124 HUXLEY 



brain of the lowest Mammals, such as the duck-billed 

 Platypus and the Opossums and Kangaroos, exhibits 

 a still more definite advance in the same direction. " 

 A step higher in the scale, among the placental Mam- 

 mals, the cerebral structure acquires a vast modifica- 

 tion in the appearance of a new structure between 

 the two halves of the brain, connecting them 

 together. 



In the lower and smaller forms of placental 

 Mammals the surface of the cerebral hemispheres is 

 either smooth or evenly rounded, or exhibits a very 

 few grooves, which are technically termed " sulci," 

 separating ridges or " convolutions " of the substance 

 of the brain, and the smaller species of all orders 

 tend to a similar smoothness of brain. But, in the 

 higher orders, and especially the larger members of 

 these orders, the grooves or sulci become extremely 

 numerous, and the intermediate convolutions pro- 

 portionately more complicated in their meanderings, 

 until, in the Elephant, the Porpoise, the higher Apes, 

 and Man, the cerebral surface appears a perfect 

 labyrinth of tortuous foldings. 1 . . . The sur- 

 face of the brain of a monkey exhibits a sort of 

 skeleton map of man's, and in the manlike apes the 

 details become more and more filled in until it is only 

 in minor characters, such as the greater excavation 

 of the anterior lobes, the constant presence of fissures 

 usually absent in man, and the different disposition 

 and proportions of some convolutions that the 

 Chimpanzee's or the Orang's brain can be structurally 

 distinguished from Man's. 2 



1 Man's Place in Nature, p. 96. 



2 Man's Place in Nature, p. 100. 



