INTRODUCTION.
It is extremely difficult to realize the variety, wealth, and grandeur of animal and plant life as they exist on the globe to-day. Even if we endeavor to recall, in imagination, all that we have seen in streams and woodland, in ponds and rivers, in meadows, and in the air; even when we call to mind the multifarious specimens we have beheld in all the museums of natural history we have visited, and remember the most vivid descriptions that we have ever read of the wealth of tropical life;—even then we have only the faintest conception of the multitude and variety of living forms, not to mention the vanished hosts of bygone ages.
It has always been the endeavor of students of nature to reduce this great host of living creatures to order by some system of classification. First one system of classification was adopted, and then another, with the progress of Botany and Zoölogy, until eventually, before the theory of evolution was entertained by scientific men, that system was adopted which naturalists likened to a tree. In this system, those lowest organisms which cannot properly be called either animals or plants, may be represented by a short trunk. Soon this short trunk divides into two large trunks, one of which represents the animal kingdom and the other the vegetable kingdom. Each of these trunks then sends off large branches representing classes, from which smaller and more numerous branches, representing orders, are given off. From these, other branches and sub-branches are separated, representing families and genera, and finally the terminal twigs or leaves represent species.
In this tree, while there is a general advance in organization from below upwards, there are many deviations in this respect. Some leaves may be growing on different branches at the same level, which means that species belonging to widely divergent classes or orders may still possess an equal grade of organization. On the same branch there may be growing leaves at different levels. This means that one species may be more highly organized than another belonging to the same class. Not only may all living species be classified in the form of a family tree, but all the extinct hosts of species that lived in the ages of the past may be similarly classified. If all the animals that have ever lived on the globe should be represented by a tree, those existing on the earth to-day would be indicated by the topmost twigs and leaves, while the extinct forms would be represented by the trunk and main branches. (Vid. [Diagram of Development].)
The detecting of this tree-like arrangement of species in nature is the progressive work of naturalists for centuries past. At about the commencement of the present century, when it was finally detected, naturalists were unable to understand the significance of it. They did not perceive the underlying principle that accounts for the fact that groups of living forms have such natural affinities that they are arranged like a family tree.
When Darwin came upon the scene and found that his predecessors had already empirically worked out the tree-like system of classification, he convinced naturalists that the great underlying principle of this system was Heredity; and that, therefore, the grouping of living and extinct organisms in a family tree according to their natural affinities is a grouping based upon genetic affinities; and further that the ultimate meaning of classification is the tracing of lines of pedigree. This signifies that all the creatures living on the globe to-day, and all the hosts that lived in the ages of the past, are blood relations in greater or less degree, and that they have all been evolved from simple microscopic creatures that appeared on the globe at the dawn of life. All organisms, then, have undergone an evolution.
The factors of fundamental importance in the study of the theory of Evolution, or the doctrine of the Transmutations of Organisms, are Cells, Heredity and Variation, Environment, Natural Selection, and Isolation.
The evolution of an organism means its descent from preceding organisms with continuous adaptation to its Environment. Its adaptation occurs chiefly, if not entirely, through the Natural Selection of its useful Variations, and the tending of these variations to be transmitted to the offspring by the forces of Heredity. The summation of the variations, by Heredity and Isolation, leads ultimately to specific, generic, ordinal, and other differences in the descendants; leads, in other words, to the transmutations of organisms.
An organism consists of two great groups of structural units called cells,—Germ-Cells and Body-Cells,—harmoniously associated together. As far as Evolution is concerned, the most fundamental parts of an organism are the Germ-Cells.
In order that the reader may have a working knowledge of the wonderful powers of Germ-Cells and their progeny, it is vitally important that some concrete illustrations of the activities of different kinds of cells should be given.