56 THE BIOLOGY OF THE FROG chap. 



surface, and the cold acts indirectly as a stimulus to increase 

 metabolism and causes the combustion of the bodily fuel 

 to proceed at a more rapid rate and thereby to compensate 

 for the loss of heat by radiation. By virtue of this mechan- 

 ism the higher animals are able to keep in an active con- 

 dition in both winter and summer. With the cold-blooded 

 animals it is different. Their temperature rises and falls 

 in correspondence with the temperature of their environ- 

 ment. In the cold their metabolism is slow, their tem- 

 perature runs down, and consequently they become sluggish 

 and inactive. As it becomes warmer, their temperature 

 rises, their metabolism increases, and they become more 

 active and alert. A lizard which is made almost stiff when 

 the weather approaches the freezing point becomes the 

 most agile of creatures in the sunshine of a hot day. 



The temperature of the cold-blooded animals is not, how- 

 ever, entirely at the mercy of the environment. Evaporation 

 from the surface of the body tends to keep the temperature 

 of the animal in warm weather below that of the surround- 

 ing atmosphere.^ And as the weather approaches the 

 freezing point, the small amount of metabolism in the ani- 

 mal serves to keep its temperature somewhat above that of 

 its environment. 



The effect of high temperatures on the frog has been 

 studied by Maurel and Lagriffe.^ At 26° to 30° C. the frogs 

 become active and restless. At 31° to ;^2'° C. they show 

 evident signs of discomfort. From 34° to 36° C. they jump 

 about wildly without any apparent sense of direction. At 

 a temperature of 37° to 39° C. they lose their sense of equi- 

 librium, and if exposed to a temperature of 39° to 40° C. 

 they die. If, however, they are exposed only for a short 

 time at the latter temperature, they may subsequently re- 



1 Maurel and Lagriffe, Comp. rend. Soc. Biol. Paris, torn. 52, 1900. 



