346 HELEN DEAN KING 



growth was attributed to a stimulation of the growth processes 

 produced by adequate nutrition following a period of semi- 

 starvation. During this period the productiveness of the females 

 was increased considerably, since the average size of the litters 

 in the group comprising the seventh to the tenth generations was 

 0.4 greater than the average for the previous generation group 

 (table 6). The period of maximum fertility in the inbred series 

 did not, however, coincide with the period of maximum growth in 

 body weight, but came at a much later time (fifteenth to the 

 twenty-second generations), when the litters contained 7.7 young, 

 on the average. The fact that the average size of the litters in 

 the last three generations of the inbred series was slightly lower 

 than the maximum can be attributed to a change in diet made 

 necessary by the economic conditions of the present time. This 

 diet does not seem to be quite as favorable to growth and fertility 

 as was the more varied diet used until the beginning of last year. 



The graph in figure 1, showing the average size of the litters 

 produced in the various generations of the inbred strain, was con- 

 structed from the data in the last column of table 3. 



Starting at the point of 6.8, the graph in figure 1 drops at the 

 third generation to 5.0, the lowest point in its course. From this 

 point it rises slowly, and after the fifth generation tends to be a' 

 fairly horizontal line, since it never falls below 6.9 nor does it rise 

 above 7.9. At more or less regular intervals the graph drops 

 slightly below the normal level. The most pronounced depres- 

 sion is at the point of the third generation; a second drop comes 

 at the ninth generation; other depressions of about the same depth 

 are found at the point of the sixteenth, the twentieth, and the 

 twenty-fourth generations. As the last three depressions in the 

 graph occur at intervals of four generations, it is evident that they 

 were not due to a chance variation in the data, but that they must 

 express periodic changes in the reproductive cycle of the females 

 that tended to reduce the number of young born. In w^hatever 

 way this reduction was effected, whether by a lessening of fecun- 

 dity or by limiting the number of embryos that were capable of 

 normal development, the cause for it, I believe, lay in the seasonal 

 changes in temperature which always have a marked effect on the 



