THE NEW EVOLUTION Wi 



is sleeping from the effect of excessive heat and in 

 the other from the effect of excessive cold, but they 

 are the same in that in both cases the plants and 

 animals are sleeping over a period when they are 

 unable to obtain sufficient water. 



Temperature affects directly only such animals as 

 are so very delicately balanced that they require a 

 fixed and usually high degree of heat for the main- 

 tenance of their internal chemical reactions. In this 

 category fall the most active vertebrates, the mam- 

 mals, birds and reptiles. 



In the mammals and birds the body is insulated 

 from the temperature changes in the air about it by 

 a layer of air which is held in place by a covering of 

 hair or feathers. Their body temperature is high and 

 constant and, excepting in the monotremes or egg- 

 laying mammals, and in hibernating mammals, it is 

 quite independent of the outer temperature. The 

 bodies of whales are insulated from the temperature 

 of the surrounding water by a layer of fat, while the 

 seals have both hair and fat. 



Reptiles require a relatively high temperature, but 

 have no mechanism for controlling it. Therefore all 

 of the more active and all of the larger reptiles are 

 tropical or subtropical, only the smaller representa- 

 tives of less active types, the turtles and the snakes, 

 occurring in the colder regions. The amphibians are 

 all relatively inactive and the frogs, toads and sala- 

 manders range far into the colder regions. 



Among the other forms of life temperature seems 

 to have the most effect in delimiting the activity of 



[5^] 



