INHERITANCE OF FECUNDITY 173 



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 enviironmental influence. Allow- 

 ance for these factors must be made in interpreting and classifying 

 results. In particular the following points must be kept in mind 

 throughout. 



(1) A zero winter production may be due to genetic causes or 

 to purely somatic (physiological) ones, and there is nothing in a 

 single record of this sort, taken by itself, to indicate to which 

 category it belongs. A bird may carry the factor or factors for 

 winter production, yet owing to purely physiological causes, such as 

 a disturbance of metabolism, or of the ovary in respect to its physi- 

 ology, or to disease, patent or obscure, it may never actually lay 

 during the winter period. Usually it will be possible to tell from 

 other considerations than the record itself, whether a given zero 

 record is 'somatic' or 'genetic' 



(2) The upper limit of the winter period at March 1 is arbi- 

 trary, and only approximately coincides with the biological 

 reality. Actually with most birds the spring or reproductive 

 cycle of production (cf . 37) begins in the latter part of February. 

 In handling the material it has been found necessary (for reasons 

 which will be obvious upon consideration of the matter) to take 

 a fixed date for the beginning of the spring cycle of laying and the 

 ending of the winter cycle. The records of the Station prior to 

 1908 are tabulated only for months (the daily records unfortu- 

 nately having been destroyed before I took charge of the work) , 

 and on this account it is necessary to take the workmg limit of 

 the winter cycle at the end of a calender month. Since March 

 1 comes the nearest to the biological limit of any date which is also 

 the beginning of a calendar month it has been chosen. The error 

 introduced by taking this arbitrary date for a point which really 

 shifts within rather narrow limits is, on the average, small. How- 

 ever, it must be recognized as a disturbing element in the individ- 

 ual case. Thus, some birds which really lack any genetic factor 

 for winter production will begin to lay in the last days of Febru- 

 ary, and consequently on the arbitrary 'March 1' basis will 

 actually be credited with a small winter production. This will 



