150 EVOLUTION OF THE EARTH 



kill themselves by overexertion. Activity accelerates the proc- 

 esses of metabolism and generates toxic poisons. In the right 

 kind of climate these are eliminated during periods of rest. In 

 a warm climate, however, the high temperature appears to 

 cause excessive chemical activity of the protoplasm just as does 

 exercise. Hence people fejel tired even without exertion. When 

 bacteria attack such people, they find a ready prey, and when 

 the effectsof activity and of heat are combined the result is often 

 fatal. The exact mechanism of the process has not yet been de- 

 termined, but some such poisoning of the system and consequent 

 elimination of unduly active types appears to be one of the 

 reasons why the negro has acquired a comparatively indolent 

 character. 



Among the backward natives of Australia the elimination of 

 energetic, nervously alert people has gone farther than among 

 the negroes. The Australians in crossing the torrid zone were 

 subjected to all the evils which have weakened the mental 

 powers of the negroes. They also suffered a terrible handicap 

 because their tropical experience was the precursor of an 

 equally strenuous repression by the desert. There sudden and 

 intense activity is at a premium when the water dries up and 

 a long march must be made to a new supply. The most 

 essential of all qualities, however, is the ability to endure 

 hunger, thirst, and heat indefinitely, a kind of endurance which 

 is much harder on people with alert nerves than upon those 

 of stolid disposition. Moreover, mental alertness loses much 

 of its importance as a factor in natural selection when the 

 environment becomes so poor that there are almost no material 

 resources. It is by no means strange, then, that the Aus- 

 tralian bushmen, even more than their fellow sufferers, the 

 Hottentots of South Africa, show, as it were, the combined 

 weaknesses of the tropical negroes and of the desert people 

 of Arabia. One might go on to discuss this theme in relation 

 to all the races of the earth. Such a discussion would appar- 

 ently strengthen the conclusion that while the mental inhei 



