342 HELEN DEAN KING 



An examination of table 4 shows that all of the litters produced 

 in the first generation group of the A series were smaller, on the 

 average, than corresponding litters in the later generation groups. 

 The relatively low fertility of the animals in the early generations 

 was not due to inbreeding, but to the fact that these individuals 

 suffered from malnutrition. As soon as the nutritive conditions 

 were improved there was at once an increase in the number and 

 in the size of the litters produced, as the data for the fifth and for 

 the sixth inbred generations show (table 1). 



As indicated in the last column of table 4, the groups compris- 

 ing the tenth to the twenty-fifth generations of the A series 

 showed, as a whole, comparatively little variation in the average 

 size of the litters. The maximum average size (7.8) came in the 

 group including the fifteenth to the eighteenth generations. This 

 maximum was, however, only 0.1 greater than the average litter 

 size for the preceding and for the following group, and therefore 

 it can have little, if any, significance. 



Litter data for various generation groups in the B series of in- 

 breds are shown in table 5. 



As the average size of the litters produced in the first gener- 

 ation group was greater than that in the second group (table 5), 

 it might appear that the fertility of the breeding females in the 

 B series was not lessened by malnutrition. In the beginning of 

 these experiments many more females of the B series than of the 

 A series were completely sterile, but the females of the B series 

 that did breed were the more productive. Malnutrition, in this 

 instance, was a selective agent that helped to eliminate the tend- 

 ency to sterility in the B series by preventing the breeding of any 

 except the most fertile females. 



In the B series the maximum average size of the litters was 

 found in the group comprising the nineteenth to the twenty- 

 second generations, but, as was the case in the A series, this max- 

 imum was not great enough to be considered significant. 



Litter data given in table 4 and in table 5 have been combined 

 in table 6. 



The data for each of the two inbred series, as well as that given 

 in table 6, shows that in all generation groups the first litter cast 



