298 HUMAN BIOLOGY 



all. But if the human race were not protected by clothing, 

 shelter and fire, it would almost certainly fail to reproduce 

 itself wherever the temperature falls far below the freezing 

 point for any great length of time. 



The Hmits imposed by humidity are clear enough at high 

 temperatures, but fade away at the most favorable tem- 

 peratures. If warm air is completely saturated, the absence 

 of evaporation and the consequent difficulty which the 

 body experiences in coohng itself make it doubtful whether 

 the human species could keep on reproducing itself even 

 though other conditions were propitious and the temperature 

 no higher than 90°f. In Japan at the end of the hot damp 

 summer the conceptions which result in living births are less 

 numerous than the deaths. How much of this is due to high 

 temperature and how much to excessive humidity it is 

 impossible to say, but humidity is of decisive importance, 

 for similiar temperatures with moderate humidity do not 

 produce any such results. If humidities and temperatures 

 like those of the summers in Japan and along the coast of 

 South China persisted indefinitely the inhabitants would 

 presumably diminish in numbers until natural selection 

 had eliminated all who were unable to endure extreme 

 humidity at the ordinary summer temperatures. Slightly 

 higher temperature and humidity might easily prevent 

 all reproduction. 



We are not yet sure whether man is excluded from any 

 part of the earth by the direct effect of a lower limit of 

 humidity, although he is obviously excluded by the indirect 

 effects upon water supplies and vegetation in places like 

 the uninhabited southern part of the Arabian interior. 

 In Death Valley, even when one does not feel uncomfortably 

 hot, the dryness of the air makes one uncomfortably thirsty 

 practically all the time. One drinks till his stomach is 

 seriously distended, and yet is never satisfied, for the moisture 

 content of the tissues cannot be kept normal. Whether such 

 conditions would permanently prevent the reproduction 

 of the human race we do not know. At lower temperatures 

 the bad effects of extreme aridity diminish and there is no 

 evidence that even complete dryness would in itself prevent 

 human existence, provided food and water were available. 



