CHAPTER IX 

 GROWTH AND LONGEVITY 



The growth of any organism is, of course, the result 

 of many interdigitating factors. An animal to increase 

 in size must have food to supply building material. It 

 must have a body temperature which permits anabolic 

 reactions to proceed at such a rate that they exceed the 

 destructive changes which are essential for living from 

 day to day. It must be free from parasites or diseases 

 which sap its vitality. Growth also depends on a con- 

 tinual balance between an animal and its environment. 

 Food must not only contain material but must furnish 

 a proper balance of calories, amino acids, and vitamins. 

 Environmental temperature should not only fall within 

 the limits where metabolism and growth may occur, but, 

 to permit an animal to really flourish, should vary so 

 that growth receives healthful stimulation. Growth is 

 a complex phenomenon which through metabolic wasting 

 and waxing results in a net increment in size. 



Rate of Growth. — The rate of growth of animals is 

 usually progressively slower with increasing age (Brody, 

 1927). There may be seasonal fluctuations or variations 

 associated with progression through the life cycle, but 

 in general the metabolic rate gradually runs down. Some 

 animals grow rather slowly and at a rather constant rate, 

 and others increase largely during intermittent periods 

 of building. 



Rubner (1924) in his admirable studies on metabolism 



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