590 GENETICS IN RELATION TO AGRICULTURE 
grees given herewith show the type of results which Pearl has secured 
by an application of this method of breeding. In Fig. 226 is given the 
pedigree of line D5D39, a high-producing line. It will be observed that 
a mean winter egg production of about sixty eggs is maintained through- 
out four generations. In every case, of course, males were selected for 
mating of a genotype corresponding to the females. The last generation 
in this pedigree shows how large a range of phenotypic fluctuation may be 
expected in breeding operations with fowls. 
For contrast with the above line we may present line D61D168, a low- 
producing line. The pedigree is given in full in Fig. 227. Here the 
mean winter egg production of the main low line is about one-third that 
of line D5D39. This line is interesting on account of the appearance of 
a high-producing individual, #248, which is probably a product of 
Mendelian segregation, but may possibly be a mutant. Not only was 
this individual herself a high producer, but she transmitted her producing 
abilities to her daughters, so that in the high line of this race, only one 
individual G495, possibly pathological, failed to exhibit a considerably 
higher winter egg production than the highest individual in the low por- 
tion of the line. These two pedigrees illustrate clearly what a systematic 
plan of breeding may accomplish after mere phenotypic selection has 
failed completely. 
We should not fail to point out as a factor to be considered in in- 
terpreting Pearl’s directions for breeding poultry for high fecundity, that 
neither Pearl nor any other scientist claims that the two factors, L and M, 
are the only ones concerned in breeding for fecundity. In any mixed 
flock of birds there must be a number of other factors, which although they 
may not have as marked an effect as the two primary factors, nevertheless 
will appreciably affect winter egg production. For this reason the 
breeder is admonished to use moderate inbreeding or line-breeding, 
because by too close inbreeding he may inadvertently breed his flock 
to a homozygous condition for unfavorable modifiers. Line-breeding 
is necessary, because by this method a like genotypic constitution is 
assured. Further, too hasty rejection of lines which do not measure 
up to standard may result inadvertently in the discarding of some line 
which had greater potentialities than those at first most productive, 
consequently the warning not to reject all but the best line. Moreover, 
we suspect that by isolating different high lines and then crossing them 
and applying the above procedure to the hybrid progeny still better 
strains might result. The directions which Pearl has given for poultry 
breeding may be applied with proper modifications to other livestock. 
They should be carefully studied by every breeder, with the distinct 
proviso that no rule of thumb, however excellent, can supply the ability 
for intelligent practical application, an indispensable feature of successful 
breeding operations. 
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