154 THE EVOLUTION OF MAN. 



how comparatively short this whole Tertiary or Csenolithic 

 Epoch was. Judging from the relative thicknesses of the 

 various strata-formations we were able to say that this 

 whole period, during which Placental Animals first appeared, 

 and assumed their respective forms, amounted at most to 

 about three per cent, of the entire duration of the organic 

 history of the earth. (Cf. p. 18.) 



All Placental Animals are distinguished from the two 

 lower Mammalian groups already considered, from the 

 Cloacal Animals and Pouched Animals, by many prominent 

 peculiarities. All these characters are present in Man ; a 

 most significant fact. For on the most accurate comparative 

 anatomical and ontogenetical researches, we may base the 

 irrefutable proposition that Man is in every respect a true 

 Placental Animal ; in him are present all those peculiarities 

 in the structure and in the development of the body which 

 distinguish Placental Animals from the lower Mammalian 

 groups, and at the same time from all other animals. 

 Among these characteristic peculiarities the higher develop- 

 ment of the brain, the organ of the mind, is especially 

 prominent. The fore-brain, or large brain (cerebrum) is 

 much more highly developed in these than in lower 

 animals. The body (corpus callosum), which, like a bridge, 

 connects the two hemispheres of the fore-brain, attains its 

 fall development only in Placental Animals ; in the Pouched 

 Animals and Cloacal Animals it exists merely as an insigni- 

 ficant rudiment. It is true that in their brain structure 

 the lowest of the Placental Animals yet resemble Pouched 

 Animals very nearly; but within the Placental group we 

 can trace a continuous series of progressive stages in the 

 development of the brain, ascending quite gradually from 



