744 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



from the stock-jar mentioned on page 735. Their broods of offspring 

 numbered 43 and 24 respectively, but the latter may have been incom- 

 plete, as the mother died within fourteen days after the culture was 

 started. At any rate, there was no indication of any increased productive- 

 ness by A females on account of their being mated with males from 

 a different stock. Two pairs, representing a cross reciprocal to the fore- 

 going (i. e., 9 from stock-jar X £ A), produced broods numbering 91 

 and 132 respectively, i. e. two or three times as many young as the 

 first mentioned crosses yielded. As this was about the usual relation of 

 broods of normal series to those of the A series, it seems that the produc- 

 tiveness of the female is unaffected by the productiveness of the race to 

 which her mate belongs. Subsequent experiments support this idea. 

 But whether the offspring of a female would be affected in productiveness 

 by the cross is an entirely different question. A single experiment was 

 made at that time which bears on this matter. A pair taken from the 

 132 young produced by the cross, 9 from stock -jar X Ji, yielded, in 

 April, 1903, 116 young. The A series produced at this time an average 

 of 32.4 young to a brood. The productiveness of the daughter was in 

 this case similar to that of her mother, rather than that of the A race 

 from which the father came. 



Experiments of W. E. Castle. 



More extensive experiments in crossing the A stock were made in 

 July and August, 1903, parallel with generation 27 of the A series, which, 

 it should be remembered, produced an average of 128 young to a brood, 

 under the favorable summer conditions. 



Cross la. D ? x A d. 



Av. for fertile pairs, 129 young. 



Pair 1 

 Pair 2 

 Pair 3 

 Pair 4 

 Pair 5 



sterile. 

 337 young. 

 315 " 

 206 " 

 163 " 



Av. for fertile pairs, 255 young. 



In Crosses 1 and la, as in the previously described experiment, 

 females of each race apparently had their productiveness unaffected by 

 the cross-mating; for A females produced an average of 129 young by 

 D mates, and 128 young by A mates. Exactly one pair in five is 

 fruitless in each sort of mating. On the other hand, D females pro- 



