172 THE KIDDLE OF THE UNIVEKSE. 



is completely bridged over by a long series of inter- 

 mediate 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 alto- 

 gether. 



II. The brain development of the lowest and earliest 

 mammals (the monotremes, marsupials, and procho- 

 riates) is closely allied to that of their paleozoic 

 ancestors, the Carboniferous amphibia (the stegoce- 

 j)hala) and the Permian reptiles (the tocosauria) . 



III. During the Tertiary period commences the 

 typical development of the cerebrum, which distin- 

 guishes the younger mammals so strikingly from the 

 older. 



IV. The special development (quantitatively and 

 qualitatively) of the cerebrum which is so prominent 

 a feature in man, and which is the root of his pre- 

 eminent psychic achievements, is only found, outside 

 humanity, in a small section of the most highly- 

 developed mammals of the earlier Tertiary epoch, 

 especially in the anthropoid apes. 



V. The differences of brain- structure and psychic 

 faculty which separate man from the anthropoid aj)e 

 are slighter than the corresponding interval between 



