48Section I.R. Pearl. 
The progressive decline of the birth rate in all, or nearly all, civilized 
countries is an obvious and impressive fact. Equally obvious and much 
more disturbing is the fact that this decline is differential. Generally it is 
true that those racial stocks which by common agreement are of high, if not 
the highest, value, to the state or nation, are precisely the ones where the 
decline in reproduction rate has been most marked. 
The causes concerned in the production of these results are, without 
question, exceedingly complex and difficult, if not impossible, of complete 
analysis. But of one thing we may be certain; somewhere in the complex 
of causes is included the biological factor as one element. Fecundity and 
fertility axe physiological characters of the organism, subject to variation 
and capable of being inherited, just in the same manner as structural 
characters. We must be in possession of definite information regarding the 
physiology of fecundity and fertility, before it will be possible to make 
safe and sure advance in the social and eugenic analysis of matters involving 
these factors, such as, for example, the declining birth-rate. 
The basic eugenic significance of that characteristic of organisms termed 
fecundity furnishes sufficient justification, I hope, for bringing to the 
attention of this Congress certain results regarding fecundity in one of the 
lower animals, namely the domestic fowl. In some particulars the results 
are, I believe, novel. They indicate, for the first time, the precise mode by 
which this complex physiological character fecundity is inherited. It will 
be the purpose of this paper to present—necessarily very briefly and without 
the detailed supporting evidence—the essential results of a study of 
fecundity in poultry, pointing out at the end some possible eugenic bearings 
of the results.* 
During the course of this investigation into the inheritance of fecundity 
in the domestic fowl, which has now involved thirteen generations and 
several thousand individuals, two definite and clear-cut results have come to 
light. These are :— 
First.—That the record of egg production or fecundity of a hen is not, 
of itself, a criterion of any value whatsoever from which to predict the 
probable egg production of her female progeny. An analysis of the records 
of production of large numbers of birds shows beyond any possibility of 
* The results set forth below were first presented at the meeting of the American Society 
of Naturalists at Princeton, N.J., in December, 1911. A complete report with full pre­ 
sentation of the experimental data will shortly be published, probably in the Journal of 
Experimental Zoology.