584 GENETICS IN RELATION TO AGRICULTURE 
which have been discussed above may be used, the only requirement is 
that the sire must always be pure-bred and of the same breed. By rigid 
selection of females which approach most nearly to the ideal type of the 
breed from which the sire is selected, grade herds after three or four gene 
rations will approach very nearly to the standard of excellence from an 
economic standpoint at least of pure-breds. 
Crossbreeding we have also described in a previous chapter. It is 
an economic procedure entirely, and is based on the uniting of favorable 
characteristics of two strains in the cross-bred animals. Along with 
crossing sometimes comes the increased vigor of hybrids, sometimes 
striking, other times only slight. Although greatly decried by breeders 
and advocates of pure-bred livestock, crossbreeding is sound in theory 
and productive of good results in practice. To reap its benefits, however, 
it must be followed systematically. The breeder must not be tempted 
to allow the excellence of cross-bred animals to overcome his better 
judgment to the extent of permitting their retention in the breeding herd. 
Increased vigor and size are not alone responsible for the adoption of cross- 
breeding by some livestock men, but the changing standards of market 
demands have sometimes favored types of livestock not represented 
in any existing breed. Two alternatives are then open to the breeder, 
to establish within existing breeds the type demanded or as it were to 
synthesize such a type by crossbreeding. The former method is pro- 
ductive of the most permanent good, but it is a slow and expensive pro- 
ject and one requiring the good judgment of an unusually critical breeder. 
It has its illustrations, however, in the establishment of the Cruick- 
shank family of Shorthorn cattle, the American type of Hereford cattle; 
and as an outgrowth of crossbreeding in the building up of the Corrie- 
dale sheep of New Zealand. Crossbreeding, however, often achieves 
the same result immediately with existing materials; and, the advantage 
of a particular cross having been established, it does not require as much 
skill in operation as the establishment of a pure breeding type. As 
agricultural science develops we may expect to see crossbreeding for 
specific purposes much more fully utilized than it is at the present time. 
Genotypic Selection.—The method of genotypic selection is a method 
based on a knowledge of the genotypic constitution of the individuals 
used in mating. Although but little breeding can be ordered along this 
line on account of the dearth of knowledge of the actual factors which are 
concerned in particular character complexes, nevertheless to all practical 
purposes intelligent application of the methods of line breeding and in- 
breeding amounts to the same thing. Thus far our knowledge of factors 
is only extensive enough to apply this method of breeding to relatively 
simple problems, such as that of producing polled breeds of cattle by the 
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