124 How Animals Changed 



fined to situations where the environment prevents rapid changes. 

 The soil, leafy ground litter, rotting logs, and similar refuges are 

 commonly sought during cold or dry periods by primitive terrestrial 

 animals. 



Homoiothermism is perhaps the most important adaptation that 

 animals have made to terrestrial life. By developing for living cells 

 a favorable internal environment which remains quite constant 

 chemically and thermally, animals have thus become more or less 

 independent of environment (Pike 6C Scott, 1915; Pearse dC Hall, 

 1928) . The continual, rapid metabolism of homoiothermic animals 

 has the disadvantage that large quantities of food must be available. 

 The development of nutritious land plants was associated with and 

 to some extent conditioned the development of homoiothermic land 

 animals (Berry, 1920) . A mouse at the same environmental tem- 

 perature produces twenty-five times as much heat per day as a fish 

 of the same size (Rubner, 1924) . A poikilothermic animal may live 

 for years without food, but a homoiothermic animal dies of starva- 

 tion in a few months. 



Benedict (1932) has made an extensive study of the differences 

 between poikilothermic and homoiothermic animals. He finds that 

 the temperature of the latter is higher than that of the former at 

 any environmental temperature. A poikilothermic vertebrate warmed 

 to 37 °C. produces only one-third to one-eighth as much heat as a 

 homoiothermic vertebrate. Benedict, after examining various possi- 

 bilities, concludes that differences between the two arc not due to 

 differences in protoplasm, chemical constitution, or metabolism, 

 though poikilothermic animals do contain more water and ash, less 

 dry matter and active protoplasmic tissue than homoiothermic ani- 

 mals. The heart rate is higher and there is more blood in homoio- 

 thermic animals. "It is therefore believed that the distribution of 

 the blood in the tissues may explain the difference in the metabolism 

 of these animals. . . . Where there is a liberal supply of blood to the 

 tissues heat production can be high. Where the blood supply is low, 



