38 HOMOIOTHERMISM 



are correspondingly less important. A monse requires 

 mucli more food per gram of body weight than an ele- 

 phant, but the differences between the food requirements 

 of a little lizard and a giant crocodile are less significant. 



As metabolism has been and is of prime importance 

 in the origin and continuance of homoiotherms this chap- 

 ter is devoted to a brief consideration of the factors 

 which are chiefly concerned with its variations in rate. 

 In this connection age, temperature, respiration, food, 

 glands, desiccation, rhythms, and nervous control will be 

 considered. 



Age. — In all animals the rate of metabolism probably 

 varies more or less at different ages. The fertilization 

 of a sea urchin egg is soon followed by an increase in 

 metabolic rate. As development proceeds the rate may 

 increase or decrease. The development of a yolk-laden 

 fish egg may proceed slowly until there are mechanisms 

 for transporting food and then progress at a more rapid 

 rate. When inert substances are being assimilated or 

 when profound reorganizations are taking place in the 

 bodies of animals, the metabolic rate may be slow. A 

 Drosophila pupa shows a decrease in its metabolism dur- 

 ing its second day of development, then the rate gradu- 

 ally increases up to the time of the emergence of the 

 adult fly (Bodine and Orr, 1925). Various arthropods 

 and chick embryos show differences in rate of heart beat 

 and respiratory movements which are characteristic of 

 particular ages (Crozier and Stier, 1927a; Henderson, 

 1927). 



In youth a fish (Knauthe, 1898) or a mammal 

 (Lusk, 1917) has a high rate of metabolism and this 

 gradually grows slower, with some variations at such 

 times as the annual spawning or the age of puberty. On 

 the other hand the metabolic rate of amphibian tadpoles 



