Il8 MAINi; AGRICULTURAL LXPE;RIMENT STATION. IQIO. 



duction in the index the relative or percentage production is 

 used, for reasons which have been discussed in a previous pub- 

 Hcation by the present writers.* 



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 infer- 

 tile 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, for 

 which detailed evidence has been presented elsewhere.** Since 

 relatively poor fertility of the eggs is an undesirable character- 

 istic c is put in the denominator of the index fraction. This 

 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 indi- 

 cation that that particular bird is not desirable as a breeder. 



Summarizing, we believe the poultry selection index de- 

 scribed, or some modification of it to suit particular needs, to 

 be a useful aid in practical breeding operations with poultry. 

 It measures in an exact and impartial manner the performance 

 of a bird as a breeder in any given season. On the basis of the 

 knowledge so gained the breeder can select birds which are to 

 be retained for further breeding. It substitutes an exact and 

 impartial measure, in the place of a rough, general impression 

 of the relative effect of several variables. 



The purpose of the paper here abstracted was to call the 

 attention of those interested in breeding operations to the use- 

 fulness of what we have called "selection index numbers" in 

 such work. The idea of such index numbers is to combine in 

 a single numerical expression the values of a series of variable 

 characters with regard to all of which the breeder wishes to 

 practice selection at the same time. The analytical expression 

 of this idea is discussed in the original paper and its adapta- 

 bility and usefulness are illustrated by examples drawn from 

 poultry and maize breeding. It is shown that selection index 

 numbers form a valuable adjunct to the score card in stock 

 judging. 



*Cf Me. Agric. Expt. Station, Bulletin No. 165, pp. 46- 

 **Me. Agr. Expt. Station, Bulletin 168. 



