150 EVOLUTION OF THE EARTH 



kill themselves by overexertion. Activity accelerates the 

 pfooesies~of metabolism and genefa"Fe % 5~to'xic poisons. In the 

 right kind of climate these are eliminated during periods of 

 rest. In a warm climate, however, the high temperature ap- 

 pears to cause excessive chemical activity of the protoplasm 

 just as does exercise. Hence people feel tired even without 

 exertion. When the effects of activity and of heat are combined 

 the result is often fatal. The exact mechanism of the process 

 has not yet been determined, but some such poisoning of the 

 system and consequent elimination of unduly active types ap- 

 pears to be the reason whj j-he ngyrnjias acquired a compara- 

 tivelyTndolent 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^jTiental alertness loses much 

 of itsimortance_as a factor in natural sele^ttoTr-when the 

 environment becomes so poor that there aTcralmoGt no material 

 resources. It is by no means strange, then, thatch e 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 inherit- 



