INTRODUCTION 3 



They are now specialized for a peculiar mode of life, a 

 life characterized by many mechanisms for maintaining 

 miiformity in temperature, metabolic rate, salt and or- 

 ganic content of the blood, and reaction of body fluids. 

 This specialization has been associated with the migra- 

 tion of vertebrates from marine and fresh water habitats 

 to land. Animals which live in the ocean are surrounded 

 by a medium which is quite constant in its composition 

 and resembles the blood of vertebrates in its salt content. 

 This medium in any particular locality does not vary 

 rapidly or extremely in temperature or in its content of 

 dissolved gases. Freshwater animals live in habitats 

 which are often limited in extent and vary widely in 

 temperature and in content of dissolved substances. 

 Land animals inhabit a medium which is very constant 

 in composition and is rich in oxygen, but varies extremely 

 in temperature. They are in continual danger of desic- 

 cation and therefore require a continual supply of water. 



The homoiothermic condition is an adaptation for 

 living a life under constant conditions in a highly vari- 

 able environment. It has enabled birds and mammals to 

 become dominant in the land habitats on the earth. Some 

 homoiotherms, like the whales and sea birds, have been 

 able to return to the home of their remote ancestors and 

 have again attained a considerable degree of success in 

 the ocean. 



This book is concerned with the physiological and eco- 

 logical aspects of the evolution of poikilothermic into 

 homoiothermic animals. In it we attempt to discuss evi- 

 dence for the origin of homoiotherms, temperature regu- 

 lation, the factors influencing rate of metabolism and 

 growth, and other matters which relate to the adjust- 

 ments associated with evolution from erratic, more or 

 less intermittent, living to uniform, constant living. 



