582 GENETICS IN RELATION TO AGRICULTURE 
not stand in any causal relation to it. Asa method of breeding, however, 
it gives abundant opportunity for a race which has any defects what- 
soever to express them, for by simplifying the genotypic constitutions of 
the animals within a family and making them like each other, it tends to 
increase the proportion of recessive defectives produced in the family. 
But if no inherent defects exist in the family, then such an effect cannot 
be produced, and the practice is on the whole to be commended. ~ 
The advantages of a system of inbreeding are found in the close ap- 
proach which this method makes to a strict method of genotypic selec- 
tion. It overcomes that difficulty of a system of phenotypic selection 
which arises from the possibility of mating different genotypes which 
are alike phenotypically. By this method the breeder is assured of 
genotypic identity in his breeding stock, because they have received 
their germinal elements from a common ancestor. Accordingly we are 
not surprised that this method has proven so notably successful in fixing 
types in the formative period of a breed’s existence, because it is just 
at this time that both genotypic and phenotypic diversities are most 
common, and the difficulties arising from their existence most baffling. 
The increase in prepotency which is universally acknowledged to ac- 
company inbreeding is in entire harmony with this interpretation—for 
by simplifying the genotypic constitutions of the individuals the tendency 
is to secure more and more individuals which are homozygous for all 
or nearly all the favorable germinal elements, and which possess in con- 
sequence of this fact superior transmitting capacity. The breeder who 
would add the practice of inbreeding to his operations must learn to 
cull with a firm hand, however, whenever defects appear for they indicate 
inevitably that some necessary constituents of the hereditary material 
have been lost. If he can do this, he has added a powerful instrument 
for improvement to his breeding equipment. 
Line-breeding.—The term line-breeding designates breeding within 
a given line of descent. By common agreement the term does not in- 
clude inbreeding; it begins with those degrees of relationship which are 
just outside the pale of inbreeding. It is, therefore, a system of breeding 
in which cousins of different degrees are mated with each other. 
No system of breeding has been so popular or so generally productive 
of good results as line-breeding. Like inbreeding, it is a method of breed- 
ing which approaches as nearly as present knowledge will permit to the 
ideal of genotypic selection. Because the individuals which are mated 
belong to the same line of descent and exhibit similar sets of characters, 
it is logically just to conclude that they possess similar sets of germinal 
elements. In this fact we have the whole explanation of the uniformity 
of progeny which is so characteristic of continued line-breeding. 
Line-breeding is popularly credited with all the excellencies of in- 
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