72 THE AMERICAN NATURALIST [Vol. LII 



tain body size, which depends upon the rate at which the 

 individual grows, as well as the limiting size for that in- 

 dividual. That is, size at a given age is the result of rate 

 multiplied by time, up to certain limiting values deter- 

 mined by the genetic composition. Cessation of growth, 

 however, does not necessarily coincide with the onset of 

 sexual maturity nor even with general bodily maturity. 

 Although it is certain that the hen is heavier in her second 

 autumn than at the beginning of egg production, our data 

 show that there is little or no growth during the first 

 winter. We must, then, distinguish between sexual ma- 

 turity, which is capable of manifesting itself as soon as 

 the body reaches a certain size, from that maturity which 

 is not attained until long after the adult size is reached. 

 At present the relation between sexual maturity and bod- 

 ily maturity has not been worked out. Some extreme 

 phases, however, of the interrelation appear a priori 

 probable. Chicks that grow very rapidly naturally tend 

 to reach sexual maturity at a very early period in their 

 life. They may or may not start in laying immediately 

 after reaching full size. Other birds grow very slowly 

 and can not lay before a certain size is reached. There- 

 fore, they must of necessity reach sexual maturity rela- 

 tively late in life. It may be impossible for birds of this 

 sort to reach sexual maturity before spring if hatched 

 during the usual breeding season (April, May). The 

 general effect of slow growth, then, will be to lower the 

 record made by such individuals, although they may be 

 otherwise identical with those that grow more rapidly. 



Combined with the factors mentioned are the factors 

 that limit the size finally reached. As pointed out above, 

 size results from rate of growth times length of period 

 through which growth continues. Each factor is deter- 

 mined in part by the environment and in part by the 

 genetic constitution of the bird. 



The following combinations of factors (Table I) and 

 their effect on egg production may be assumed. Each 

 factor is treated as though it were wholly independent 



