192 Experimental Zoology 



times as great and showed no signs of sterility." The productive- 

 ness of the A-series was not due to inbreeding, but was inherent 

 in this stock from the beginning. That this is the explanation of 

 the results is shown by comparing the A-series with other inbred 

 series; and also by the sudden rise in productiveness of the 

 A-series at the end of the third year. 



Two other inbred series, M and N, were from the beginning 

 more productive than the A-series. The average of eggs never 

 fell below 200, except in a single generation, and was usually 

 nearer 300, occasionally rising to 400 or more. The productive- 

 ness of this series, carried through 28 generations, is shown in the 

 next diagram. No sterile individuals w^re found in the N-series 

 and only one in the seventh, one in the tenth, and one in the four- 

 teenth of the M-series. In the M-series "the average brood of 

 the first inbred generation was 213, while the average for the 

 series as a whole is 280, an apparent increase of fertihty under 

 inbreeding. In the N-series the first generation gave an aver- 

 age of 231 young, while the series as a whole gave 278.5 young to 

 a brood." In both cases the first inbred generation may have 

 happened to be lower than the average because of less skillful 

 treatment. Hence a fairer comparison is between the first and 

 the second half of the series, thus : — 



This comparison shows an increase in the productiveness in the 

 M-series and a decrease in the N-series. 



An interesting experiment consisted in crossing the M- and 

 the N-series with the A-series. After crossing the two broods 

 the offspring wxre mated inter se (brother and sister) for several 

 generations in order to observe the effects on subsequent genera- 

 tions. The results showed that when a female of the A-series is 

 mated with a male of the M- or N-series, her productiveness is not 

 increased. The daughters, however, produce more young than 



