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.