R. Pearl.Biology and Eugenics.53 
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 (up1 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 F will denote the 
presence of the ovary or the 9 sex determiner. Then f will denote the 
absence of femaleness and the absence of an ovary. Obviously a separate 
letter is not needed for this “ anatomical factor ” since the presence of an 
ovary is the objective criterion of the existence of the female sex, and its 
absence of the existence of the male sex. 
(2) The “ first production ” factor. This is the primary physiological 
factor which in coexistence with F makes the bird lay eggs during the winter 
period. Quantitatively it may be taken as determining a winter production 
of more than zero eggs and less than 30. The presence of this factor will 
be denoted by Lx. 
(3) The “ second production ” factor. This is a second physiological 
factor, which in coexistence with F and Lx leads to high fecundity. The 
presence of this factor will be denoted by L2 and its absence by the 
corresponding small letter. When F and Lx are present the addition of