PRINCIPLES OF LIVESTOCK BREEDING. 67 
Straight selection is sometimes effective in fixing characters, but 
a single unfortunate cross is likely at any time to upset much previous 
work. 
The primary effect of inbreeding is the automatic fixation of some 
combination of hereditary factors present in the stock. This leads 
to uniformity of type or function, if such uniformity is possible. 
When not possible, owing to variation which is not genetic, the 
hereditary potentialities of different lines are at least clearly brought 
out. Decline in vigor is a common but not a necessary consequence 
of the fixation of heredity. 
The first generation of a cross is as uniform in character as the 
parent stocks, and, in general, shows increased vigor. The type is 
usually, but not always, intermediate between that of the parents. 
In the second generation there is increased variability, the character- 
istics of the grandparents being combined, as a rule, in all combina- 
tions and degrees. 
Aside from mere increase in numbers, the principal objects of 
breeding are to produce uniformity of a desired type and improve- 
ment. 
Uniformity of type depends on close breeding accompanied by 
selection, and may either be fixed within a line or secured in the first 
generation of a cross. 
Radical improvement depends on crossbreeding followed by close 
breeding and selection in order to fix the desired combination of 
characteristics when obtained. 
Improvement of inferior types, whether scrub or purebred, de- 
pends on the consistent use of prepotent males of the same breed or 
line within the bred. 
Selection of breeding stock requires good judgment in estimating 
the merits and properly weighing the claims of the animal's per- 
formance, his conformation, pedigree, and previous success as a 
breeder. 
The most direct tests of performance are the best. 
The best evidence of the value of an animal in breeding is his past 
record in this respect, if sufficiently extensive. 
In judging the value of a pedigree, the worth and similarity in 
type of the progeny produced by the three or four closest top-cross 
males in the ancestry in connection with their own worth and their 
relationship to one another are the most important considerations. 
The sire should be given twice as much weight as the dam's sire, and 
so on. 
The pure breeds of livestock are the successful results of past 
efforts at improvement and should form the basis for further progress. 
