CASTLE. — INBREEDING, CROSS-BREEDING, AND SELECTION. 743 



was usually nearer 300, occasionally rising to 400 or even higher. The 

 M series shows a few sporadic cases of sterility ; the N series, none 

 whatever. The M series shows on the whole a rise of the average during 

 the period under discussion ; the N series, a fall. With the subsequent 

 history of these series we are not at present concerned. Comparison of 

 the three experiments shows, that in Drosophila close inbreeding during 

 fourteen successive generations is not necessarily attended by either de- 

 creased productiveness or sterility, but that particular degrees of produc- 

 tiveness are characteristic of particular races. Subsequent experiments 

 indicate that the productiveness of series M and that of N, during this 

 period, are much nearer the normal than that of series A. A more 

 complete account of generations 1-14 of the M and N series will be 

 found on page 757. 



Other Families of Drosophila. 



In the summer of 1903 observations were made on the productiveness 

 of flies obtained from two other sources. A stock, Z>, was reared from 

 eggs found in a banana purchased in a Cambridge fruit store. Another 

 stock, F, was formed from flies captured about a garbage can at a dis- 

 tance of half a mile from the laboratory, and at more than a quarter 

 of a mile from the fruit store. 



Four pairs from the D stock produced broods numbering respectively 

 164, 204, 304, and 241 young, the average number being 228. Six 

 pairs from the F stock produced broods which were only in part recorded. 

 The smallest number credited to a brood was 85 ; the largest, 220. It 

 is probable, therefore, that the productiveness of this stock was similar to 

 that of stock D, though perhaps a little lower. Both stocks, D and F, 

 resemble M and N, rather than A, in productiveness. 



Effects of Cross-breeding. 



The question now arises, if a stock characterized by low productive- 

 ness, like A, is crossed with a normal stock, what will be the produc- 

 tiveness of the offspring ? Further, if Drosophila is indifferent to a 

 considerable amount of inbreeding, will it be indifferent also to cross- 

 breeding? To throw light on these questions, a number of crosses with 

 the A stock have been made at various times. 



Experiments of A. H. Clark. 



In March, 1903, when the A series was producing an average of 43J 

 young to a pair, two females from the A stock were mated with males 



