VARIATION IN DOMESTIC ANIMALS 



457 



ing investigations in egg production have been carried on for about 

 20 years. An attempt was made during the period from 1899 to 

 1907 to increase egg production by a system of mass selection. For 

 breeding stock, pullets were selected which had produced 160 or more 

 eggs during the first year of production, and they were mated to males 

 the mothers of which had produced 200 or more eggs in their first year of 

 production. The results of this rigid system of selection are shown 

 graphically in Fig. 185. The solid line connecting the dots represents the 

 line of actual average winter egg production of the entire flock during 

 the period from 1899 to 1915. The fitted straight line for the period 

 1899 to 1907 shows clearly that during this period of mass selection there 

 was an actual decrease, rather than an increase, in average winter egg 

 production. Pearl and Surface point out that certain environmental 

 factors may have had something to do with this decrease in production, 

 but even when such factors are accorded a fair maximum effect, the 

 evidence favors the interpretation that this system of mass selection 

 has had absolutely no effect upon the average winter production of eggs. 

 That this conclusion is a sound one is also supported by correlation 

 studies of egg production of mothers and daughters in these flocks. In 

 Table LIX the winter egg productions of 192 daughters are entered 



TABLE LIX. CORRELATION BETWEEN DAUGHTERS AND MOTHERS WITH RESPECT 



TO WINTER EGG PRODUCTION, r = 0.068 + 0.048 (Data of Pearl and Surface) 



MOTHER'S EGQ PRODUCTION 



