travel. Man remains man, whether we are in utter ignorance of his mode of origin, or whether we know all about his ancestry and about the factors that have made him human. It is because our species appears to occupy a superior and isolated position above the rest of nature that the mind seems reluctant to follow the guidance of science when it conducts its investigations into the history of seemingly privileged human nature. And it is feared also, that if evolution is proven for man as well as for all other kinds of animals, our cherished ideas and our outlook upon many departments of human life must be profoundly affected. This may be so, but science endeavors only to find out the truth; it cannot alter truth, nor does it seek to do so. We might well wish that the world were different in many respects and that we were free from the control of many natural laws besides that of evolution, but if the real is what it is, then our duty is plain before us; as we think more widely and deeply on the basis of ripened experience, it becomes ever clearer that a knowledge of human history gives the only sure guidance for human life.

To the zoölogist it seems strange that so many are opposed to a scientific inquiry into the facts of human evolution, and to the conclusions established by such an inquiry,—though, to be sure, this opposition is directly proportional to ignorance or misunderstanding of the nature and purpose of scientific investigation and of human evolution. The naturalist comes to view our species as a kind of animal, and as a single one of the hundreds of thousands of known forms of life; thus the question of human origin is but a small part of organic evolution, which is itself only an episode in the great sweep of cosmic evolution, endless in past time and in the future. Were we some other order of beings, and not men, human evolution would appear to us in its proper scientific proportions, namely, as a minute fraction of the whole progress of the world.

While the foregoing statements are true, it is nevertheless right that a close study should be made of the particular case of mankind. No doubt much of the naturalist's interest in nature at large is due to his conviction that the laws revealed by the organisms of a lower sphere must hold true for man, and may explain many things that cannot be so clearly discerned when only the highest type is the subject of investigation. It is only too evident that little more than a general outline can be given of the wide subject or group of subjects included under the head of human evolution. We must divide the subject logically into parts, so that each one may be taken up without being complicated by questions relating to topics of another category, although the findings in any one department must surely be of importance for comparison with the results established in another section; for if evolution is universally true, the main conclusion in any case must assist the investigation of another, just as comparative anatomy and embryology supplement and corroborate each other in the larger survey of organic evolution. As before, the illustrations of each department of the subject must be selected from the stock of everyday observation and information that we already possess, for we gain much when we realize that evolution includes all the happenings of everyday life and thought, as well as the occurrences of the remote past.

For the present, then, the questions relating to the higher aspects of human life must be put aside, only that they may be taken up at the last. Social evolution likewise finds its place in a later section, after the phenomena of mind and mental evolution receive due attention and description. At the present juncture, the human species presents itself as a subject for organic analysis and classification, merely as a physical organism. Just as the study of locomotives must begin with the detailed structure of machines in the workshop before they can be profitably understood as working mechanisms, so the physical evolution of mankind must first be made intelligible before it is possible to prosecute successfully the studies dealing with the psychology, social relations, and higher conceptions that seem at first to be the exclusive properties of our species.

The problems of physical evolution of man and of men fall into two groups. Those of the first deal with the origin of the human species as a unit, and its comparative relation to lower organisms, while those of the second part are concerned with the further evolution of human races that have come to be different in certain details of structure since the human type as such arose. In the first part, all men will be assumed to be alike and the members of a homogeneous species whose fundamental attributes are to be compared with those of other animals; only afterwards will attention be directed to the differences, previously ignored, that divide human beings into well-marked varieties. It must be evident even at this point that the mode of evolution demonstrated by the first investigation will be likely to bear some close relation to the methods by which human races have evolved to their present diverse anatomical situations.

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The foregoing classification of the problems concerned with the nature and origin of the human species renders it possible to restrict the immediate inquiry to a definite and precise question. It is this: does the evidence relating to the physical characteristics of our species prove that man is the product of a supernatural act of creation, or does it show that man's place in nature has been reached by a gradual process of natural evolution? In order to obtain an equally precise and definite answer to this question, referring to the particular case of most concern to us, it is obvious that the method to be employed is the one which has given us an understanding of organic evolution as an all-inclusive natural process. The data must be verified, related, and classified, so that their meaning may be concisely stated in the form of scientific principles. What are the facts of human structure, comparatively treated? How does the human body develop? Does palæontology throw any light on the antiquity of man? Do the rules of nature's order control the lives of men? Our course is now clear; we shall take up serially the anatomy, embryology, and fossil history of the human species, in order to see that there is ample proof of the actual occurrence of evolution, and then, as before, we may look about for the causes which have produced this result by natural methods.

