240 WORLD-POWER AND EVOLUTION 



ship of human progress. We started when man's ancestors were 

 still spineless creatures living in the vast ocean. We have seen 

 that again and again great crises have depended largely upon cli- 

 matic conditions. When the primitive animals came out upon the 

 land, when they began to crawl in the mud and thus develop the 

 limbs which finally became hands, and when they at last evolved 

 the power of warming their own bodies, the circumstances which 

 brought about the change appear each time to have been largely 

 climatic. Other factors must unquestionably have played most 

 vital parts, but none stands out more clearly than climate. In 

 later days man's brain, the most sensitive of all his organs, made 

 by far its most rapid evolution under the stress of great climatic 

 extremes. The greater the climatic changes, the more rapidly new 

 types were evolved not only among plants and animals, but in the 

 human species. Whatever may be the inner causes of the rise and 

 extinction of new forms of life, there can be no question that these 

 have occurred most rapidly at times when the climate of the world 

 swung rapidly from one extreme to another. 



Side by side with this great fact stands another. Today man's 

 body is more sensitive to the temperature and humidity of the air 

 than to any other feature of his environment. This is proved by 

 our daily experiences of discomfort because the air is too hot, too 

 cold, too dry, too stuffy, or too actively in motion. It is 

 also proved by the measured work of thousands of factory oper- 

 atives and students, who achieve most under certain narrowly de- 

 fined limits of temperature and humidity. Further and still more 

 conclusive proof is found in the fact that deaths, which all physi- 

 cians recognize as by far the most sensitive index to the general 

 health of a community, are least frequent when the conditions of 

 the air approach closely to the optimum for physical activ- 

 ity. Moreover, not man alone, but every plant and animal that 

 has yet been carefully tested possesses the same close adaptation 

 to certain distinct conditions of climate. Such an adaptation 

 among all living creatures appears to be the inevitable result of 



