GROWTH AND LONGEVITY 71 



mones and endocrine secretions play an important role, 

 especially in liomoiotherms. Larger animals in general 

 require less energy than smaller, on account of surface- 

 mass relations. Fishes have little physical regulation of 

 body temperature because they live in water. Amphib- 

 ians commonly regulate their body temperature through 

 their lungs and skin. The growth of homoiotherms and 

 poikilotherms is similar. Energy requirements depend 

 on size and rate of growth, and rate of growth in many 

 cases depends on the temperatures at which metabolic 

 reactions take place. 



Mammals may differ among themselves in rate of 

 growth. Brody (1928) has recently made careful studies 

 of several domestic animals and compares them with 

 man. ' * The length of the juvenile period in man is about 

 10 years (4 to 14 years). This relatively enormous 

 length of the juvenile period appears to be the most dis- 

 tinguished feature of the growth curve of man. ... In 

 the curve of man, the major inflection (at puberty) oc- 

 curs when the body weight is, roughly, two-thirds of the 

 mature weight ; in other animals it occurs when the body 

 weight is, roughly, one-third of the mature weight. . . . 

 There are no radical quantitative or qualitative differ- 

 ences between the growth curves of man and other ani- 

 mals during the phase of growth following puberty. . . . 

 If the percentage rate of growth for a given group of 

 children is relatively low during earlier ages, then there 

 is usually an acceleration between 12 and 15 years; if 

 it is high, there is no acceleration. The pubertal accel- 

 eration, when present, appears to be in the nature of a 

 compensatory growth for earlier deficiency. ... It 

 should be said that growth in length takes place at an 

 approximately constant time rate when growth in weight 

 takes place at a constant percentage rate. This is evi- 



