THE EVOLUTION OF LIFE
Inorganic and organic evolution—Biogenesis and cosmogenesis—Mechanical evolution—Mechanics of phylogenesis—Theory of selection—Theory of idioplasm—Phyletic vital force—Theory of germ-plasm—Progressive heredity—Comparative morphology—Germ-plasm and hereditary matter—Theory of mutation—Zoological and botanical transformism—Neo-Lamarckism and Neo-Darwinism—Mechanics of ontogenesis—Biogenetic law—Tectogenetic ontogeny—Experimental evolution—Monism and biogeny.
I fully explained in my General Morphology (1866) the profound importance of the science of evolution in relation to our monistic philosophy. A popular synopsis of this is given in my History of Creation, and is briefly repeated in the thirteenth chapter of the Riddle. I must refer the reader to these works, especially the latter, and confine myself here to a consideration of some of the principal general questions of evolution in the light of modern science. The first thing to do is to compare the conflicting views on the nature and significance of biogenesis which still face each other at the beginning of the twentieth century.
The essential unity of inorganic and organic nature, which I endeavored to establish in the second book of the General Morphology, and the significance of which I explained in the fourteenth chapter of the Riddle, is found through the whole course of its development, in the causes of phenomena and their laws. Hence, in dealing with the evolution of organisms, we reject vitalism and dualism, and maintain our conviction that it can always be traced to physical forces (and especially chemical energy). As we regard plasm as the basis of it (chapter vi.), we may say that organic evolution depends on the mechanics and chemistry of the plasm. We postulate no supernatural vital force for the explanation of physiological functions, and we are just as far from admitting it as regulator or agency of the biogenetic process.
If we understand by biogeny the sum total of the organic evolutionary processes on our planet, by geogeny the processes at work in the formation of the earth itself, and by cosmogony those that produced the whole world, biogeny is clearly only a small part of geogeny, and this in turn only a small section of the vast science of cosmogony. This important relation is evident enough, yet often overlooked; it holds both of time and space. Even if we suppose that the biogenetic process occupied more than a hundred million years, this period is probably much shorter than that which our planet has needed for its development as a cosmic body—from the first detachment of the nebular ring from the shrinking body of the sun to its condensation into a rotating sphere of gas, and from this to the formation of the incandescent globe, the stiffening of the crust at its surface, and finally the downpour of fluid water. It was not until this last stage that carbon could begin its organogenetic activity and proceed to the formation of plasm. But even this long geogenetic process is, as regards space and time, only a very small part of the illimitable history of the world. If we further assume that organic life develops on other cosmic bodies (Riddle, chapter xx.) in the same way as on our earth under like conditions, the whole sum of all these biogenetic processes is only a small part of the all-embracing cosmogenetic process. The vitalistic belief that its mechanical course was interrupted from time to time by the supernatural creation of organisms is opposed to pure reason, the unity of nature, and the law of substance. We must, therefore, hold fast above all to the conviction that all biogenetic processes are just as reducible to the mechanics of substance as all other natural phenomena.
The mechanical and natural character of the development of inorganic nature, the earth and the whole material world, was established mathematically at the end of the eighteenth century by the great atheist Laplace in his Mécanique Céleste (1799). The similar cosmogony which Kant had expounded in 1755 in his General Natural History and Theory of the Heavens only obtained recognition at a later date (Riddle, chapter xiii.). But the possibility of giving a mechanical explanation of organic nature was not seen until Darwin provided a solid foundation for the theory of descent by his theory of selection in 1859. I made the first comprehensive attempt to do this in 1866 in my General Morphology, the aim of which is expressed in the title: "General outlines of the science of organic forms, mechanically grounded on Darwin's improvement of the theory of descent." Especially in the second volume of the work, the "General Evolution of Organisms," I endeavored to show that both sections of the science, ontogeny (or embryology) and phylogeny, can be reduced to physiological activities of the plasm, and so explained mechanically, in the wider meaning of the word.
When I stated the nature and the aim of phylogeny in 1866, most biologists regarded my attempt as unjustifiable, as they did Darwinism itself, of which it was a natural consequence. Even the famous Émil Dubois-Reymond, to whom as a physiologist it should have been welcome, described it as "a poor romance"; he compared my first attempts to construct the genealogical tree of the organic classes, on the evidence of paleontology, comparative anatomy, and ontogeny, to the hypothetical labors of philologists to draw up the genealogical tree of the legendary Homeric heroes. As a matter of fact, I had myself described my imperfect effort as merely a provisional sketch, as a temporary hypothesis that would open the way for later and better research. A single glance at the immense literature of phylogeny to-day shows how much has been done since in this province, and how far we have advanced in the establishment of the features of evolution by means of the united labors of numbers of able paleontologists, anatomists, and embryologists. Ten years ago I attempted, in the three volumes of my Systematic Phylogeny, to give a comprehensive statement of the results attained. My chief aim was, on the one hand, to construct a natural system of organisms on the basis of their ancestral history, and on the other hand to prove the mechanical character of the phylogenetic process. All the activities of organisms which are at work in the transformation of species and the production of new ones in the struggle for existence may be reduced to their physiological functions—to growth, nutrition, adaptation, and heredity; and these again to the mechanics and chemistry of the plasm. The struggle for life is itself a mechanical process, in which natural selection uses the disproportion between the excess of germs and the restricted means of existence, in conjunction with the variability of species, in order to produce new purposive structures mechanically and without any preconceived design. This teleological mechanicism has no need of a mysterious design or finality; it takes its place in the general order of mechanical causality which controls all the processes in the universe. Natural finality is only a special instance of mechanical causality. The one is subordinate to the other, not opposed to it, as Kant would have it.
