GROWTH AND LONGEVITY 73 



grow to the full extent of its capabilities. A particular 

 species which occurs in two lakes only a few miles apart 

 may be uniformly of small size in one lake and large in 

 the other (Pearse, 1918; Pearse and Achtenberg, 1920). 

 In nature, animals in particular habitats may never be 

 able to exercise their full capacity to grow. Peterson 

 (1918) found that in one locality off the coast of Den- 

 mark the plaice did not grow for two-thirds of a year, 

 but if individuals were transferred to another locality 

 they grew at normal rates. Sundstroem (1922) kept 

 mice in an ^* artificial tropical" environment where there 

 was little circulation of air and where other conditions 

 were monotonous. He found that they did not grow as 

 well as mice reared where conditions were variable. 



Methods of Growth. — While in general percentage 

 additions become less and less as an animal grows, there 

 are usually more or less intermittent periods of rapid 

 and slow growth which are characteristic of species and 

 of individuals. Young tissues grow fastest and perhaps 

 old animals grow more slowly because they come to con- 

 tain a considerable bulk of inert, differentiated, and com- 

 paratively stable tissues. *' Accompanying differentia- 

 tion there is, therefore, a decrease in the activities of 

 metabolism'' (Beer, 1924). An animal may, however, 

 show an increased rate of metabolism during degrowth, 

 or decrease in size. 



Kobertson (1923) in his comprehensive work on 

 growth observes that most animals increase in size by 

 spurts with periods of slower growth between. He be- 

 lieves this is due to an autocatalyzed process. He found 

 that there were usually three growth cycles. Eobertson 

 believes that the rate of growth reactions depends on 

 (1) the specific reaction velocity of the process, (2) the 

 concentration of autocatylist, (3) the concentration of 



