THE TEMPERATURE OF THE BODY 1307 



producing heat and thus raising its temperature above that of its 

 surroundings, one might expect to find that the higher the external 

 temperature the greater would be the difference between this and the 

 temperature of the animal, until finally the latter would rise to such 

 a height that the animal would die of heat-stroke, its tissues being 

 destroyed by the actual temperature attained. A certain protection 

 is afforded to most cold-blooded terrestrial animals by the fact 

 that their surface is moist, and that with a rise of external tempera- 



* External temp. C. 



FIG. 542. Effect of alterations in the temperature of the surrounding medium 

 on output of C0 2 in cold-blooded (poikilothermic) animals. (C. J. MABTIN.) 



ture the rate of evaporation on the surface increases, so that the 

 increase in the rate of cooling by evaporation more than corresponds 

 to the rate of increase in the heat production, which would tend 

 to raise the body temperature. Most of these animals, how- 

 ever, escape from any extreme rise of external temperature by 

 burrowing underground or taking to the water, while in plants a 

 rise of external temperature assists transpiration to such an extent 

 that the temperature of the plant is generally several degrees below 

 that of the surrounding atmosphere. The extreme variability 

 in the metabolism of such animals implies a state of dependence of 

 all the activities of the body on the environment, which would pre- 

 vent the utilisation to the full of the available sources of energy. 

 An animal whose metabolism was more or less independent of the 

 surrounding temperature must have a great advantage over an 



