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, M7, 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 fazled, 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 poultryman, 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. 
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