434 NATURE AND NURTURE 



714. When we compare one modern species with another, the 

 more intelligent type invariably has, proportionately to the size 

 of its body, a larger brain. Thus the dog has a larger brain than 

 a rabbit, and a man than a dog. Presumably, therefore, intelli- 

 gence and size of brain are correlated in some degree at least. 

 Doubtless the correlation is not absolute ; for brains vary in their 

 constituent elements, for instance, in the relative amounts of grey 

 and white matter. " The leading feature in the development and 

 separation of man from amongst other animals is undoubtedly the 

 relatively enormous size of the brain in man and the corresponding 

 increase in its activity and capacity. It is a very striking fact 

 that it was not in the ancestors of man alone that this increase in 

 the size of the brain took place at the same period, viz. the 

 Miocene. The great mammals, such as the titanotherium, which 

 represented the rhinoceros in early Tertiary times, had a brain 

 which was in proportion to the bulk of the body, not more than 

 one-eighth of the volume of the brain of the modern rhinoceros. 

 Other great mammals of the earlier Tertiary period were in the 

 same case ; and the ancestors of the horse, which are better known 

 than those of any other modern animal, certainly had much 

 smaller brains in proportion to the size of their bodies than has 

 their descendant." 1 



715. Biometricians, however, founding their opinions on 

 statistical data, have denied that the law that intelligence is 

 correlated to size of brain obtains in the case of human races and 

 individuals. According to them " there is no marked correlation 

 between skull capacity and intellectual power." What, however, 

 is the real meaning of their facts, for there can be little doubt that 

 the facts are correct? The mind manifestly grows under the 

 stimulus of experience. Mental facts are correlated to cerebral 

 facts. Therefore, we have every reason to believe that the brain 

 grows under the stimulus of use. We know, in fact, that if one 

 hemisphere of the brain atrophies from any cause, such as disease, 

 the opposite hemisphere tends to hypertrophy ; and we cannot 

 account for this increase except by supposing that it results from 

 increase of function. After birth, then^the growth of the mind and 

 brain is conditioned, on the one hand, by the inborn capacity to 

 grow under the stimuli of experience and use, and on the other, by 

 the amount of stimulus. Individuals may differ both with respect 

 to the capacity and the amount of stimulus their minds and brains 

 receive. It follows, within limits that vary with the individual, 



1 The Kingdom of Man, by Sir Ray Lankester, p. 22. 



