I4O F. B. SUMNER, M. E. MCDANIEL AND R. R. HUESTIS. 



King and Stotsenburg likewise believe that there is a relation 

 between breeding activity and the sex ratio. It is stated that 

 "the rat breeds more readily in the spring than in any other 

 season of the year, and there is a second, less pronounced, period 

 of sexual activity in the early fall," while relatively fewer litters 

 are produced in July and August. Their graphs show low sex 

 ratios in the spring, followed by high ones in the summer, and 

 low ones again in the fall. As already stated, the results for 

 winter are contradictory with one another. 



Our records for Pcromyscus are unfortunately not adapted to 

 revealing definite periods of increased or diminished reproductive 

 activity, since the matings were to a large extent controlled in 

 accordance with the demands of the breeding experiments. 1 But 

 it is to be noted that the seasonal fluctuations which we have found 

 in the sex ratio of Peromyscus correspond as little with those re- 

 ported by Heape as they do with those reported by King and 

 Stotsenburg. The relations which Heape believes to be shown 

 by the Cuban statistics, even if applicable to man in that particular 

 locality, cannot be generalized for all mammals nor for all places. 



It is worth remarking at this point that the differences just 

 discussed, between our findings and those of certain other writers, 

 are typical of the conflicting results which pervade the entire 

 literature of sex determination. 



SIZE OF THE BROODS. 



The 1,567 broods here recorded ranged in size from i to 9, 

 though we have only nine records of broods containing over 6 

 young (seven of 7, one of 8, and one of 9). The mean size of 

 all these broods was 3.22. This figure naturally varied somewhat 

 according to the year (2.57 to 3.67), the season (2.96 to 3.53), 

 and the race (2.76 to 3.78), etc. 



The size of the litter will first be considered in relation to its 

 possible influence upon the sex ratio. This part of our discussion 



i The numbers born in November, December and January are well below 

 those born in any other months of the year, and it is likely that this fact is 

 due, in part, to an actual slowing down of reproduction, at least under the 

 conditions of our experiments. But it is questionable whether any other seas- 

 onal differences in fecundity can be fairly inferred from our records. 



