II. Second stage--the _cyclostomata_: the oldest group of the craniota, now only represented by the _petromyzontes_ and _myxinoides_: the fore-termination of the cord expands into a vesicle, which then subdivides into five successive parts--the great-brain, intermediate-brain, middle-brain, little-brain, and hind-brain: these five cerebral vesicles form the common type from which the brain of all craniota has evolved, from the lamprey to man. III. Third stage--the _primitive fishes_ (_selachii_): similar to the modern shark: in these oldest fishes, from which all the gnathostomata descend, the more pronounced division of the five cerebral vesicles sets in. IV. Fourth stage--the _amphibia_. These earliest land animals, making their first appearance in the Carboniferous period, represent the commencement of the characteristic structure of the _tetrapod_ and a corresponding development of the fish-brain: it advances still further in their Permian successors, the _reptiles_, the earliest representatives of which, the _tocosauria_, are the common ancestors of all the amniota (reptiles and birds on one side, mammals on the other). V.-VIII. Fifth to the eighth stages--the _mammals_. I have exhaustively treated, and illustrated with a number of plates, in my _Anthropogeny_, the evolution of our nervous system and the correlative question of the development of the soul. I have now, therefore, merely to refer the reader to that work. It only remains for me to add a few remarks on the last and most interesting class of facts pertaining to this--to the evolution of the soul and its organs within the limits of the class mammalia. In doing so, I must remind the reader that the _monophyletic origin_ of this class--that is, the descent of all the mammals from one common ancestral form (of the Triassic period)--is now fully established. The most important consequence of the monophyletic origin of the mammals is the necessity of deriving the human soul from a long evolutionary series of other mammal souls. A deep anatomical and physiological gulf separated the brain structure and the dependent psychic activity of the higher mammals from those of the lower: this gulf, however, is completely bridged over by a long series of intermediate stages. The period of at least fourteen (more than a hundred, on other estimates) million years, which has elapsed since the commencement of the Triassic period, is amply sufficient to allow even the greatest psychological advance. The following is a summary of the results of investigation in this quarter, which has recently been very penetrating: I. The brain of the mammal is differentiated from that of the other vertebrates by certain features, which are found in all branches of the class; especially by a preponderant development of the first and fourth vesicles, the cerebrum and cerebellum, while the third vesicle, the middle brain, disappears altogether.