While it is necessary to treat the subject directly, namely, by examining the actual evidences relating to the particular case in question, it is worthwhile before doing so to point out that, as the whole includes a part, human evolution has already been proved beyond question. This conclusion must be accepted, unless reasons can be given for excluding mankind from the rest of the living world as an absolutely unique type, supreme and isolated because of some peculiar endowments not shared with the rest of animate nature. If these reasons are lacking, and the unity of organic nature be recognized, human evolution cannot be denied unless some interpretation more reasonable and logical than evolution can be given for the whole mass of facts exemplified and discussed in the foregoing chapters. We may accordingly approach the main questions by asking if there are any reasons for regarding the human species as a unique and isolated type of organism.

At the outset, we must recognize that in so far as the human body is material, its movements and mass relations are controlled by physical principles, like all other masses of matter. It is well, indeed, that this is so, for if gravitation and the laws of inertia were not consistent and reliable principles holding true at all times and not intermittently, it would be difficult to order our lives with confidence. In the next place, the general principles of biology hold true for the structure and physiology of the human species as they do for all other living things. A human body is composed of eight systems of organs, whose functions are identical with the eight vital tasks of every other animal. All these organs are made up of cells as ultimate vital units, and the materials of which human cells are composed belong to the class of substances called protoplasm. Human protoplasm, like all other living materials, must replenish itself, and respire and oxidize in obedience to biological laws that have been found to be uniform everywhere. Thus the human organism is no more unique in fundamental organic respects than it is apart from the world of physical processes and laws.

How does the matter stand when the general structural plan of a human being is examined? Is it entirely different from everything else? It is a fact of common knowledge that the human body is supported by a bony axis, the vertebral column, to which the skull is articulated and to which also the skeletal framework of the limbs is attached. These characteristics place man inevitably among the so-called vertebrata; he is certainly not an invertebrate, nor is the basic structure of his body such that a third group, outside the invertebrata and vertebrata, can be made to include only the single type—man.

Passing now to the classes that make up the group of vertebrates, we meet first the lampreys or cyclostomes without jaws, and the others with jaws, such as the fishes, amphibia, reptiles, birds, and mammals, each class distinguished by certain definite characters in addition to the vertebral column. The fishes have gills and scales; amphibia of to-day are scaleless, and they are provided with gills when they are young and lungs as adults; reptiles have scales and lungs; birds are warm-blooded and feathered; while mammals are warm-blooded and haired. Is the human species a unique kind of vertebrate, or does it find a place in one of these classes? The occurrence of hair, of a four-chambered heart which propels warm blood, of mammary glands, and of other systematic characters marks this species as a kind of mammal and not as a vertebrate in a section by itself.

The members of the class mammalia differ much among themselves; and now that we recognize clearly that man is a mammalian vertebrate, the next question is whether an order exists to which our type must be assigned, or whether we have at last reached a point where it is justifiable to establish an isolated division to contain the human species alone. We are familiar with many representatives of different mammalian orders and with the kind of structural characteristics that serve as convenient distinctions in denoting their relationships. Horses and cattle, sheep, and goats and pigs resemble one another in many respects besides their hoofs, and they form one natural order; the well-developed gnawing teeth of rats and rabbits and squirrels place these forms together in the order rodentia; the structures adapting their possessors for a flesh-eating and predatory life unite the tribes of the lion, wolf, bear, and seal, in the order carnivora. Among these and other orders of mammalia is one to which the lemurs, monkeys, and apes are assigned, because all these forms agree in certain structural respects that place them apart from the other mammalia, in the same way, for example, that the races of white men may be recognized as a group distinct from the black and red races. But comparative studies, prosecuted not only by those who have been forced to adopt the evolutionary interpretation, but also by believers in special creation like Linnæus and Cuvier and other more modern opponents of evolution, have shown that the peculiar qualities of this order are shared by the human species. Indeed, the name of primates was given to this section by Linnæus himself, because the human body found a place in the array which begins at the lower extreme with the lemurs and the monkeys and ends with man at the other end. Again it is found that no separate order of mammals exists to include only the genus Homo.

