FERTILITY AND STERILITY IN DROSOPHILA 355 



the proper combinations were made. It seems probable in the 

 case of fertility, as in many other characters, that inbreeding gives 

 a chance for defects to be brought to the surface; and that low 

 fertility is likely to accompany close inbreeding provided it is not 

 guarded by rigorous selection. When a stock has reached a low 

 degree of fertility it seems strange that that same stock should be 

 able to rise again in fertility. Yet this is exactly what may happen. 

 Take for example the truncate fly which has been selected for 75 

 generations and has its fertility reduced to about 20 per cent ; and 

 yet that fly can throw a form, the long wings, the fertility of which 

 is more than twice as great as its truncate brothers and sisters. 



The fact that different individuals, brothers and sisters of the 

 same stock, should differ in such a marked degree (so that one is 

 actually able to separate the more fertile ones from the less fertile 

 flies by inspection) is submitted as evidence to show how selection 

 may operate in controlling the fertility in these strains. 



BIBLIOGRAPHY 



Darwin, C. Cross and self-fertilization in the vegetable kingdom. 

 Morgan, T. H. The origin of five mutations in eye color in Drosophila and their 

 modes of inheritance. Science, vol. 33. 



