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