documents"--palæontology, ontogeny, and morphology--qualified me to extend it. I had already, in my _General Morphology_ (at the end of the fifth book), described the close causative connection which exists, in my opinion, between the two branches of organic evolution as one of the most important ideas of transformism, and I had framed a precise formula for it in a number of "theses on the causal nexus of biontic and phyletic development": "_Ontogenesis is a brief and rapid recapitulation of phylogenesis_, determined by the physiological functions of heredity (generation) and adaptation (maintenance)." Darwin himself had emphasized the great significance of his theory for the elucidation of embryology in 1859, and Fritz Müller had endeavored to prove it as regards the Crustacea in the able little work, _Facts and Arguments for Darwin_ (1864). My own task has been to prove the universal application and the fundamental importance of the biogenetic law in a series of works, especially in the _Biology of the Calcispongiae_ (1872), and in _Studies on the Gastraea Theory_ (1873-1884). The theory of the homology of the germinal layers and of the relations of _palingenesis_ to _cenogenesis_ which I have exposed in them has been confirmed subsequently by a number of works of other zoologists. That theory makes it possible to follow nature's law of unity in the innumerable variations of animal embryology; it gives us for their ancestral history a common derivation from a simple primitive stem form. The far-seeing founder of the theory of descent, Lamarck, clearly recognized in 1809 that it was of universal application; that even man himself, the most highly developed of the mammals, is derived from the same stem as all the other mammals; and that this in its turn belongs to the same older branch of the ancestral tree as the rest of the vertebrates. He had even indicated the agencies by which it might be possible to explain man's descent from the apes as the nearest related mammals. Darwin, who was, naturally, of the same conviction, purposely avoided this least acceptable consequence of his theory in his chief work in 1859, and put it forward for the first time in his _Descent of Man_ in 1871. In the mean time (1863) Huxley had very ably discussed this most important consequence of evolution in his famous _Place of Man in Nature_. With the aid of comparative anatomy and ontogeny, and the support of the facts of palæontology, Huxley proved that the "descent of man from the ape" is a necessary consequence of Darwinism, and that no other scientific explanation of the origin of the human race is possible. Of the same opinion was Karl Gegenbaur, the most distinguished representative of comparative anatomy, who lifted his science to a higher level by a consistent and ingenious application of the theory of descent.