352 ROSCOE K. HYDE 



The second question under consideration is; What is the effect 

 on fertility when stocks from different sources separately inbred 

 are crossed? The relations expressed by the right hand side of 

 diagram A throw some light upon this question. In two of the 

 combinations high fertility brought the low fertility up to its level 

 while in four of the combinations there is an appreciable rise in 

 fertility beyond that shown by the parents, although by no means 

 as high as one might expect from the history of the crosses made 

 with the truncates in Part 11. But since the rise in fertility occurs 

 here in the combinations expressed by the right side of the diagram 

 and not those on the left it looks as if the rise in fertility in this 

 case is significant. It rhust be admitted, however, that the rise in 

 fertility in these crosses is not great enough to base a final conclu- 

 sion upon, in regard to a point as far-reaching as this. It is to be 

 remembered that Woods Hole, Massachusetts, is only four miles 

 distant from Falmouth and it is altogether probable that the two 

 strains had not been separated by many generations when taken 

 into captivity. Consequently, this material would present after 

 all a picture very much like the first case. If the environment can 

 influence the different strains in respect to the 'factors' that bring 

 about fertility the influence in this case has been slight, and, after 

 all, from stocks so closely related this is probably what we should 

 expect to find. In any case it is certain that the high-producing 

 stocks can bring the fertility of the low-producing stocks to their 

 level whether descended from the same or from different germ 

 plasms. It might seem from this (although I am far from con- 

 tending at present that such is necessarily the case) that there is 

 a set of factors of some sort for fertility and that when a loss in 

 fertility occurs in a stock (other than that which occurs in the case 

 of mutations) it is the same set of factors that is lost or is changed 

 and this accounts for the fact that there is no rise in fertility on 

 crossing. The stock that has the highest fertility (the largest 

 number of factors) acts as a dominant character and brings the 

 lower set of factors up to its level. In the case of a sudden rise in 

 fertility on crossing such as occurs in the case of some of the muta- 

 tions it is probable that the stocks have lost different factors for 



