112 INBREEDING AND OUTBREEDING 



indication of reduction in size when inbred flies were 

 compared with random mated stock reared under the 

 same conditions. Far from being exterminated by in- 

 breeding, however, the flies at the end of the experi- 

 ment were apparently fully equal to those with which it 

 was begun,. 



These experiments showed clearly that inbreeding 

 results in strains of unequal fertility. The less fertile 

 tended to be eliminated by differential productiveness, so 

 that only the more fertile remained. The occurrence of 

 absolute sterility was pronounced in the first part of the 

 experiment, but almost entirely disappeared in the later 

 generations. The figures as calculated from their table 

 are as follows : 



Per cent. 



of matings 



totally sterile 



Generations 6 to 24 17.80 



Generations 25 to 42 18.47 



Generations 43 to 59 3.37 



Such a result is to be expected when it is remembered 

 that inbreeding produces homozygous individuals, and 

 these, whenever sterile, are, of course, eliminated. 



Moenkhaus,^^^ Hyde,^^ and likewise Wentworth,^^^ 

 by similar inbreeding experiments with Drosophila 

 found sterility, though increased in the first stages of 

 inbreeding, tended to be eliminated after the process 

 was long continued. 



The only other experiments on invertebrates which 

 ought to be cited here are those of Whitney ^^^ and A. F. 

 ShuU ^^^ on the rotifer Hydatina senta. Both of these 

 investigators found that inbreeding had a considerable 

 adverse effect on the size of family, number of eggs laid 



