After the works of teratologists, after the experiments of Geoffroy, so ably resumed and completed by M. Dareste, the part played by pathogenic causes, even by external causes, in arrested development cannot be denied. Now microcephalism is nothing else than _arrested development_ acting on the cranium and its contents. But this is not an isolated case. Other organs and functions in microcephali suffer in the same manner. They have been proved to be always sterile, and certainly sterility is not a phenomenon which can be referred to atavism. Thus among microcephali a teratogenic cause is clearly proved to have acted on part of the organism, viz., the generative organs. What reason can be alleged for attributing alterations of the cranium and brain to an entirely different cause? By virtue of what principle can two facts be separated, which observation has shown to be so intimately connected with each other? Why should the first be appealed to as an argument and nothing said about the second? Is it not evident that this is an entirely arbitrary kind of procedure, and actuated solely by the requirements of theory? The general plan of the brain is fundamentally the same in all the mammalia and in man. Upon this point, as upon every other, the resemblance is greatest when the latter is compared with the anthropomorphous apes. When, for some reason or other, his brain is altered and reduced, as in the microcephali, is it at all surprising that fresh resemblances should arise. The contrary would be unintelligible. This is a fact upon which Vogt has especially insisted, and he has described from this point of view several interesting details which render less general some of the results obtained by M. Gratiolet. But it is a remarkable fact that these new relations are not established with the most highly developed apes, but with the tailed apes of the new world, with the _platyrrhini_, which are excluded by Haeckel and Darwin from the human ancestral series. Thus, the Darwinian theory itself protests against the comparison between the microcephali and our pretended pithecoid ancestors. The relations which we are discussing do not, moreover, reach a similarity which would authorise the conclusions of the Genevese savant. The brains of microcephali, though often less voluminous and less convoluted than those of the anthropoid apes, according to Gratiolet, do not become at all similar to them. This proposition is confirmed by the work of Vogt.