• THE INHERITANCE OF FECUNDITY 373 



zygous or heterozygous form causes about the same degree of low 

 fecundity to be manifested. 



5. That one of these factors, namely L 2 , is sex-limited or sex-corre- 

 lated in its inheritance, in such way that in gametogenesis any gamete 

 which bears the female sex determinant F does not bear L 2 . 



6. That there is a definite and clear-cut segregation of high fecun- 

 dity from low fecundity, in the manner set forth above. 



These conclusions are fully and independently substantiated by 

 long-continued breeding experiments involving the breeding together 

 of (1) Barred Plymouth Rock males and females (a breed of generally 

 high fecundity), (2) Cornish Indian Game males and females (a breed 

 of generally low fecundity, (3) the F x and F 2 offspring from reciprocal 

 crosses of Barred Plymouth Eocks and Cornish Indian Games and all 

 possible matings inter se and with the parent forms of the cross-bred 

 F 1 and F 2 offspring. 



While these results may have no direct eugenic bearing, they do, I 

 believe, have an important indirect connection with eugenic problems. 

 In the first place, these results furnish a novel conception of the mode 

 of inheritance of fecundity. They show that this highly variable physi- 

 ological character is inherited in accord with simple Mendelian prin- 

 ciples. They further show that simple selection of highly fecund 

 females alone is not sufficient to ensure high fecundity in the race. 



From the eugenic standpoint they suggest, though of course they do 

 not prove, that possibly some part of the observed decline in human 

 fecundity in highly civilized races may be due to the dropping out or 

 loss of one or more of the genes upon which high fecundity depends, 

 this loss being coincident with the complete cessation of the natural 

 selection of highly fecund types. 



Finally, these results on fecundity in fowls not only emphasize the 

 importance of analytical studies to determine the precise mode of in- 

 heritance of human fecundity, but they also furnish a guide and stimu- 

 lus for the conduct of such studies. If, as in the actual fact, it can be 

 shown that in one animal belonging to the same great phylum to which 

 man himself belongs (the vertebrate) fecundity is inherited in a simple 

 Mendelian fashion, it encourages one to hope that some time a solution 

 of the same problem may be reached for man. It at least points the 

 way to a mode of attacking this complex problem which gives greater 

 promise of leading ultimately to a solution than does any method which 

 has hitherto been applied to it. 