The effort that the great Lamarck made in 1809, in his Philosophie Zoologique, to establish transformism deserves high appreciation from monists, because it was the first attempt to give a natural explanation of the origin of the countless species of organic forms which inhabit our planet. Up to that time it had been the fashion to attribute their origin to a miraculous intervention of the Creator. This metaphysical creationism had now to face physical evolutionism. Lamarck explained the gradual formation of organic species by the interaction of two physiological functions—adaptation and heredity. Adaptation consists in the improvement of organs by use, and degeneration by disuse; heredity acts by transmitting the features thus acquired to posterity. New species arise by physiological transformation from older species. The fact that this great thought was overlooked for half a century does not detract from its profound significance. But it only obtained general recognition when Darwin had supplemented it and filled up its causal gaps by the theory of selection in 1859. Apart from this specifically Darwinian feature (whether it be true or not), the fundamental idea of transformism is now generally received; it is admitted to-day even by metaphysicians who maintained a spirited opposition to it thirty years ago. The fact of the progressive modification of species is only intelligible on Lamarck's theory that the actual species are the transformed descendants of older species. In spite of all the learning and zeal with which the theory has been attacked, it has proved irrefutable; nor can any one suggest a better theory to replace it. This may be said particularly of its chief consequence—the descent of man from a series of other mammals (proximately from the apes).
The high value of Darwin's theory of selection for the monistic biology is now acknowledged by all competent and impartial authorities on the science. In the course of the forty-four years since it found its way into every branch of biology, it has been employed in more than a hundred large works and several thousand essays in explaining biological phenomena. This alone is enough to show its profound importance. Hence it is mere ignorance of the subject and its literature to say, as has been done several times of late, that Darwinism is in decay, or even "dead and buried." However, absurd writings of this kind (such as Dennert's At the Death-bed of Darwinism) have a certain practical influence, because they fall in with the prevailing superstition in theology and metaphysics. Unfortunately, they also seem to obtain notice from the circumstance that a few botanists persistently attack the Darwinian theory. One of the most conspicuous of these is Hans Driesch, who affirms that all Darwinists (and therefore the great majority of modern biologists) have softening of the brain, and that Darwinism is (like Hegel's philosophy) the delusion of a generation. The arrogance of this conceited writer is about equal to the obscurity of his biological opinions, the confusion of which is covered by a series of most extravagant metaphysical speculations. All these attacks have lately been met very ably by Plate in his work, On the Significance of the Darwinian Principle of Selection and the Problem of the Foundation of Species (second edition, 1903). The most thorough of recent defences of Darwinism is that made by August Weismann in his Lectures on the Theory of Descent (1902) and other works. But the distinguished zoologist goes too far when he seeks to prove the omnipotence of selection and wishes to ground it on an untenable molecular hypothesis—the theory of germ-plasm, which we will consider presently. Apart from these or other exaggerations, we may say with Weismann that Lamarck's theory of descent received a sound causal basis by Darwin's theory of selection. Its real foundations are these three phenomena: heredity, adaptation, and the struggle for existence. All three are, as I have often said, of a purely mechanical and not a teleological nature. Heredity is closely bound up with the physiological function of reproduction, and adaptation with nutrition; the struggle for life follows logically and mathematically from the disproportion between the number of potential individuals (germs) and of actual individuals that grow to maturity and propagate the species.
When I had, in my General Morphology, endeavored to gain acceptance for Darwin's theory of selection, and had presented evolution as a comprehensive theory from the point of view of the monistic philosophy, a number of works, sometimes of value, appeared, which made special studies of the various parts of the immense province. Eighteen years afterwards a greater work was published, which started from the same monistic principles, but reached the same conclusion by a different way. In 1884 Carl Nägeli, one of our ablest and most philosophic botanists, issued his Mechanical-physiological Theory of Evolution. This interesting book consists of various parts. It is especially notable that evolution is presented in it as the one possible and natural theory of the origin of species; even morphology and classification are treated explicitly as "phylogenetic sciences." The chapter on archigony—a dark and dangerous problem that is generally avoided by scientists!—is one of the best that has been written on the subject. On the other hand, Nägeli rejects Darwin's theory of selection altogether, and would explain the origin of species by an inner "definitely directed variation," independently of the conditions of existence in the outer world. As Weismann has properly observed, this internal principle of evolution, which dispenses with adaptation in the true sense of the word, is at the bottom merely a "phyletic vital force." It is not made more acceptable by Nägeli when he builds up a subtle metaphysical system on it and postulates a special "principle of isagitation." But the idioplasm theory he connects with it is of some value, since it goes more fully into the differentiation of the cell-plasm into two physiologically different parts—the idioplasm of the hereditary matter and the trophoplasm as nutritive matter of the cell.
