" The Descent of Man" 441 



would almost certainly become larger. No one, I presume, doubts 

 that the large proportion which the size of man's brain bears to his 

 body, compared to the same proportion in the gorilla or orang, is 

 closely connected with his higher mental powers." " With respect to 

 the lower animals," he says 1 , "M. E. Lartet 2 , by comparing the crania 

 of tertiary and recent mammals belonging to the same groups, has 

 come to the remarkable conclusion that the brain is generally larger 

 and the convolutions are more complex in the more recent form." 



Sir E. Ray Lankester has sought to express in the simplest terms 

 the implications of the increase in size of the cerebrum. " In what," 

 he asks, "does the advantage of a larger cerebral mass consist?" 

 "Man," he replies "is born with fewer ready-made tricks of the nerve- 

 centres these performances of an inherited nervous mechanism so 

 often called by the ill-defined term 'instincts' than are the monkeys 

 or any other animal. Correlated with the absence of inherited ready- 

 made mechanism, man has a greater capacity of developing in the 

 course of his individual growth similar nervous mechanisms (similar 

 to but not identical with those of 'instinct') than any other animal... 

 The power of being educated ' educability ' as we may term it is 

 what man possesses in excess as compared with the apes. I think we 

 are justified in forming the hypothesis that it is this ' educability ' 

 which is the correlative of the increased size of the cerebrum." 

 There has been natural selection of the more educable animals, for 

 "the character which we describe as 'educability' can be trans- 

 mitted, it is a congenital character. But the results of education 

 can not be transmitted. In each generation they have to be acquired 

 afresh, and with increased 'educability' they are more readily ac- 

 quired and a larger variety of them.... The fact is that there is no 

 community between the mechanisms of instinct and the mechanisms 

 of intelligence, and that the latter are later in the history of the 

 evolution of the brain than the former and can only develop in 

 proportion as the former become feeble and defective 3 ." 



In this statement we have a good example of the further develop- 

 ment of views which Darwin foreshadowed but did not thoroughly 

 work out. It states the biological case clearly and tersely. Plasticity 

 of behaviour in special accommodation to special circumstances is of 

 survival value ; it depends upon acquired characters ; it is correlated 

 with increase in size and complexity of the cerebrum ; under natural 

 selection therefore the larger and more complex cerebrum as the 

 organ of plastic behaviour has been the outcome of natural selection. 

 We have thus the biological foundations for a further development of 

 genetic psychology. 



1 Descent of Man (Popular edit.), p. 82. 2 Comptes Eendus des Sciences, June 1, 1868. 



3 Nature, Vol. LXI. pp. 624, 625 (1900). 



