26. THE INFLUENCE OF CLIMATE 

 UPON MAN 



Two basic biological principles have limited the evolu- 

 tion of man in the tropics. The first principle is Berthe- 

 lot's law. Because of the operation of this law, protoplasm, 

 like the nitroexplosives, is detonated at a fixed temperature 

 level; therefore it is the sensitive nitro group in protoplasm 

 that limits the temperature in warm-blooded animals. It 

 would follow that the human being who evolved in the 

 tropics, with its intense external heat, would be limited to a 

 smaller brain, a smaller thyroid gland and blood volume 

 than the human being who became adapted to the colder 

 temperatures. As a corollary, the average basal metabolism 

 of man of the tropics is lower than that of man of the 

 temperate zones, and the basal metabolism of man of the 

 temperate zones is lower than the basal metabolism of man 

 of the arctic. 



The second biological principle that has interfered with 

 the evolution of the thinking brain to higher levels in 

 man in the tropics as compared with man in the temperate 

 zones is the principle of struggle and survival. The thinking 

 brain, like other organs, owes its size solely to its survival 

 value. In the tropics, where food, shelter, and clothing are 

 at the lowest requirement for man, the thinking brain has 

 the lowest survival value, in contrast to the highest survival 

 value in the temperate zones, where there is an abundance 

 of food in the summer that the thinking brain of man must 

 cultivate and store against the lifeless winter. 



What about man in the arctic? In the arctic, with the 

 extreme cold, evolution centered upon protection of the 

 warm-blooded state by lessening the surface area through 

 an evolution of a short thick-set body in contrast to the 

 usual tall, lithe, fat-free native of Africa. The brain of the 

 Eskimo is relatively larger than any other human brain. 



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