502 GENETICS IN RELATION TO AGRICULTURE 
methods of many a successful animal breeder. But prepotency, if it 
indicates anything, points to the supreme importance of genotypic con- 
stitution as the measure of excellence, and not to augmented excellence 
from development, performance, or any other factor, for these prepotent 
animals are often not themselves superior in individual excellence to 
many other animals of the breed although the latter have failed to 
impress the breed so strongly with their characteristics. 
It is safe to say, therefore, that the isolation view of selection is 
sufficient to account for any of the results which have been obtained in 
practical breeding operations. Since practical breeding methods have 
often been very successful it follows as a matter of of course that the 
adoption of such an interpretation does not involve of necessity any altera- 
tion in livestock-breeding methods. Why then emphasize the importance 
of this interpretation? The answer to the question should be clear. It 
follows that, if a method of breeding is not creative with respect to 
addition of new and better elements to the hereditary material, full 
utilization must be made of those existing elements of the germinal 
material which are of most value commercially. It, therefore, follows 
that no method of breeding, however excellent, can attain a full measure 
of success unless the very best existing foundation stock is utilized, for 
in such individuals alone are contained in the very best combinations 
those hereditary elements upon the utilization of which the breeder must 
rely for success. The importance of extreme care in the selection of 
foundation stock cannot, therefore, be overestimated. 
Selection Indices.—In all selection work, as indeed in all practical 
reeding, it is necessary for the stockman to have his ideal thoroughly in 
mind. In the absence of such a definite ideal, it is difficult to understand 
how any breeding operations can possibly have an orthogenetic trend. 
The requirements of efficient selection, therefore, demand first an ac- 
curate method of judging the comparative worth of aseries of animals with 
respect to certain definite characters, and second a method of weighting 
different characters in the same individual according to their comparative 
value from a breeding standpoint. It is here that science may be ex- 
pected to give definitely useful contributions to practical breeding 
methods in the determination of mathematically accurate means of 
comparing data. 
With respect to the first point, the comparative value of a given 
character in a series of individuals, several factors must be considered. 
Only one definite case can be considered here, that of milk production 
in dairy cows. Obviously with cows kept under identical conditions, 
two factors have a great influence on the amount of milk produced within 
a given period, namely, the age of the cow and the stage in lactation. 
Pearl has shown that the amount of milk produced by a cow within a 
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