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
