R. Pearl.Biology and Eugenics.57 
4. That either of these factors when present alone, whether in 
homozygous or heterozygous form, causes about the same degree of low 
fecundity to be manifested. 
5. That one of these factors, namely L2, 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 L2. 
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 Fx and F2 offspring from reciprocal crosses of Barred 
Plymouth Rocks and Cornish Indian Games and all possible matings 
inter se and with the parent forms of the cross-bred Fx and F2 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 simpile 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 is 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 simple Mendelian fashion, 
it encourages one to hope that sometime a solution of the same problem may 
be reached for man. It at least points the way to a mode of attacking this 
compilex problem which gives greater p<romise of leading ultimately to a 
solution than does any method which has hitherto been applied to it.