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 zooelogical 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.