258 EAYMOND PEARL 



be individually identified, and in the second place those that 

 were killed and carried off by predaceous enemies (chiefly crows, 

 hawks, and rats). 



The mortality is considered in two groups according to the 

 age at death. In the first group are included all individuals 

 dying before attaining the age of 180 days; in the second group 

 all mortality at higher ages. Finer grouping might of course 

 be made but for present purposes seems unnecessary. Six 

 months covers practically the whole of the life on free range 

 as a chick. To that age all individuals are given every oppor- 

 tunity to live if they are able to do so. 



It has not been thought necessary to deal "with each indi- 

 vidual mating separately in table 5. Instead the matings of a 

 particular class are grouped together. Table 6 gives the mor- 

 tality, in 1915, of the chicks from three different sets of matings 

 of normal, untreated parents, to serve as controls on the treated 

 matings of table 5. The first set of controls consist of the ran- 

 dom sample of 22 matings in which the female was a pure Barred 

 Plymouth Rock. These are the same 22 matings that were 

 used in table 2. The second set of control data includes all Fi 

 matings, regardless of the breeds crossed, which were made in 

 1915. The degree of heterozygosity of the chicks from these 

 matings is the same as that of the chicks of treated parents dealt 

 with in table 5. The third set of data for control comparison 

 consists of the mortality results from 22 selected matings of 1915, 

 the basis of the selection being the amount of mortality. The 

 22 matings included were the ones showing the lowest mortality 

 losses of all the matings of the year. 



From these tables we note the following points: 



1. Taking the grand total figures from table 5 it appears 

 that the average percentage of mortality during the first six 

 months of life was distinctly lower in the case of chicks, one or 

 both of whose parents were treated, than in the case of any chicks 

 from normal, untreated parents, except as compared with those 

 coming from the selected best matings. These selected best 

 matings are practically identical with the treated matings in 

 respect of average mortality, differing by less than 2 per cent. 



