No. 511] 



SELECTION INDEX NUMBERS 



393 



which died within three weeks from the date of 

 hatching. 



A brief discussion will make clear the reasons why 

 these particular variables are chosen for the index and 



A bird's value as a breeder increases as the percentage 

 of her fertile eggs which hatch increases. Therefore a 

 should go into the numerator of the index fraction. 

 Similarly a bird's value increases as a breeder in pro- 

 portion to her egg production in the breeding season. A 

 bird which produces few eggs during the breeding season 

 (whatever she may have done before) ipso facto can not 

 produce many chickens. Instead of using the actual egg 

 production in the index the relative or percentage produc- 

 tion is used, for reasons which have been discussed in a 

 previous publication by the present writers. 3 



Now in distinction to the factors so far discussed it is 

 clear that a hen's value as a breeder decreases as the 

 number of infertile eggs which she produces in a given 

 time increases. To put this factor into the breeding 

 index is, of course, equivalent to asserting that the hen 

 plays at least an equal part with the cock in determining 

 fertility. This is undoubtedly the case, though it is not 

 the place here to present detailed evidence on the point. 

 Since relatively poor fertility of the eggs is an unde- 

 sirable characteristic c is put in the denominator of the 

 index fraction. The case is the same in regard to d. 

 If the chicks produced by a particular bird are weak and 

 die early when given the same treatment as that under 

 which other chicks thrive, it is an indication that that 

 particular bird is not desirable as a breeder. 



In order to give a concrete idea of the values which 

 this poultry selection index may take in actual cases Table 

 I has been prepared. This table gives the value of I x 

 for a number of pullets which were tested as breeders 

 in the spring of 1908, as well as the values of the com- 

 ponent factors in each case. The cases given in the table 



