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 



14 







'12 



1,0 



! s 







*6 



4 



7 



Nov. Dec. Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. 



FIG. 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 hi 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- 