The vitalist and teleological idea of an internal principle of evolution, that determines the origin of animal and plant species independently of the environment and its conditions, is not only found in the "mechanical-physiological" theory of Nägeli, but also in several other attempts to explain the agencies of the transformation of species. All these efforts are welcomed by the academic philosophers with their Kantist dualism (mechanicism on the right, teleology on the left), and who are particularly anxious to save the supernatural element, Reinke's "cosmic intelligence," or the wisdom of the Creator, or the divine creative thought. All these dualistic and teleological efforts have the same fault: they overlook, or fail to appreciate properly, the immense influence of the environment on the shaping and modification of organisms. When, moreover, they deny progressive heredity and its connection with functional adaptation, they lose the chief factor in transformation. This applies also to the theory of germ-plasm.
The desire to penetrate deeper into the mysterious processes that take place in the plasm in the physiological activities of heredity and adaptation has led to the formulation of a number of molecular theories. The chief of these are the pangenesis theory of Darwin (1878), my own perigenesis theory (1876), the idioplasm theory of Nägeli (1884), the germ-plasm theory of Weismann (1885), the mutation theory of De Bries, etc. As I have already dealt with these in the sixth chapter (as well as in the ninth chapter of the History of Creation), I may refer the reader thereto. None of these or similar attempts has completely solved the very difficult problems in question, and none of them has been generally received. There is, however, one of them that we must consider more closely, because it is not only regarded by many biologists as the greatest advance of the theory of selection since Darwin, but it also touches the roots of several of the chief problems of biogeny. I mean the much-discussed germ-plasm theory of August Weismann (of Freiburg), one of our most distinguished zoologists. He has not only promoted the theory of descent by his many writings during the last thirty years, but has also put in its proper light the great importance and entire accuracy of the theory of selection. But, in his efforts to provide a molecular-physiological basis for it, he has proceeded by way of metaphysical speculation to frame a quite untenable theory of the plasm. While fully recognizing the ability and consistency and the able treatment which Weismann has shown, I am compelled once more to dissent from him. His ideas have recently been completely refuted by Max Kassowitz (1902) in his General Biology, and Ludwig Plate in the work I mentioned on the Darwinian principle of selection. We need not go into the details of the complicated hypothesis as to the molecular structure of the plasm which Weismann has framed in support of his theory of heredity—his theory of biophora, determinants, ideas, etc.—because they have no theoretical basis and are of no practical use. But we must pass some criticism on one of their chief consequences. In the interest of his complicated hypotheses, Weismann denies one of Lamarck's most important principles of transmutation—namely, the inheritance of acquired characters.
When I made the first attempt in 1866 to formulate the phenomena of heredity and adaptation in definite laws and arrange these in series, I drew a distinction between conservative and progressive heredity (chapter ix., History of Creation). Conservative heredity, or the inheritance of inherited characters, transmits to posterity the morphological and physiological features which each individual has received from his parents. Progressive heredity, or the inheritance of acquired characters, transmits to offspring a part of those features which were acquired by the parents in the course of their individual lives. The chief of these are the characters that are caused by the activity of the organs themselves. Increase in the use of the organs causes a greater access of nourishment and promotes their growth; decrease in the exercise of organs has the contrary effect. We have examples at hand in the modification of the muscles or the eyes, the action of the hand or throat in painting or singing, and so on. In these and all the arts the rule is: Practice makes perfect. But this applies almost universally to the physiological activity of the plasm, even its highest and most astounding function—thought; the memory and reasoning capacity of the phronema are improved by constant exercise of the cells which compose this organ, just as we find in the case of the hands and the senses.
