92 HUMAN BIOLOGY 



have disproportionately small brains and small animals 

 relatively large ones. In the cat, an animal of medium size 

 and therefore appropriate as a standard, the weight of the 

 brain is about i per cent of that of the body. In the Indian 

 elephant, whose bodily weight may be from 6000 to 7000 

 pounds, the brain, large though it is, is only about 0.2 per 

 cent of this weight, or one-fifth the percentage of the cat's 

 brain. The condition in the elephant represents fairly well 

 that characteristic of most large mammals. 



The opposite extreme is clearly illustrated by small 

 animals like the rats and the mice, whose brains are large 

 compared with their bodies. Thus the brain of the harvest 

 mouse, whose bodily weight is about 7 grams, represents 

 over 5 per cent of this weight, or five times the percentage 

 of the cat's brain. Disproportionately large brains are 

 common among small mammals. The same seems to be 

 true of birds; witness the relatively large size of the brain 

 in the smallest of these, the humming bird. And this prin- 

 ciple also appears to apply to insects, for among the castes 

 of worker ants the brain is rather uniformly large even when 

 the body is very small. 



Apparently each phase of animal life requires a certain 

 minimum of brain wherewith to carry on its nervous and 

 mental activities, and when for one reason or another the 

 body as a whole suffers an exceptional reduction, the brain 

 does not undergo a corresponding decrease. When on the 

 other hand the size of a stock of animals through evolu- 

 tionary growth becomes excessively large, as in the case 

 of the elephants or the whales, the brain follows this trend 

 to a certain extent, in response to increased skin surface and 

 musculature, but only in a restricted way, for the sense 

 organs and muscles of a large animal are after all not much 

 more complicated or appreciably more numerous than those 

 of a smaller one. Hence the necessity of proportional increase 

 in the central nervous organs of such a stock does not 

 obtain. Thus in such an evolutionary growth as that of an 

 elephant or any other large creature, the central nervous 

 organs, though they undergo some increase, fall noticeably 

 behind the general growth of the animal as a whole, with the 

 result that the proportional size of these organs is markedly 



