FERTILITY AND HATCHING OF EGGS. 153 



11. There is no evidence that the character "fertihty of 

 eggs" (measured by per cent, of eggs infertile) is in any degree 

 or manner inherited. 



12. The character "hatching quahty of eggs" (measured by 

 per cent, of fertile eggs hatched) is definitely inherited in the 

 female line and probably also in the male line. 



In considering these results as a whole there are certain mat- 

 ters of general significance which need some further considera- 

 tion. In the first place, taking all the results of the paper 

 together it is evident that fertility and hatching quality of eggs 

 are very different characters. While there are great individual 

 differences among different females in respect to the fertility 

 of their eggs, even when mated to the same male, it still remains 

 the fact that this character, as compared with hatching quality 

 of eggs, is to a very large degree influenced by external circum- 

 stances. Thus we have seen that the same relative degree of 

 fertility is not characteristic of the same bird in two successive 

 seasons ; nor is this character affected by winter egg produc- 

 tion. It is not inherited. 



On the other hand, the hatching quality of eggs is an innate 

 constitutional character just as much intrinsic as any other 

 physical character such as shape of body or length of limb. 

 Relatively the same intensity or degree of this character is per- 

 sistent in the same bird in successive breeding seasons. It is 

 adversely affected by heavy winter egg production. It is 

 inherited. 



These facts raise the question as to what the hatching quality 

 of eggs depends upon. We have used as a quantitative meas- 

 ure of this quality the percentage of fertile eggs hatched. But 

 this measurable quantity depends on underlying innate biological 

 factors. As to what these factors //; detail are, data are lack- 

 ing. It will, however, be of some value to attempt to list such 

 general factors as are known to have some bearing on the case. 

 At the start of such a list it can probably be safely said that 

 any factor which tends to reduce or impair the general constitu- 

 tional vigor of breeding birds in general tends also to reduce 

 the hatching quality of the eggs from these birds. The relative 

 "condition" or vigor of breeding birds may be im])aired in 

 variety of ways. For example, improper feeding may bring 

 about this result. Houssay (1907) in his very thorough study 