To one unacquainted with the facts of vertebrate comparative anatomy, the distinguishing characteristics of the primates seem to be trivial in nature. It is surprising to find how insignificant are the details to which appeal must be made in order to draw a line between our own division of mammalia and the others. It is well to review them as they are given in the standard text-books of comparative anatomy. Primates are eutheria, or true mammalia possessing a placental attachment of the young within the parent. The first digits, namely, the "great toe" and the "thumb," are freely movable and opposable to the others, so that the limbs are prehensile and clasping structures; usually but not always the animals of this order are tree-dwellers in correlation with the grasping powers of the feet and hands. The permanent teeth succeed a shorter series of so-called "milk teeth," and they are diverse in structure, being incisors, canines, or "eye teeth," premolars, and molars; the particular numbers of each kind are almost invariable throughout the order and markedly different from those of other orders. The number of digits is always five, and with few exceptions they bear nails instead of claws. The clavicles, or "collar bones," are well developed in correlation with the prehensile nature of the fore limbs; a bony ring surrounds the orbit or eye socket. Finally there are two mammary glands by which the young are suckled. It is because any other details of difference between man and other forms are far less marked than the agreements in these respects, that the human species must be regarded as a primate mammalian vertebrate.

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The comparative study of the human organism as a structural type has now been narrowed down to a review of the various members of the order of primates. It is the duty of science to arrange these organisms according to the minor differences beneath the agreements in major qualities, and to show how they are related in an order of evolution. It will appear, when this is done, that the supreme place is given to the human species on account of four and only four characteristics; these are (1) an entirely erect posture, (2) greater brain development, (3) the power of articulate speech, and (4) the power of reason. As we are treating the human body as a subject for comparative structural study, the third and fourth characters do not concern us here; but it is well to point out that they depend entirely upon the second, and that they are the functional concomitants of the improved type of brain belonging to the highest type. Two characters remain, and in both cases it is significant that differences in degree only are to be found by even the closest analysis. The human brain is the same kind of brain that lower primates possess; its structure is unique in no general respect. And as regards the first-mentioned character, comparative anatomy shows, in the first place, that this also is something differing only in degree, and in the second place, that it is due directly to the development of the brain. For these reasons a survey of the various members of the order of primates must deal largely with the progressive elaboration of the brain and the entailed effects of this enlargement.

The order of primates is subdivided as follows :—

Sub-order 1. PROSIMII. Lemurs.
Sub-order 2. ANTHROPOIDEA.
Family 1. Hapalidæ. The marmosets.
Family 2. Cebidæ. The American or tailed monkeys.
Family 3. Cercopithecidæ. The baboons.
Family 4. Simiidæ. The true apes.
Family 5. Hominidæ. The human species. Primates

Each one of these subdivisions is interesting in its own way, either because its members depart from the typical condition of the whole order in some respects, or because of some character that foreshadows and leads to a more developed element of the animals placed in the higher sections.

The lemurs are small animals very much like squirrels in their general form and in their tree-climbing habits. They live now almost exclusively on the island of Madagascar, but palæontology shows that they were more widely spread at an earlier time. Their teeth are exactly like our own, except that there is one more premolar on each side of each jaw. The "fingers" and "toes" bear nails like ours, again with an exception in the case of the second digits of the hind limbs, which bear claws. The details of structure that set these animals apart from all the rest of the primates are too small to deserve comment in the present connection.

Passing to the true anthropoids, or man-like primates and man himself, the first forms encountered are the little marmosets, which are like the lemurs in some ways, but in other respects they resemble the familiar tailed monkeys. They are peculiar in having three premolars and two molars on either side of both upper and lower jaws, and also in the fact that the "thumb" is not opposable to the other fingers, while all the digits except the "great toes" bear claws instead of manlike nails. The proportion of brain-case and face does not differ much from that in the lemurs and even lower forms like cats, for the brain has not increased greatly in total mass, though the cerebrum is more convoluted than in the lower forms.