BREEDING POULTRY FOR EGG PRODUCTION. 1 29 



From the table and diagram the following points are to be 

 noted : 



1. As would be expected, the percentage mortality is seen 

 to fluctuate in amount from year to year. These variations are 

 without doubt to be accounted for by differences in general en- 

 vironmental factors in different years, and to accidents.* 



2. The general trend, however, of the percentage mortality 

 is plainly dozmiward during the period covered by the experi- 

 ment in mass selection. That is, during the period from 1899 

 to 1908, when the egg production was showing a slightly down- 

 ward trend the adult mortality was also distinctly diminishing. 



3. There is no evidence from the figures of adult mortality 

 to indicate either that artificial incubation and rearing or any 

 other environmental condition adversely affected the constitu- 

 tional vigor of the strain during the course of the mass selec- 

 tion experiment, or (b) that in such action is to be found in 

 the ex:planation of the failure of that experiment to result in 

 increased annual average egg production. 



The percentage mortality figures are given here only for the 

 period covered by the selection experiment in order that direct 

 comparison may be made between the trend of the egg produc- 

 tion during that period (see fig. 80, p. 157) and the mortality 

 curve. It may be said, however, that since the laying year 

 i907-'o8 the mortality of adult birds has fluctuated about the 

 same average as during the three years preceding. During the 

 laying, year 1910-11 the adult mortality has been quite excep- 

 tionally low. 



Having now considered adult mortality as an index of gen- 

 eral vitality and constitutional vigor attention may next be 

 turned to the records of the hatching quality of eggs and the 

 mortality of chicks. It needs but little practical acquaintance 

 with poultry for anyone to recognize that in these two things 

 exists one of the most precise measures of the general vitality 

 or constitutional vigor of a strain or flock that it is possible to 

 get. If a relatively- large percentage of eggs hatch, and the 

 chickens are strong and vigorous, and only a small proportion 

 of them die it puts the question of the vitality of the stock 

 beyond cavil. 



*Cf. (2^. pp. T6-TC. 



