According to the theory of evolution every structure of to-day finds its meaning in some condition of the past. The inside of an animal tells what it really is, for it bears the record of heredity. The outside of an animal tells where its ancestors have been, for it bears record of concessions to environment. Similarity in essential structure is known as homology. By the theory of evolution homology, wherever it is found, is proof of blood-relationship.
The theory of organic evolution through natural law was first placed on a stable footing by the observations and inductions of Darwin. It has therefore been long known as Darwinism, although that term has been usually associated with the recognition of natural selection as the great motive power in organic change. Darwinism was at first regarded as a “working hypothesis.” It is now an integral part of biological science, because all opposing hypotheses have long since ceased to work. It is as well attested as the theory of gravitation, and its elements are open to less doubt. All investigations in biology must assume it, as without it most such investigations would be impossible. Naturalists could no more go back to the old notion of special creation for each species and its organs than astronomers could go back to the old notion of guiding angels as directors of planetary motion. Without the theory of organic development through natural selection, the biological science of to-day would be impossible.
Evolution as a Method of Study. In a third sense the word evolution is applied to a method of investigation. It is the study of present conditions in the light of the past. The preliminary work of science is the descriptive part. This involves accuracy of observation and precision of statement, but makes no great demands on the powers of logical analysis and synthesis. The easy work of science is largely already done. Those who would continue investigation must study not only facts and structures, but the laws that govern them. In the words of John Fiske, “Whether plants or mountains or mollusks or subjunctive moods or tribal confederacies be the things studied, the scholars who have studied them most fruitfully were those who have studied them as phases of development. Their work has directed the current of thought.” The most difficult problems in life are susceptible of more or less perfect solution if approached by the method of evolution. They cannot be even stated as problems in any other terms. In every science worthy of the name the history of origins and the study of developing forces must take a leading part.
Evolution as a System of Cosmic Philosophy. In a fourth sense the word evolution has been applied to the philosophical conceptions to which the theory of evolution gives rise. Philosophy is not truth. When it is so it becomes science. At the best it points the way to truth. The broader the inductive basis of any system of philosophy, the greater its value as an intellectual help. The system of Herbert Spencer, the greatest exponent of the philosophy of evolution, is based wholly on the results of scientific investigation. It consists of a series of more or less broad and more or less probable deductions from the facts and laws already known. Systems like these, which rest on scientific knowledge, do not rise high above it. They can therefore be revised or rewritten as knowledge increases. They provide the means for their own correction. Systems resting on aphorisms or assumptions or definitions must disappear as knowledge increases.
Philosophy is never wholly identical with truth. The partial truth which it may contain becomes wholly error with the advance of science. The growth of exact knowledge transforms the truth in philosophy into science, leaving the absolute falsehood as the final residuum.
From this necessary fact comes the ultimate decay of all creeds or philosophic formulæ. Throughout the ages science and philosophy have been in conflict. Science is the same to all minds capable of grasping its conclusions. Philosophy changes with the point of view. It is the evanescent perspective in which the facts and phenomena of the universe are seen. This can never be the same under changing times and conditions. With the larger knowledge of to-morrow, there will be large modifications in the accepted philosophy of evolution. Each succeeding generation will give to the applications of the laws of organic life a different philosophical expression.
II. What Evolution Is Not.
In these four senses the word evolution is used with some degree of accuracy. But in the current literature of the day the word has many other meanings, some of them very far from any just basis. Some things which evolution is not we may here notice briefly.
Evolution is not a theory that “man is a developed monkey.” The question of the immediate origin of man is not the central or overshadowing question of evolution. This question offers no special difficulties in theory, although the materials for exact knowledge are in many directions incomplete. Homologies more perfect than those connecting man with the great group of monkeys could not exist. These imply the blood-relationship of the human race with the great host of apes and monkeys. As to this there can be no shadow of a doubt. And as similar homologies connect man with all members of the group of mammals, similar blood-relationship must exist. And homologies, less close but equally unmistakable, connect all backboned animals one with another; and the lowest backboned types are closely joined to worm-like forms not usually classed as vertebrates.
It is perfectly true that, with the higher or anthropoid apes, the relations with man are extremely intimate. But man is not simply “a developed ape.” Apes and men have diverged from the same primitive stock, apelike, manlike, but not exactly the one or the other. No apes or monkeys now extant could apparently have been ancestors of primitive man. None can ever “develop” into man. As man changes and diverges, race from race, so do they. The influence of effort, the influence of surroundings, the influence of the sifting process of natural selection, acts upon them as it acts upon man.