CHAPTER XVI 

 THE EVOLUTION OF MIND^ 



THAT the amount of intelligence mani- 

 fested by any vertebrate animal depends 

 to a certain extent upon the amount of 

 nerve tissue integrated in its cephalic ganglia, 

 and especially in the cerebrum, is a truth famil- 

 iar to every one, though often crudely stated 

 and incorrectly interpreted. In the lowest ver- 

 tebrate, the amphioxus, there is no brain at all. 

 In fishes, the cerebrum and cerebellum are much 

 smaller than the optic lobes ; the cerebrum 

 being in many large fishes about the size of a 

 pea, though in the shark it reaches the size of 

 a plum. Continuing to grow by the addition 

 of concentric layers at the surface, the cerebrum 

 becomes somewhat larger in birds and in the 

 lower mammals. It gradually covers up the 

 optic lobes, and extends backwards as we pass 

 to higher mammalian forms, until in the an- 

 thropoid apes and in man it covers the whole 

 upper surface of the cerebellum. In these high- 

 est animals it begins also to extend forwards. In 

 the chimpanzee and gorilla the anterior portion 

 1 [See Introduction, § 21.] 

 194 



