THE UNIQUE MAMMAL 9 



species of veritable super-men, with six-pint brains instead of 

 the present three-pinters that would then seem by comparison 

 so pitifully small and inadequate. 



But we must not pursue this speculation, though nothing 

 now known of genetics or brain embryology and anatomy can 

 definitively rule it out as impossible. There is, however, one 

 solid fact about which there can be no disagreement. It is that 

 the greater size of brain, with its associated enhanced mental 

 capacity, has been of enormous practical advantage to man in 

 his struggle for survival, and in bringing him forward from 

 the level on which he emerged as a new genus of mammals, in 

 respect of all those arts, crafts, and techniques of living gen- 

 erally that are peculiarly and indeed uniquely human. Just as 

 in the case of upright posture, however, a biological debit was 

 incurred by this encephalic specialization. The more highly 

 organized human brain breaks down under the stresses of living 

 more easily, and leads more often to the death of the whole 

 organism than is the case with the simpler central nervous 

 system of the lower vertebrates. While comprehensive and pre- 

 cisely accurate statistics on the point are not available a rough 

 approximation suggests that if we take as unity the proportionate 

 mortality in the reptiles and the birds that can be assigned to 

 breakdown of the central nervous system from whatever cause, 

 the corresponding figure for infra-human mammals is about 2.5. 

 That is to say proportionately two and a half times more 

 mammals than birds and reptiles die from causes affecting the 

 nervous system. The corresponding figure for more primitive 

 human groups is about 18.0. That for the most highly civilized 

 and culturally most advanced human groups is about 27.5. In 

 other words it appears that in the evolutionary progress from 

 reptiles and birds to the most advanced sorts of men the relative 

 mortality assignable to breakdown of the central nervous system 

 has been multiplied more than twenty-seven fold. These figures 

 are not to be regarded as absolutely precise appraisals, but 

 roughly they do indicate something of the biological price that 

 man has to pay for his high-toned brain. Also of interest in this 



