68 THE PHILOSOPHY OF BIOLOGY 



cosmopolitan marine animals are the warm-blooded 

 whales. Heat-production in the mammals and birds 

 is therefore a direct object of the metabolism of the 

 animal ; it is a means whereby the latter acquires a 

 more complete mastery over its environment. That it 

 is not necessarily a step in the transformation of 

 chemical into mechanical energy we see by consider- 

 ing the metabolism of the cold-blooded animals. In 

 these poikilothermic organisms the body preserves 

 the temperature of the medium. The temperature in 

 such animals may be a degree, or a fraction of a degree, 

 higher than that of the environment, but, in the absence 

 of exact calorimetric experiments, we cannot say what 

 proportion of the energy of the food of these animals 

 passes into unavailable food energy. Probably it is 

 a very small fraction of the whole, and we are thus 

 justified in saying that in the cold-blooded animal 

 chemical energy does not, to a significant extent, 

 become transformed into heat. The result is, of course, 

 that the vital processes in these organisms keep pace, 

 so to speak, with the temperature of the environment, 

 since the chemical reactions of their metabolism are 

 affected by the external temperature. We find there- 

 fore that hibernation, the formation of resting stages, 

 and a general slowing down of metabolic processes 

 are more characteristic of the cold-blooded animal 

 during the colder seasons than of the warm-blooded 

 animal. The former has not that mastery over the 

 environment attained by the mammal or bird. 



The metabolism of the animal therefore resembles 

 the energy process of the heat-engine only in the 

 general way, that in both series of transformations 

 chemical energy descends from a condition of high 

 potential to a condition of low potential, transforming 

 into mechanical energy in so doing, and thus perform- 



