. 
192 Experimental Zoology ‘ 
times as great and showed no signsof 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 were 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 fertility 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: — 
| GENERATION 1-7 GENERATION 8-14 
M-series 263 296 
N-series 317 240 
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 were 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 
