It would take up too much space to attempt to give even a general survey of the remarkable and enormous literature of the subject that has accumulated in the last three decades in the shape of thousands of learned treatises and popular articles. The greater part of these works have been written under the influence of conventional religious prejudice, and without the necessary acquaintance with the subject, that can only be obtained by a thorough training in biology. The most curious feature of them is that most of the authors restrict their genealogical interests to the most manlike apes, and do not deal with their origin, or with the deeper roots of our common ancestral tree. They do not see the wood for the trees. Yet it is far easier and safer to penetrate the great mysteries of our animal origin, if we look at the subject from the higher standpoint of vertebrate phylogeny and go deeper into the earlier records of the evolutionary history of the vertebrates. Since the great Lamarck established the idea of the vertebrate at the beginning of the nineteenth century (1801), and his Parisian colleague, Cuvier, shortly afterwards recognised the vertebrates as one of his four chief animal groups, the natural unity of this advanced section of the animal world has not been contested. In all the vertebrates, from the lowest fishes and amphibians up to the apes and man, we have the same type of structure, the same characteristic disposition and relations of the chief organs; and they differ materially from the corresponding features in all other animals. The mysterious affinities of the vertebrates induced Goethe, 140 years ago, long before Cuvier, to make prolonged and laborious studies in their comparative anatomy at Jena and Weimar. Just as he had, in his _Metamorphosis of Plants_, established the unity of organisation by means of the leaf as the common primitive organ, he, in the metamorphosis of the vertebrates, found this common element in the vertebral theory of the skull. And when Cuvier established comparative anatomy as an independent science, this branch of biology was developed to such an extent by the classic research of Johannes Müller, Carl Gegenbaur, Richard Owen, Thomas Huxley, and many other morphologists, that Darwinism found its most powerful weapons in this arsenal. The striking differences of external form and internal structure that we find in the fishes, amphibians, reptiles, birds, and mammals, are due to _adaptation_ to the various uses of their organs and their environments. On the other hand, the astonishing agreement in their typical character, that persists in spite of their differences, is due to _inheritance_ from common ancestors.