408 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 31 



without the aid of artificial means for keeping the body temperature 

 within suitable limits. 



The advantage of a mechanism to free the organism from the 

 effects of still another variable of its environment need not be further 

 stressed. In analogy with the possible coimection between certain 

 of the chemical characters of the living organism and the composi- 

 tion of the medium in which it lived at different stages of its devel- 

 opment, it might be suggested that the constant temperature of 

 warm-blooded animals is also a perpetuation of conditions which 

 prevailed while this mechanism was being developed. But even such 

 scanty data as those on which the previously mentioned suppositions 

 have been based are in this case lacking. 



The living organism is constantly liberating energy, part of which 

 appears as heat. To preserve a constant body temperature it must, 

 therefore, maintain a balance between the rates at which heat is lost 

 and produced. This balance can be maintained by the exercise of a 

 control over one or both of these rates. As one of the principal 

 objects of adaptive mechanisms seems to be to protect the organism 

 against adventitious variations of the rate of its activities, it might 

 be expected that the control of temperature would be effected by 

 regulation of the rate of heat loss rather than by variation of the 

 rate of heat production. This expectation has, to a considerable 

 extent, been realized in the warm-blooded animals which have been 

 studied from this point of view in sufficient detail, the dog, and man. 



It has been found that when these animals are examined under 

 comparable states of activity, the rate at which they produce heat is 

 affected only to a minor extent, even by considerable variations of 

 the temperature of their surroundings. Not only is the organism 

 of these animals able to restrict the loss of heat from their bodies 

 when the external temperature falls, but they are also able to con- 

 tinue to lose heat to their surroundings, even when the external tem- 

 perature is above that of their bodies. There are, of course, limits 

 beyond which this mechanism becomes ineffective. If the external 

 temperature rises too high, or if the conditions of humidity are such 

 as to restrict unduly the loss of heat by evaporation, then heat pro- 

 duction will exceed heat loss, and the body temperature will rise. 

 The organism is incapable of decreasing its production of heat below 

 a certain value even under these circumstances. 



If the external temperature falls to a low enough level, the mech- 

 anism for regulating body temperature by controlling the heat loss 

 also becomes ineffective. The body under these circumstances, how- 

 ever, does not lose its power of maintaining a constant temperature, 

 because it is able to increase its heat production until its heat loss 

 is again balanced. The object of the constant body temperature, the 



