AND PERSONALITY 



plasm will disintegrate in heatstroke, and if it burns so low 

 that the temperature falls much below the normal his 

 protoplasm will be killed by freezing, since, according to 

 van't HofFs law, each degree centigrade of temperature 

 change is followed by a corresponding 10 per cent rise or 

 fall in chemical, hence electric, activity. Therefore, it is 

 between these two dead lines that the brain, the thyroid 

 gland, and the heart of man, like a bird balancing itself on 

 a wire during a storm, maintain their equilibrium. 



In the preceding pages we have called attention to the 

 fact that evolution took from man the hairy covering of 

 the ape. A rabbit loses heat 35 per cent faster when shaved 

 of its normal furry covering. Therefore, man, shorn of the 

 furry covering of his arboreal ancestor, could sustain 

 greater activity without succumbing to heatstroke than the 

 ape. 



Evolution has also endowed man with the greatest surface 

 area of any other animal of his size. Thus the long legs and 

 arms of man, the hands and long fingers permit a relatively 

 larger heating mechanism brain and thyroid gland for 

 man than for any other warm-blooded land animal of 

 comparable size without death from heatstroke. 



In addition, evolution endowed man not only with the 

 power to modify his internal heat through sweating but 

 also with a vasomotor mechanism that transfers blood to 

 the surface for cooling. The horse and man are the only 

 animals that sweat all over. 



These mechanisms evolved for adaptation to internal 

 heat have enabled man, with his larger-than-ape brain and 

 thyroid gland, to survive the stagnant warmth of the tropics 

 as well as the cold of the ice age and to make alternating 

 adaptation to the hot summer and the cold winter of the 

 north temperate zone. 



The thinking brain and the versatile hand constitute the 

 greatest gift of evolution to man, for the brain and the 

 hand gave to man the substitute for hair, fur, fat, and 



239 