Lamarck recognized the great morphological significance of this physiological use of the organs, and did not doubt that the modification caused was transmitted to offspring to a certain extent. When I dealt with this correlation of direct adaptation and progressive heredity in 1866, I laid special stress on the "law of cumulative adaptation" (General Morphology, ii., p. 208). "All organisms undergo important and permanent (chemical, morphological, and physiological) changes when acted on by a change in its life-conditions, slight in itself, but continuing for a long time or being frequently repeated." At the same time I pointed out that in this case two groups of phenomena are closely connected which are often separated—namely, cumulative heredity: firstly external, by the action of the external conditions (food, climate, environment, etc.), and secondly internal, by the reaction of the organism, the influence of internal conditions (habit, use and disuse of organs, etc.). The action of outer influences (light, heat, electricity, pressure, etc.) not only causes a reaction of the organism affected (energy of movement, sensation, chemosis, etc.), but it has an especial effect as a trophic stimulus on its nutrition and growth. The latter element has been particularly studied by Wilhelm Roux; his functional adaptation (1881) coincides with my cumulative adaptation, the close relation of which to correlative adaptation I had pointed out in 1866. Plate has recently given this "definitely directed variation" the name of ectogenetic orthogenesis, or, briefly, ectogenesis.
The controversy about progressive heredity still continues here and there. Weismann completely denies it, because he cannot bring it into harmony with his germ-plasm theory, and because he thinks there are no experimental proofs in support of it. A number of able biologists agree with him, led away by his brilliant argumentation. However, many of them foolishly lay great stress on experiments in heredity which prove nothing; for instance, the fact that the offspring of a mammal that has had its tail cut off do not inherit the feature. A number of recent observations seem to prove that in a few cases even defects of this sort (when they have caused profound and lasting disease of the part affected) may be transmitted to offspring. However, as far as the formation of new species is concerned, the fact is of no consequence; in this it is a question of cumulative or functional adaptation. Experimental proofs of this are difficult to find, if one wants a strict demonstration of the type of physical experiments; the biological conditions are generally too complicated and offer too many weak points to rigorous criticism. The beautiful experiments of Standfuss and C. Fisher (Zurich) have shown that changes in the environment (such as temperature or food) can cause striking modifications that are transmitted to offspring. In any case, there are plenty of luminous proofs of progressive heredity in the vast arsenal of morphology, comparative anatomy, and ontogeny.
Comparative anatomy affords a number of most valuable arguments for other phylogenetic questions as well as progressive heredity; and the same may be said of comparative anatomy and comparative ontogeny. I have collected and illustrated a good many of these proofs in the new edition of my Anthropogeny. However, in order to understand and appreciate them aright, the reader must have some acquaintance with the methods of critical comparison. This means not only an extensive knowledge of anatomy, ontogeny, and classification, but also practice in morphological thinking and reasoning. Many of our modern biologists lack these qualifications, especially those "exact" observers who erroneously imagine they can understand vast groups of phenomena by accurate description of detailed microscopic structures, etc. Many distinguished cytologists, histologists, and embryologists have completely lost the larger view of their work by absorption in these details. They even reject some of the fundamental ideas of comparative anatomy, such as the distinction between homology and analogy; Wilhelm His, for instance, declared that these "academic ideas" are "unreliable tools." On the other hand, physiological experiments ought to contribute to the solution of morphological problems, and of these they can say nothing. To show the incalculable value of comparative anatomy for phylogeny, I need only point to one of its most successful departments, the skeleton of the vertebrates, the comparison of the various forms of the skull, the vertebral column, the limbs, etc. It is not in vain that for more than a hundred years gifted scientists, from Goethe and Cuvier to Huxley and Gegenbaur, have devoted years of laborious research to the methodical comparison of these similar yet dissimilar forms. They have been rewarded by the discovery of the common laws of structure, which can only be explained in the sense of modern evolution by descent from common ancestors.
We have a striking example of this in the limbs of mammals, which, with the same internal skeletal structure, show a very great variety in outer form—the slender bones of the running carnivora and ungulates, the oar-bones of the whale and seal, the shovel-bones of the mole and hypudæus, the wings of the bat, the climbing bones of the ape, and the differentiated limbs of the human body. All these different skeletal forms have descended from the same common stem-form of the oldest Triassic mammals; their various forms and structures are adapted in scores of ways to different functions; but they rise through these functions, and all these functional adaptations can only be understood by progressive heredity. The theory of germ-plasm gives no causal explanation whatever of them.
The majority of recent biologists are of opinion that of the two chief constituents of the nucleated cell the cytoplasm of the cell-body discharges the function of nutrition and adaptation, while the caryoplasm of the nucleus accomplishes reproduction and heredity. I first advanced this view in the ninth chapter of the General Morphology (in 1866); and it was afterwards solidly and empirically established by the excellent investigations of Eduard Strasburger, the brothers Oscar and Richard Hertwig, and others. The elaborate finer structures which these observers discovered in cell-division led to the theory that the colorable part of the nucleus, chromatin, is the real hereditary matter, or the material substratum of the energy of heredity. Weismann added the theory that this germ-plasm lives quite separately from the other substances in the cell, and that the latter (the soma-plasm) cannot transmit to the germ-plasm the characters it has acquired by adaptation. It is on the strength of this theory that he opposes progressive heredity. The representatives of the latter (including myself) do not accept this absolute separation of germ-plasm from body-plasm; we believe that even in the process of cell-division in the unicellular organism there is partial blending of the two kinds of plasm (caryolysis), and that in the multicellular organism of the histona also the harmonious connection of all the cells by their plasma-fibres makes it possible enough for all the cells in the body to act on the germ-plasm of the germ-cells. Max Kassowitz has shown how we can explain this influence by the molecular structure of the plasm.
