Darwin's second contribution to this attainment was his proof of the possibility of a physiological explanation of the process of descent itself. Of this possibility he fully convinced his contemporaries, but in indicating the particular means by which the change of species has been brought about, he has not succeeded in securing universal acceptation. Quite on the contrary, objections have been raised from the very outset, and with such force as to compel Darwin himself to change his views in his later writings. This however, was of no avail, and objections and criticisms have since steadily accumulated. Physiologic facts concerning the origin of [5] species in nature were unknown in the time of Darwin. It was a happy idea to choose the experience of the breeders in the production of new varieties, as a basis on which to build an explanation of the processes of nature. In my opinion Darwin was quite right, and he has succeeded in giving the desired proof. But the basis was a frail one, and would not stand too close an examination. Of this Darwin was always well aware. He has been prudent to the utmost, leaving many points undecided, and among them especially the range of validity of his several arguments. Unfortunately this prudence has not been adopted by his followers. Without sufficient warrant they have laid stress on one phase of the problem, quite overlooking the others. Wallace has even gone so far in his zeal and ardent veneration for Darwin, as to describe as Darwinism some things, which in my opinion, had never been a part of Darwin's conceptions. The experience of the breeders was quite inadequate to the use which Darwin made of it. It was neither scientific, nor critically accurate. Laws of variation were barely conjectured; the different types of variability were only imperfectly distinguished. The breeders' conception was fairly sufficient for practical purposes, but science needed a clear understanding of the [6] factors in the general process of variation. Repeatedly Darwin tried to formulate these causes, but the evidence available did not meet his requirements. Quetelet's law of variation had not yet been published. Mendel's claim of hereditary units for the explanation of certain laws of hybrids discovered by him, was not yet made. The clear distinction between spontaneous and sudden changes, as compared with the ever-present fluctuating variations, is only of late coming into recognition by agriculturists. Innumerable minor points which go to elucidate the breeders' experience, and with which we are now quite familiar, were unknown in Darwin's time. No wonder that he made mistakes, and laid stress on modes of descent, which have since been proved to be of minor importance or even of doubtful validity.