One other point in this history must be noted. As the development of the complication of the machine progressed the possibility of further progress has been constantly narrowed. When the history of these machines began as a simple mass of cells, there was a possibility of an almost endless variety of methods of organization. But as a distinct type of organization was adopted by one and another line of descendants all subsequent productions were limited through the law of heredity to the general line of organization adopted by their ancestors. With each age the further growth of such machines must consist in the further development in the perfection of its parts, and not in the adoption of any new system of organization. Hence it is that the history of the living machine has shown a tendency toward development along a few well-marked lines, and although this complication becomes greater, we still see the same fundamental scheme of organization running through the whole. As the ages have progressed the machines have become more perfect in the adjustment of their parts, i.e., they have become more perfect machines, but the history has been simply that of perfecting the early machines rather than the production of new types. ==Evidence for this History.==--As just outlined, we see that the living machines have been gradually brought into their present condition by a process which has been called organic evolution. But we must pause for a moment to ask what is our evidence that such has been the history of the living machine. The whole possibility of understanding living nature depends upon our accepting this history and finding an explanation of it. At the outset we have the question of fact, and we must notice the grounds upon which we stand in assuming this history to be as outlined. This problem is the one which has occupied such a prominent place in the scientific world during the last forty years, and which has contributed so largely toward making modern biology such a different subject from the earlier studies of natural history. It is simply the evidence for organic evolution, or the theory of descent. The subject has for forty years been thoroughly sifted and tested by every conceivable sort of test. As a result of the interest in the question there has been disclosed an immense mass of evidence, relevant and irrelevant. As the evidence has accumulated it has become more and more evident that the evolution theory must be recognized as the only one which is in accord with the facts, and the outcome has been a practical unanimity among thinkers that the theory of descent must be the foundation of our further study. The evidence which has forced this conclusion upon scientists we must stop for a moment to consider, since it bears very directly upon the subject we are studying.