The biogenetic law applies just as much to the brain, the organ of mind, as to any other organ of the human body. On the strength of the ontogenetic facts, which fall under direct observation, we infer that there was a corresponding development in the phylogenetic series of our animal ancestors. A significant confirmation of this inference is found in comparative anatomy. It shows that in all the skull-animals (_craniota_)--from the fishes and amphibia up to the apes and man--the brain is developed in the same way, as a vesicular distension of the ectodermal medullary tube. This simple oval cerebral vesicle first divides into three and afterwards five successive vesicles by transverse constriction (_Anthropogeny_, chapter xxiv., plate 24). It is the first of these vesicles, the cerebrum, that afterwards becomes the chemical laboratory of the mind. In the lower craniota (fishes and amphibia) the cerebrum remains very small and simple. It only reaches a notably higher stage in the three chief classes of the vertebrates, the amniotes. As these land-dwelling and air-breathing craniota have more difficult work to do in the struggle for life than their lower aquatic ancestors, we find much more varied and complex habits among them. These hereditary habits are gradually converted into instincts by functional adaptation and progressive heredity; and with the further development of consciousness in the higher mammals we have at last the appearance of reason. The gradual unfolding of the mental life is accompanied step by step with the advance of its anatomic organ, the phronema in the cortex. Recent careful investigations of the ontogeny and histology of the origin of mind (by Flechsig, Hitzig, Edinger, Ziehen, Oscar Vogt, etc.) have given us an interesting insight into the mysterious processes of its phylogeny.