At the beginning of the twentieth century a new biological theory aroused a good deal of interest, and was welcomed by some as an experimental refutation of Darwin's theory of selection and by others as a valuable supplement to it. The distinguished botanist Hugo de Bries (of Amsterdam) gave an interesting lecture at the scientific congress at Hamburg in 1901 on "The Mutations and Mutation-periods in the Origin of Species." Supported by many years of experiments in selection and some ingenious speculations, he thinks he has discovered a new method of the transformation of species, an abrupt modification of the specific form at a bound, and so discredited Darwin's theory of their gradual change through long periods of time. In a large work on Experiments and Observations on the Origin of Species in the Plant Kingdom (1903), De Bries has endeavored to demonstrate the truth of his theory of mutation. The warm approval which it won from a number of eminent botanists, and especially vegetal physiologists, was not shared by zoologists. Of these Weismann, in his Lectures on the Theory of Descent (1902, ii. p. 358), and Plate in his Problems of Species-formation (1903, p. 174), have dealt fully with the theory of mutation, and, while appreciating the interesting observations and experiments of De Bries, have rejected the theory he has built on them. As I share their opinion, I may refer the reader who is interested in these difficult problems to their works, and will restrict myself here to the following observations. The chief weakness of the theory of mutation of De Bries is on its logical side, in his dogmatic distinction between species and variety, mutation and variation. When he holds the constancy of species as a fundamental "fact of observation," we can only say that this (relative) permanence of species is very different in the different classes. In many classes (for instance, insects, birds, many orchids and graminea) we may examine thousands of specimens of a species without finding any individual differences; in other classes (such as sponges, corals, in the genera rubus and hieracium) the variability is so great that classifiers hesitate to draw up fixed species. The marked difference between various forms of variability which De Bries alleges cannot be carried through; the fluctuating variations (which he takes to be unimportant) cannot be sharply distinguished from the abrupt mutations (from which new species are supposed to result at a bound). De Bries's mutations (which I distinguished in the General Morphology as "monstrous changes" from other kinds of variation) must not be confused with the paleontological mutations of Waagen (1869) and Scott (1894) which have the same name. The sudden and striking changes of habit which De Bries observed only in one single species of œnothera very rarely occur, and cannot be regarded as common beginnings of the formation of new species. It is a curious freak of chance that this species bears the name œnothera Lamarckiana; the views of the great Lamarck on the powerful influence of functional adaptation have not been refuted by De Bries. It must be carefully noted, in fact, that De Bries is firmly convinced of the truth of Lamarck's theory of descent, like all competent modern biologists. This must be well understood, because recent metaphysicians see in the supposed refutation of Darwinism the death of the whole theory of transformism and evolution. When they appeal in this sense to its most virulent opponents, Dennert, Driesch, and Fleischmann, we may remind them that the curious sermons of these minor sophists are no longer noticed by any competent and informed scientist.
Not only in the brilliant speculations of De Bries and Nägeli, but also in many other botanical works that have lately attempted to advance the theory of descent, we find a striking difference from the prevailing views of zoologists in the treatment of a number of general biological problems. This difference is, of course, not due to a disproportion of ability in the two great and neighboring camps of biology, but to the differences in the phenomena that we observe in plant life on the one hand and animal life on the other. It must be noted particularly that the organism of the higher animals (including our own) is much more elaborately differentiated in its various organs and much more exposed to our direct experience than that of the higher plants. The chief properties and activities of our muscles, skeleton, nerves, and sense-organs, are understood at once in comparative anatomy and physiology. The study of the corresponding phenomena in the bodies of the higher plants is much more difficult. The features of the innumerable elementary organs in the cell-monarchy of the animal body are much more intricate, yet at the same time much more intelligible, than those of the cell-republic of the higher plant-body. Thus the phylogeny of the plants encounters much greater difficulties than that of the animals; the embryology of the former says much less in detail than that of the latter. We can understand, therefore, why the biogenetic law is not so generally recognized by botanists as by zoologists. Paleontology, which provides such valuable fossil material for many groups of the animal kingdom that we can more or less correctly draw up their ancestral tree on the strength of this, gives us very little for most groups of the plant kingdom. On the other hand, the large and sharply demarcated plant-cell, with its various organella, is much more valuable in connection with many problems than the tiny animal-cell. For many physiological purposes, in fact, the higher plant body is more accessible to exact physical and chemical research than the higher animal body. The antithesis is less in the kingdom of the protists, as the difference between animal and vegetal life is mostly confined to difference of metabolism, and finally disappears altogether in the province of the unicellular forms of life. Hence, for a clear and impartial treatment of the great problems of biology, and especially of phylogeny, it is imperative to have a knowledge of both zoological and botanical investigation. The two great founders of the theory of descent—Lamarck and Darwin—were able to penetrate so deeply into the mysteries of organic life and its development because they had extensive attainments both in botany and zoology.
