Page:Popular Science Monthly Volume 81.djvu/379

Rh 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-correlated 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 fecundity 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 $$F_1$$ 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 physiological character is inherited in accord with simple Mendelian principles. 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 inheritance of human fecundity, but they also furnish a guide and stimulus 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.