Full text: papers communicated to the first International Eugenics Congress held at the University of London, July 24th to 30th, 1912

R. Pearl.Biology and Eugenics.49 
doubt that, in general, there is no correlation between the egg production 
of individuals and either their ancestors or their progeny. 
Second.—That, notwithstanding the fact just mentioned, fecundity is, 
in some manner or other, inherited in the domestic fowl. This must clearly 
be so, to mention but a single reason, because it has been possible to isolate 
and propagate from a mixed flock “ pedigree lines ” or strains of birds 
which breed tiue, generation after generation, to definite degrees of 
fecundity. Some of these lines breed true to a high condition or degree of 
the character fecundity; others to a low state or degree. 
Definite as these results are they give no clue as to how fecundity is 
inherited; what the mechanism is. It is believed that now a first 
approximation to the solution of this problem has finally been reached. 
While there remain obscure points yet to be cleared up, and more data are 
needed definitely to decide between certain alternatives, yet the results now 
in hand appear to indicate quite clearly the general character of the 
mechanism of the inheritance of fecundity, and to show what lines further 
investigation of the problem may most profitably take. 
At the outstart it will be well to understand clearly what is meant by the 
term fecundity as here used. I have used the term “ fecundity ” only to 
designate the innate potential reproductive capacity of the individual 
organism, as denoted by its ability to form and separate from the body 
mature germ cells. Fecundity in the female will depend upon the 
production of ova and in the male upon the production of spermatozoa. 
Fecundity is obviously a character depending upon the interaction of 
several factors. In the first place the number of ova separated from the 
body by a hen or any other animal must depend, in part at least, upon an 
anatomical basis, namely the number of ova present in the ovary and 
available for discharge. Further there must be involved a series of 
physiological factors. It has been possible to prove that the mere 
presence of an anatomically normal reproductive system, including a normal 
ovary with a full complement of ova, and a normal oviduct, is not enough 
to insure that a hen shall lay eggs, that is, exhibit actual as well as potential 
fecundity. While comparatively very rare, cases do occur in which a bird 
possesses a perfect ovary and perfect oviduct and is in all other respects 
entirely normal and healthy, yet never lays even a single egg in her life­ 
time. Such cases as these prove: first, that what we may call the 
anatomical factor is not alone sufficient to make potential fecundity actual; 
and second, that the anatomical and physiological factors are distinct, in 
the sense that the normal existence of one in an individual does not neces­ 
sarily imply the co-existence of the other in the same individual. 
Turning now to the physiological factors involved in fecundity it would 
appear that there are at least two such factors or groups of factors. The 
first of the physiological factors involved may be designated the “ normal 
ovulation ” factor. By this is meant the complex of physiological E

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