Of the various tendencies that have recently made their appearance among zoologists and botanists in the discussion of the theory of descent, we frequently find Neo-Lamarckism and Neo-Darwinism distinguished as opposing schools. This opposition has no meaning unless we understand by it the alternatives of transformism—with or without the theory of selection. The one principle that distinguishes Darwinism proper from the older Lamarckism is the struggle for existence and the theory of selection based on it. It is quite wrong to make the test an acceptance or rejection of progressive heredity. Darwin was just as firmly convinced as Lamarck or myself of the great importance of the inheritance of acquired characters, and particularly of the inheritance of functional adaptations; he merely ascribed to it a more restricted sphere of influence than Lamarck. Weismann, however, denies progressive heredity altogether, and wants to trace everything to "the omnipotence of natural selection." If this view of Weismann and the theory of germ-plasm he has based on it are correct, he alone has the honor of founding a totally new (and in his opinion very fruitful) form of transformism. But it is quite wrong to describe this Weismannism as Neo-Darwinism, as frequently happens in England. It is just as wrong to call Nägeli, De Bries, and other modern biologists who reject selection Neo-Lamarckists.
If the theory of descent is right, as all competent biologists now admit, it puts on morphology the task of assigning approximately the origin of each living form. It must endeavor to explain the actual organization of each by its past, and to recognize the causes of its modification in the series of its ancestors. I made the first attempt to achieve this difficult task in founding stem-history or phylogeny as an independent historical science in my "General Evolution" (in the second volume of the General Morphology). With it I associated as a second and equally sound part ontogeny; I understood by this the whole science of the development of the individual, both embryology and metamorphology. Ontogeny enjoys the privileges (especially in the way of certainty) of a purely descriptive science, when it confines itself to the faithful description of the directly observed facts, either the embryonic processes in the womb or the later metamorphic processes. The task of phylogeny is much more difficult, as it has to decipher long-past processes by means of imperfect evidence, and has to use its documents with the utmost prudence.
The three most valuable sources of evidence in phylogeny are paleontology, comparative anatomy, and ontogeny. Paleontology seems to be the most reliable source, as it gives us tangible facts in the fossils which bear witness to the succession of species in the long history of organic life. Unfortunately, our knowledge of the fossils is very scanty and often very imperfect. Hence the numerous gaps in its positive evidence have to be filled up by the results of two other sciences, comparative anatomy and ontogeny. I have dealt fully with this in my Anthropogeny. As I have also spoken of the general features of these phyletic evidences in the sixteenth chapter of the History of Creation, I need do no more here than repeat that it is necessary to make equal and discriminating use of all three classes of documents if we are to attain the aim of phylogeny correctly. Unfortunately, this necessitates a thorough knowledge of all three sciences, and this is very rare. Most embryologists neglect paleontology, most paleontologists embryology, while comparative anatomy, the most difficult part of morphology, involving most extensive knowledge and sound judgment, is neglected by both. Besides these three sources of phylogeny there is valuable proof afforded by every branch of biology, especially by chorology, œcology, physiology, and biochemistry.
Although there has been very extensive phylogenetic research during the last thirty years, and it has yielded a number of interesting results, many scientists still seem to look on them with a certain distrust; some contest their scientific value altogether, and say that they are nothing but airy and untenable speculations. This is especially the case with many physiologists who look upon experiment as the only exact method of investigation, and many embryologists who think their sole task is description. In view of these sceptical strictures, we may recall the history and the nature of geology. No one now questions the great importance and the various uses of this science, although in it there is no possibility of directly observing the historical processes as a rule. No scientist now doubts that the three vast successive formations of the Mesozoic Period—the Triassic, Jurassic, and Cretaceous—have been formed from sea-deposits (lime, sandstone, and clay), though no one was a witness to the actual formation; no one doubts to-day that the fossil skeletons of fishes and reptiles which we find in these groups are not mysterious freaks of nature, but the remains of extinct fishes and reptiles that lived on the earth during those millions of years long ago. And when comparative anatomy shows us the genealogical connection of these related forms, and phylogeny (with the aid of ontogeny) constructs their ancestral trees, their historical hypotheses are just as sound and reliable as those of geology; the only difference is that the latter are much simpler, and thus easier to construct. Phylogeny and geology are, in the nature of the case, historical sciences.
