INHERITANCE OF FECUNDITY IN DOMESTIC FOWI,. 359> 



At the outstart it should be noted that in spite of the fact 

 that as many F- birds were hatched and reared as the available 

 facilities would permit, nevertheless, the number of adult daugh- 

 ters available for fecundity study is small in case of some of 

 the matings. There are several reasons for this. Besides the 

 obvious one such as mortality, depredations of thieves, hawks, 

 crows, rats and the like, there is another important but not so 

 obvious one. This is the failure or great difficult}^ experienced 

 in getting certain of the F2 cross breds to grow into normal, full- 

 sized, healthy adult birds. After rather wide experience in 

 handling cross-bred chicks, I am convinced that certain gametic 

 combinations which are to be expected on Mendelian theory, and 

 can be produced in the expected numbers in the breeding pen,, 

 are nevertheless physiologically abnormal or unsound. Such 

 birds do not make a normal growth, but in spite of the best 

 care and attention grow up into stunted weaklings, which always 

 show, both in th.eir structure and their physiological economy, 

 the effect of this retarded, abnormal development. I am further 

 convinced that this result is primarily due to the hereditary 

 constitution of the individuals in question. Certain combina- 

 tions of hereditary factors do not produce physiological sound- 

 and vigorous zygotes. 



Of course, there is nothing novel in such a result. It is of a 

 piece, for example, with the parts respecting the relation be- 

 tween hereditary constitution and physiological vigor in maize,, 

 which have been so clearly set forth and anah^zed by Shull 

 (45, 46, 47) and East ('9). Other examples of the same phe- 

 nomenon might be cited. The whole phenomenon is precisely 

 what would be expected from Johannsen's general conception of 

 inheritance and ontogeny (22). 



This relationship between hereditary constitution and physio- 

 logical constitution or normality takes on particular significance 

 wnen one is dealing with fecundity. As has been pointed out 

 earlier in this paper one cannot expect to get a normal somatic 

 expression, of the hereditary constitution in respect to fecundity 

 unless the bird is a physiologically normal, well-developed indi- 

 vidual. Stunted, under-developed, or physiologically unsound 

 birds will lay but very few if any eggs, regardless of what fe- 

 cundity factors it may carry. A marked difference is here ap- 

 parent between structural and physiological characters so far as 



