METHODS OF BREEDING 585 



use of polled mutants, mule-footed breeds of hogs, hornless sheep, or 

 particular coat colors in horses, cattle, and swine. Nevertheless it is a 

 very useful conception to add to the stock-breeder's fund of knowledge. 

 The method of breeding for increased fecundity in poultry devised by 

 Pearl is the best existing illustration of the employment of genotypic 

 selection in attacking a problem of economic importance. We have 

 pointed out how Pearl on the basis of investigations of winter egg pro- 

 duction in fowls established the fact that two dominant factors for high 

 winter egg production existed. One of these factors, L, determines the 

 production of pullets which lay somewhat less than thirty eggs during the 

 winter period; the other factor, M, which is sex-linked, adds to this so 

 that birds possessing both these factors lay over thirty eggs during the 

 winter cycle. The breeder's problem, therefore, starting with a mixed 

 flock, is to isolate and breed from individuals of the genetic constitutions 

 (ZM)(ZM}LL for males and (ZM)WLL for females, to the end that the 

 flock will consist entirely of individuals of these genotypes. So valuable 

 are the specific directions which Pearl has given that they are printed in 

 full below. 



1. Selection of all breeding birds first on the basis of constitutional vigor and 

 vitality making the judgment of this so far objective as possible. In particular the 

 scales should be called on to furnish evidence, (a) There ought to exist, for all 

 standard breeds of fowls, normal growth curves, from which could be read off the stan- 

 dard weight which should be attained by a sound, vigorous bird, not specially fed 

 for fattening, at each particular age from hatching to the adult condition. These 

 curves we shall sometime have. (6) Let all deaths in shell, and chick mortality, be 

 charged against the dam, and only those females used as breeders a second time which 

 show a high record of performance in respect to the vitality of their chicks, whether 

 in egg or out of it. This constitutes one of the most valuable measures of constitu- 

 tional vigor and vitality which we have. If for no other reason than to measure their 

 breeding performance, a portion of the females each year should be pullets. In this 

 way one can in time build up an elite stock with reference to hatching quality of eggs 

 and viability of chicks, (c) Let no bird be used as a breeder which is known ever to 

 have been ill, to however slight a degree. In order to know something about this, 

 why not put an extra leg-band on every bird, chick, or adult, when it shows the first 

 sign of indisposition? This then becomes a permanent brand, which marks this 

 individual as one which failed, to a greater or less degree, to stand up under its environ- 

 mental measures of constitutional vigor, (d) Many of the bodily stigmata by which 

 the poultry man, during the last few years, has been taught to recognize constitutional 

 vigor, or its absence, have, in my experience, little if any real significance. Longevity 

 is a real and valuable objective test of vigor and vitality, but it is of only limited 

 practical usefulness, because of the increasing difficulty with advancing age of breeding 

 successfully on any large scale from old birds of the American and other heavy 

 types. 



2. The use as breeders of such females only as have shown themselves by trap- 

 nest records to be high producers, since it is only from such females that there can be 

 any hope of getting males capable of transmitting high-laying qualities. 



