72 HOMOIOTHERMISM 



dent from geometrical considerations when growth in 

 length is strictly terminal. It is also evident from physi- 

 ological considerations; constant percentage rates of 

 growth in volume and constant time rate of terminal 

 growth both imply that the physiological environment 

 with respect to the growth-limiting process remains con- 

 stant." MacDowell, Allen, and MacDowell (1927) have 

 compared the growth rate of the monse, guinea pig, and 

 chick. They conclude that ** growth of different animals 

 may be compared more accurately if, instead of either 

 birth age or conception age, embryo age is used." 



It is difficult to compare the growth of homoiotherms 

 and poikilotherms and often difficult to compare that of 

 animals in the same class or order. A guinea pig or a 

 snake is able to shift for itself soon after birth ; a robin 

 or a child has a long youthful period during which it is 

 nourished and protected by its parents. 



As has been stated, some types of animals grow at a 

 rather constant rate ; others grow in spurts ; and others 

 grow rapidly during youth and then more slowly. A 

 painted turtle {Chrysemys belli) doubles its weight and 

 length during its second year of life, but after twelve 

 years increases at one-thirtieth the rate for the first two 

 years (Pearse, 1923; Pearse, Lepkovsky, and Hintze, 

 1925a). The sea turtle, Caretta caretta, at hatching 

 weighs 20 grams; at 43^4 months, 800 grams; and at 3 

 years, 19 kilograms (Parker, 1926). The Pacific striped 

 bass grows 150 per cent in length during its second year, 

 but during its twelfth year of life, only two per cent 

 (Scofield, 1928). A carp may weigh 500 grams when it 

 is two years of age and then gain 750 grams between 

 the beginning of May and the middle of August, when it 

 may attain a weight of 1250 grams (Piitter, 1909). How- 

 ever, a fish living in natural conditions often does not 



