162 



RAYMOND PEARL 



TABLE 3 



Showing the -percentage of the whole flock producing in the winter period more than 

 certain specified numbers of eggs, in the case (a) of Barred Plymouth Rocks and 

 {b) of Cornish Indian Games 



THE INDICATED PERCENTAGE OF THE FLOCK PRODUCE.? 

 IN THE WINTER PERIOD 



6 or more eggs, 



12 or more eggs 



18 or more eggs 



24 or more eggs , 



30 or more eggs 



36 or more eggs 



42 or more eggs 



48 or more eggs 



54 or more eggs 



60 or more eggs, 



66 or more eggs 



72 or more eggs 



78 or more eggs 



84 or more eggs 



90 or more eggs 



96 or more eggs 



102 or more eggs 



108 or more eggs 



114 or more eggs 



120 or more eggs 



tion cycle. These added periods of productivity are what may be 

 called (cf. 37, 28, 30) the winter cycle and the summer cycle. 

 The winter cycle is the more important of these. It is the best 

 practical measure of relative fecundity which we have and has 

 been used as the chief unit of fecundity in these studies. It con- 

 stitutes a distinct and definite entity in fecundity curves. The 

 existence of this added fecundity, in high laying birds must de- 

 pend upon some additional physiological factor or mechanism 

 besides that which suffices for the normal reproductive egg pro- 

 duction. Given the basic anatomical and physiological factors 

 the bird only lays a large number of eggs if an additional factor 

 is present. 



As to the nature of this physiological mechanism we can only 

 speculate. It probably involves fundamentally such matters as 



