46 
BULLETIN 905, U. S. DEPARTMENT OF AGEICULTUEE. 
vigor and a basis for further improvement. The great body, even of 
the owners of purebreds, however, will own stock which is distinctly 
below the best of its breed. For them improvement and fixation of 
type can go hand in hand. The method is the consistent use of pre- 
potent males of a given superior line. 
GRADING UP. 
Common stock can be improved and fixed in type by the same 
methods as those described last. The process in this case is known 
as grading up. Even two or three crosses with superior purebred 
Fig. 12.— A Merino ram, an example of the fine-wool type of sheep. 
males should raise the level practically to that of the average pure- 
bred, if not better, so far as individual qualities are concerned. With 
five or six such crosses it would doubtless be possible to produce 
animals better than most purebreds both in their own characteristics 
and in their breeding power. 
It is not, however, considered practicable to permit registration of 
grades as purebreds, no matter how many top crosses there may be, 
with superior purebred males of the same breed. In all but this 
respect, however, a large stock of common females can be rapidly 
converted into a herd as good as a purebred herd by grading. It is 
in the grading up of the common stock of the country that scientific 
breeding can render its greatest service. 
