344 ROSCOE R. HYDE 



INTRODUCTION 



This paper, which is the third of the series on ''Fertihty and 

 SteriUty in Drosophila ampelophila,"" considers the evidence that 

 bears on the effects of crossing on fertility when : (1) Stocks from 

 the same source (descended from the same grandparents) sepa- 

 rately inbred are crossed; (2) Stocks from different sources sepa- 

 rately inbred are crossed; (3) When the mutations, having pink 

 eyes and white eyes are crossed with each other and with a wild 

 stock which has been inbred (brothers and sisters) for 26 gener- 

 ations. 



The evidence so far presented goes to show that some of these 

 stocks have, up to the time the present experiments were carried 

 out, lost in fertility on inbreeding. The evidence, however, does 

 not show conclusively that the loss is due to the process of inbreed- 

 ing as such. The fact remains, that the different strains did up to 

 this time lose in fertility on continuous inbreeding, and it will now 

 be shown what effect out-crossing has on these reducedstrains. It 

 is to be recalled that 'fertility' is here used in the sense of eggs 

 that reach maturity. 



There are some experimental data that bear out the belief that 

 the crossing of different races or strains is beneficial to the fertility 

 of the stock itself. This relation has been demonstrated beyond 

 any doubt in the second part of this paper in so far as it applies to 

 the truncate mutant for the fertility of the female was more than 

 doubled when mated to a wild stock, while the fertilizing powers of 

 the truncate male were more than three times as great with a wild 

 female as compared to his own mate. 



I wish now to consider the evidence that bears on the question 

 as to whether or not a rise in fertility will result when one strain of 

 Drosophila is crossed with any other strain. The evidence deals 

 especially with the fertility of two mutations and two wild stocks 

 that originally came from two different sources. It will facilitate 

 treatment of this subject to give a brief resume of the history of 

 these stocks. 



