EFFECT OF EXTERNAL TEMPERATURE 511 



blooded animal by a study of the response of different animals 

 to changes in the temperature of their surroundings ; the con- 

 sequent variations in the respiratory exchange indicate the 

 changes in the production of heat. Several interesting ques- 

 tions now arise. What are the relative advantages of the 

 different effects produced by changes in external temperature 

 upon the respiratory exchange of warm-blooded and cold-blooded 

 animals during their struggle for existence ? Is it possible to 

 reduce the warm-blooded animal to the condition of a cold- 

 blooded one ? 



It has already been shown that life, both animal and vegetable, 

 is accompanied by constant chemical change, the production of 

 heat and gaseous interchange. The living tissues require a high 

 internal temperature for the proper manifestation of their acti- 

 vities ; the warm-blooded animals maintain this favourable con- 

 dition, an optimum temperature, by a constant struggle against 

 cold and heat ; the cold-blooded animals have not acquired this 

 power, and within certain limits the activity of their tissues is 

 determined by the external temperature. Both of these conditions 

 are advantageous to the respective groups of animals in the 

 struggle for existence ; the warm-blooded animals are able to 

 live a more ardent life, unbroken by long periods of inactivity 

 or torpor, and within wide limits are independent of the tempera- 

 ture of their surroundings. The constant exercise of the different 

 organs of the body, especially during the search for food in cold 

 weather, leads to a greater development of their nervous system 

 and intelligence. The cold-blooded animal, on the other hand, 

 depends for its food supply upon the external temperature ; a 

 period of torpidity with the activities of its tissues reduced to the 

 lowest ebb consistent with the retention of life tides it over the 

 cold winter months, when food is scarce, and prolongs its life 

 beyond one^ brief summer. It has already been mentioned that 

 even some of the mammals, hibernating animals such as the 

 marmot, hedgehog, dormouse, and bat, have retained this ancestral 

 characteristic for their better preservation in the struggle for 

 existence. 



The intensity of life is dependent upon a high and constant 

 temperature, and the power to maintain this temperature is asso- 

 ciated chiefly with the control of the nervous system over the 



