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 

 breeding, 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 a series 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 hi 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 