Hypotheses are necessary in phylogeny and geology, where the empirical evidence is incomplete, as in every other historical science. It is no detraction from the value of these to urge that they are sometimes weak and have to be replaced by better and stronger ones. A weak hypothesis is always better than none. We must, therefore, protest against the foolish dread of hypotheses which is urged against our phylogenetic methods by the representatives of the exact and descriptive sciences. This shrinking from hypotheses often hides a defective knowledge of other sciences, an incapacity for synthetic thought, and a feeble sense of causality. The delusions into which it leads many scientists may be seen from the fact that chemistry, for instance, is reckoned an "exact" science; yet no chemist has ever seen the atoms and molecules of compounds with which he is occupied daily, or the complicated relations on the assumption of which the whole of modern structural chemistry is based. All these hypotheses rest on inferences, not on direct observation.
I have, from the first, insisted on the close causal connection between ontogeny and phylogeny, ever since I distinguished these two parts of biogeny in the fifth book of the General Morphology. I also laid stress on the mechanical character of these sciences, and endeavored to give a physiological explanation of their morphological phenomena. Until then embryology had been regarded as a purely descriptive science. Carl Ernst Baer, who had provided a solid foundation for it in his classic Animal Embryology (1828), was convinced that all the phenomena of individual development might be reduced to the laws of growth; but he was quite unconscious of the real direction of this growth, its "purposiveness," the real causes of construction. The distinguished Würtzburg anatomist, Albert Kölliker, whose Manual of Human Embryology (1859) gave the first comprehensive treatment of the science from the cellular point of view, adhered, even in the fourth edition (1884), to the opinion that "the laws of the development of the organism are still completely unknown." In opposition to this generally received opinion, I endeavored, in 1866, to prove that Darwin had, by his improvement of the theory of descent, not only solved the phylogenetic problem of the origin of species, but, at the same time, given us the key to open the closed doors of embryology, and to learn the causes of the ontogenetic processes as well. I formulated this view in the twentieth chapter of the General Morphology, in forty-four theses, of which I will quote only the following three: 1. The development of organisms is a physiological process, depending on mechanical causes, or physico-chemical movements. 40. Ontogenesis, or the development of the organic individual, is directly determined by phylogenesis, or the evolution of the organic stem (phylon) to which it belongs. 41. Ontogenesis is a brief and rapid recapitulation of phylogenesis, determined by the physiological functions of heredity and adaptation. The pith of my biogenetic principle is expressed in these and the remaining theses on the causal nexus of biontic and phyletic development. At the same time I make it quite clear that I reduce the physical process of ontogenesis, and also phylogenesis, to a pure mechanics of the plasm (in the sense of the critical philosophy).
The comprehensive fundamental law of organic development was briefly formulated by me in the fifth book of the General Morphology and in the tenth chapter of the History of Creation (developed more fully in the fourteenth chapter of the tenth edition, 1902). I afterwards sought to establish it securely in two different ways. In the first place, I proved in my Studies of the Gastræa Theory (1872-1877) that in all the tissue-animals, from the lowest sponges and polyps to the highest articulata and vertebrates, the multicellular organism develops from the same primitive embryonic form (the gastrula), and that this is the ontogenetic repetition, in virtue of heredity, of a corresponding stem-form (the gastræa). In the second place, I made the first attempt in my Anthropogeny (1874) to illustrate this recapitulation theory from the instance of our own human organism, by trying to explain the complex process of individual development, for the whole frame and every single part of it, by causal connection with the stem-history of our animal ancestors. In the latest edition of this monistic "ontogeny of man" I gave numbers of illustrations (thirty plates and five hundred engravings) of these intricate structures, and endeavored to make the subject still plainer by the addition of sixty genetic tables. I may refer the reader to this work,[10] and not dwell any further here on the biogenetic law, especially as one of my pupils, Heinrich Schmidt (of Jena), has recently described its biological significance and its earlier history and present position in a very clear and reliable little work (Haeckel's Biogenetic Law and its Critics). I will only add a word or two on the struggle that has taken place for thirty years over the complete or partial recognition of the biogenetic law, its empirical establishment, and its philosophic application.
In the very name, "fundamental law of biogeny," which I have given to my recapitulation theory, I claim that it is universal. Every organism, from the unicellular protists to the cryptogams and cœlenteria, and from these up to the flowering plants and vertebrates, reproduces in its individual development, in virtue of certain hereditary processes, a part of its ancestral history. The very word "recapitulation" implies a partial and abbreviated repetition of the course of the original phyletic development, determined by the "laws of heredity and adaptation." Heredity brings about the reproduction of certain evolutionary features; adaptation causes a modification of them by the conditions of the environment—a condensation, disturbance, or falsification. Hence I insisted from the first that the biogenetic law consists of two parts, one positive and palingenetic and the other restrictively negative and cenogenetic. Palingenesis reproduces a part of the original history of the stem; cenogenesis disturbs or alters this picture in consequence of subsequent modifications of the original course of development. This distinction is most important, and cannot be too often repeated in view of the persistent misunderstanding of my opponents. It is overlooked by those who (like Plate and Steinmann) grant it only a partial validity, and by those who reject it altogether (like Keibel and Hensen). The embryologist Keibel is the most curious of these, as he has himself afforded a good many proofs of the biogenetic law in his careful descriptive-embryological works. But he has so little mastered it that he has never understood the distinction between palingenesis and cenogenesis.
