314 THE AMERICAN NATURALIST [Vol. LII 



to either of PearPs factors, for early-maturing birds 

 almost invariably lay more than the required number of 

 eggs even when their production is interrupted in some 

 way. Since, however, the theory demands that both 

 genetic factors be present for high production, the assign- 

 ment of one factor to rate and the other to maturity can 

 not be made. 



The Shape of Winter Egg Production Curves 

 The curves of winter egg production are clearly com- 

 pound curves, probably belonging to the "S" type de- 

 scribed by Pearl and Surface for monthly egg produc- 

 tion in Barred Plymouth Rocks. 



In the case of the winter egg production curves, it can 

 be shown that this type of curve is due primarily to the 

 variability in age at first egg, plus the date at winch the 

 census of the flock is taken. The curve, however, is 

 modified somewhat by the variability in rate and by the 

 fact that the birds were not all hatched at the same time. 

 If a flock of birds were all hatched on the same day and 

 all laid at some uniform rate, say an egg per day, it is 

 clear that, on any given date between the date the first 

 pullet commenced to lay and the date the last one began, 

 the frequency polygon showing the number of eggs pro- 

 duced would consist of a zero class plus the remaining 

 portion of the polygon, beginning at class 1 and proceed- 

 ing through classes 2, 3, 4, etc., to the upper end of the 

 range. Now the number of birds laying one egg apiece 

 would depend upon the date chosen for the census and 

 would be the number of birds that reached a given age on 

 that date as shown by the curve of age at first egg. Thus, 

 if Fig. 1 be taken as our standard, and some date early 

 in the season be chosen for the census, say the date on 

 which the flock becomes 186 days of age, the zero com- 

 ponent of the egg curve would have a value of 99 per 

 cent, and the 1-egg class a value of .6 per cent. At mid- 

 season (256 days) the zero component would have an 

 ordinal value of 55.3 per cent, and the 1-egg class a value 



