54 The Destiny of Man. 



cal variations came to be of more use to 

 the species than variations in bodily struc- 

 ture. The gulf by which the lowest known 

 man is separated from the highest known 

 ape consists in the great increase of his 

 cerebral surface, with the accompanying 

 intelligence, and in the very long duration 

 of his infancy. These two things have 

 gone hand in hand. The increase of cere- 

 bral surface, due to the working of natural 

 selection in this direction alone, has en- 

 tailed a vast increase in the amount of 

 cerebral organization that must be left to 

 be completed after birth, and thus has pro- 

 longed the period of infancy. And con- 

 versely the prolonging of the plastic pe- 

 riod of infancy, entailing a vast increase 

 in teachableness and versatility, has con- 

 tributed to the further enlargement of the 

 cerebral surface. The mutual reaction of 

 these two groups of facts must have gone 

 on for an enormous length of time since 

 man began thus diverging from his simian 

 brethren. It is not likely that less than a 



