748 PROCEEDINGS OF THE AMERICAN ACADEMY. 



The three crosses, 5, 6, and 7, show a result entirely normal for the 

 A. series ; and the reciprocal crosses, ba and 6a, show a result normal for 

 the M and N series. In each case the productiveness of the mother is 

 unaffected by the character of her mate, precisely as in crosses 1-4, and 

 their reciprocals. Pairs of cross-bred individuals produced by crosses 

 5-7, when mated brother with sister, showed a marked increase in 

 productiveness as compared with that of their respective inbred mothers. 



Average Mother's Average Daughter's 

 Brood. Brood. 



Cross 5 42 187 (omitting three sterile pairs). 



Cross 6 45 229 



Cross 7 62 275 



In the crosses reciprocal to 5 and 6, viz. ba and 6a, the mothers came 

 from races unusually fertile and not extensively inbred ; no increase of 

 fertility was observable among their offspring as a result of a cross with 

 the A series. 



The cross-breds, like their mothers, showed a high productiveness. 

 This was probably neither increased nor diminished materially by the 

 cross with the unproductive A race. The apparent falling off on the part 

 of the young produced by Cross ba, from 447, the average mother's 

 brood, to 353, the average daughter's brood, is due to the unusual size 

 of the maternal average. This in turn was due to the occurrence of a 

 single very large brood, 597, in Cross ba, the other broods numbering 

 369 aud 379 respectively, which is not very different from the average 

 daughter's brood, 353. While the cross-breds from Cross ba were 

 producing broods of this size, the uncrossed M race was producing broods 

 averaging 273. While the cross-breds from Cross 6a were producing 

 broods averaging 274 in number, the uncrossed Wrace produced a single 

 complete brood of 277 young. We conclude that in the early generations 

 of the M series and up to the ninth generation at least of the N series, 

 the inbreeding had not materially affected the productiveness of those 

 races, for, among other reasons, a cross with an unrelated and more 

 extensively inbred race (A) did not affect the productiveness of the 

 cross-breds, which remained normal, like that of the pure M or traces 

 respectively. 



