Lamarck says, that “Nature, in producing, successively, all the species of animals, beginning with the most imperfect, or the most simple, and terminating with the most perfect, has gradually complicated their organization. These animals becoming scattered throughout the habitable regions of the globe each species has received from the influences of its surroundings its present habits, and the modifications of the parts the use of which we recognize.” Such are Lamarck’s views and a fairly complete statement of the facts from which he draws his conclusions. His illustrations appear naïve, and often not a little ludicrous, but it must be admitted that, despite their absurdities, his theory appears in some cases to account wonderfully well for the facts. The long legs of wading birds, the long neck and disproportionately long fore-legs of the giraffe, the structure of the sloth, and particularly the degeneration of the eyes of animals living in the dark, seem to find a simple explanation in the principle of the inheritance of acquired characters. But the crucial point of the entire theory is passed over in silence, or rather is taken for granted by Lamarck, namely, the inheritance in the offspring of the characters acquired through use or disuse in the parent. He does not even discuss this topic, but in several places states unreservedly that the increase or decrease of a part reappears in the next generation. It is here that Lamarck’s theory has been attacked in more modern times, for as soon as experimental proof was demanded to show that the results of use or of disuse of an organ is inherited, no such proof was forthcoming. Yet the theory is one that has the great merit of being capable of experimental test, and it is astonishing to find that, with the immense amount that has been written by his followers, so few attempts have been made to give the theory a thorough test. The few results that have been obtained are not, however, favorable to the theory, but almost the only attempts at experiment that have been made in this direction have been those of mutilating certain parts; and were it not for popular belief to the effect that such mutilations are inherited, one would least expect to get evidence for or against the theory in this direction. Lamarck himself believed that the changes were slowly acquired, and I think modern Lamarckians are justified in claiming that the validity of the theory can only be tested by experiments in which the organism is subjected to influences extending over a considerable period, although Lamarck appears to have believed that the first results may appear quite soon. Before expressing any opinion in regard to the probability of the theory, let us examine what the followers of Lamarck have contributed in the way of evidence to the theory, rather than the applications that they have made of the theory. We shall also find it profitable to consider some of the modern criticism, to which the theory has been