THE SELECTION PROBLEM IN ANIMAL BREEDING 505 
45.2, those birds having the highest. breeding value which have the highest 
selection index. By this method it is possible to substitute for a vague 
personal impression of breeding value, an exact numerical expression 
which is an accurate measure of the breeding value of any individual. 
It is possible to devise such selection index numbers for other purposes, 
and they should prove of utility in practical breeding operations. 
Another line in which still further necessity for strictly scientific 
analysis is exemplified is that of detailed study of curves of production. 
Thus Pearl and Surface have made a detailed biometrical study of the 
seasonal distribution of egg production in domestic fowls. From this 
study they find that the polygon of monthly egg production is of the form 
shown in Fig. 195. They find that with pullets the normal season of egg 
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Mean Egg Production 
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Noy, Dec, Jan, Feb. Mar, Apr, May June July Aug, Sept. Oct, 
Fic. 195.—Diagram showing the weighted mean monthly egg production for each month 
of the pullet year. (After Pearl and Surface.) 
production begins in November. The mean rises rapidly during the 
following 2 months, but in February there is a characteristic slackening 
up in egg production. In March and April egg production is at a maxi- 
mum, and after that it decreases fairly regularly until it reaches a mini- 
mum in October, with the exception of a slight, but significant, indenta- 
tion in May. These data taken together with certain other facts which 
have been determined during the course of the Maine Station investi- 
gations of egg production indicate that the laying year may be broken 
up into four periods which correspond broadly with natural cycles of 
egg production in the domestic fowl. The first of these periods begins in 
November and ends at about March 1. The end of this winter-laying 
period is marked rather definitely in the curve of annual egg production 
by the distinct slackening of increase in egg production during February. 
The winter period of laying is in a sense an added period for it is not nor- 
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