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. 1 At 26 to 30 C. the frogs 

 become active and restless. At 31 to 33 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. 