It is especially unfortunate that one of our most distinguished embryologists, Oscar Hertwig, of Berlin, who provided a good deal of evidence in favor of the biogenetic law thirty years ago, has lately joined the opponents of it. His supposed "correction" or modification of it is, as Keibel has rightly said, a complete abandonment of it. Heinrich Schmidt has partly explained the causes of this change in his work on the biogenetic law. They are not unconnected with the psychological metamorphosis which Oscar Hertwig has undergone at Berlin. In the discourse on "The Development of Biology in the Nineteenth Century," which he delivered at the scientific congress at Aachen in 1900, he openly accepted the dualist principles of vitalism (although he says they are "just as unreliable as the chemico-physical conception of the opposing mechanical school"). The views which he has lately advanced on the worthlessness of Darwinism and the unreliability of phylogenetic hypotheses are diametrically opposed to the opinions he represented at Jena twenty-five years ago, and to those which his brother, Richard Hertwig, of Munich, has consistently maintained in his admirable Manual of Zoology.
In opposition to the mechanical ontogeny which I formulated in 1866 and embodied in the biogenetic law, a number of other tendencies in embryology afterwards appeared, and, with the common title of "mechanical embryology," branched out in every direction. The chief of these to attract attention thirty years ago were the pseudo-mechanical theories of Wilhelm His, who has rendered great service to ontogeny by his accurate descriptions and faithful illustrations of vertebrate-embryos, but who has no idea of comparative morphology, and so has framed the most extraordinary theories about the nature of organic development. In his Study of the First Sketch of the Vertebrate-body (1868), and many later works, His endeavored to explain the complicated ontogenetic phenomena on direct and simple physical lines by reducing them to elasticity, bending, folding of the embryonic layers, etc., while explicitly rejecting the phylogenetic method; he says that this is "a mere by-way, and quite unnecessary for the explanation of the ontogenetic facts (as direct consequences of physiological principles of development)." As a matter of fact, nature rather plays the part of an ingenious tailor in His's pseudo-mechanical and tectogenetic speculations, as I have shown in the third chapter of the Anthropogeny. Hence they have been humorously called the "tailor theory." However, they misled a few embryologists by opening the way to a direct and purely mechanical explanation of the complex embryonic phenomena. Although they were at first much admired, and immediately afterwards abandoned, they have found a number of supporters lately in various branches of embryology.
The great success that modern experimental physiology achieved by its extensive employment of physical and chemical experiments inspired a hope of attaining similar results in embryology by means of the same "exact" methods. But the application of them in this science is only possible to a slight extent on account of the great complexity of the historical processes and the impossibility of "exactly" determining historical matters. This is true of both branches of evolution, individual and phyletic. Experiments on the origin of species have very little value, as I said before; and this is generally true of embryological experiments also. However, the latter, especially careful experiments on the first stages of ontogenesis, have yielded some interesting results, particularly in regard to the physiology and pathology of the embryo at the earliest stages of development. The Archiv für Entwickelungsmechanik, which is edited by the chief representative of this school, Wilhelm Roux, contains, besides these valuable inquiries, a good number of ontogenetic articles, which partly rely on and partly ignore the biogenetic law.
Psychology and biogeny have been up to the present regarded as the most difficult branches of biology for monistic explanation, and the strongest supports of dualistic vitalism. Both departments become accessible to monism and a mechanico-causal explanation by means of the biogenetic law. The close correlation which it establishes between individual and phyletic development, and which depends on the interaction of heredity and adaptation, makes it possible to explain both. In regard to the first, I formulated the following principle thirty years ago in my first study of the gastræa theory: "Phylogenesis is the mechanical cause of ontogenesis." This single principle clearly expresses the essence of our monistic conception of organic development:
In the future every student will have to declare himself for or against this principle, if in biogeny he is not content with a mere admiration of the wonderful phenomena, but desires to understand their significance. The principle also makes clear the wide gulf that separates the older teleological and dualistic morphology from the modern mechanical and monistic science. If the physiological functions of heredity and adaptation are proved to be the sole causes of organic construction, every kind of teleology, and of dualistic and metaphysical explanation, is excluded from the province of biogeny. The irreconcilable opposition between the leading principles of the two is clear. Either there is or is not a direct and causal connection between ontogeny and phylogeny. Either ontogenesis is a brief compendium of phylogenesis or it is not. Either epigenesis and descent—or pre-formation and creation.
In repeating these principles here, I would lay stress particularly on the fact that, in my opinion, our "mechanical biogeny" is one of the strongest supports of the monistic philosophy.
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