EFFECTS OF INBREEDING ON FERTILITY AND VIGOR 347 



physical condition of the animals. During the summer months 

 rats suffer severely from excessive humidity and from high tem- 

 perature, since their mechanism for heat regulation, under these 

 conditions, is inadequate. At this season their sexual activity 

 is at its lowest point, and the litters that are produced tend to be 

 relatively small. Severe cold checks reproduction, but litters 

 born under these conditions are usually of normal size and the 



8 9 10 II, 12 13 14 16 16 J., 17 18 19 20 S( 22 23 24 25 



Fig. 1 Graph showing the average size of the litters produced in the various 

 generations of the inbred strain (data in table 3). 



individuals strong and vigorous. At about every fourth genera- 

 tion the majority of litters produced in the inbred strain were 

 born at the most unfavorable season of the year, the summer and 

 early fall. In this generation, as the records show, the litters 

 were smaller, as a rule, than the litters in the preceding and in the 

 following generations. This decrease in size was sufficient to 

 account for all of the depressions in the graph in figure 1, except 

 the first one, which was doubtless due to the fact that the maxi- 



