The data now in hand, however, indicate clearly, it seems to me, that there must be some other factor than the anatomical one involved in the existence of different degrees of actual fecundity in the domestic fowl. It evidently is the case that when one bird has a winter record of twice what another bird has it is not because the first has twice as many oocytes in the ovary. On the contrary, it appears that all birds have an anatomical endowment entirely sufficient for a very high degree of fecundity, and in point of fact quite equal to that possessed by birds which actually accomplish a high degree of fecundity. Whether or not such high fecundity is actually realized evidently depends then upon the influence of additional factors beyond the anatomical basis. As has already been indicated in the preceding section, it is reasonable to suppose that these factors are physiological in nature.
The Mechanism of the Inheritance of Fecundity as Measured by Winter Egg Production
A study of numerous statistics shows that hens fall into three well defined classes in respect to winter production. These classes include (a) those birds which lay no eggs whatever in the winter period (up to March of the laying year); (b) those that lay but have a production during the period of something under about 30 eggs; and finally (c) those whose production exceeds 30 eggs in the winter period. The division point between the two latter classes is not sharply defined in every case, but it is plainly at about 30 eggs in the case of the breeds and strains used in these experiments. Since in the analysis some fixed point must be taken for this boundary, a production of 30 has been chosen for this purpose and will be used throughout. This is an arbitrary choice only in the sense that it is a convenient round number lying very near where the biological division point falls, at least in the strains of domestic fowls used in these experiments. The analysis could doubtless be carried through nearly or quite as well by taking the division point at a production of 29 or 31, but 30 is a more convenient figure.
In making the division of winter egg production into three groups it must be remembered that this is a character subject to purely somatic fluctuations and environmental influence. Allowance for these factors must be made in interpreting and classifying results.
Turning now to the symbolic analysis, we have to deal with three factors. These are:
1. An anatomical factor. This is basic. It consists in the presence of a normal ovary, the primary organ of the female sex. In the genetic analysis a separate letter need not be used for the designation of this factor, but instead it will be understood to be included in the letter denoting the presence of the female sex or its determiner. That is,