34 HELEN DEAN KING 



When the reproductive period is long, litters cast at its be- 

 ginning and near its close tend to be small. Increased fertility 

 leads not only to the production of a greater number of litters, 

 but also, in most cases, to the casting of larger litters during 

 the height of reproductive activity. 



The summary of data in table 9 is shown by the graph in 

 figure 10, which indicates a very symmetrical distribution of 

 the various litter groups around the modal point. 



Litter size in captive grays, as in other varieties of rats, 

 is not affected by the season of the v'ear in which birth 



Fig. 10 Litter size in the second to the twenty-sixth generation. 



occurs (King, '27). In this respect, findings for the rat 

 accord with those for the mouse (Parkes, '26 b), the pig 

 (Machens, '15; Carmichael and Rice, '20; Parkes, '26 a) and 

 the dog (Dighton, '22). 



Since a previous investigation has shown that the age of 

 the mother has a pronounced effect on both litter production 

 and litter size in rats (King, '16 a), a brief analysis of the 

 series of data for gray rats with reference to this point is 

 given here. Data for litters cast in the second to the twenty- 

 sixth generation, arranged according to the age of the mothers 

 at the time of parturition, are shown in table 10. 



