238 THE BREEDING OF ANIMALS 



Among the domestic animals it is quite probable that 

 the effects of in-breeding on different species will differ 

 materially. 



220. How long is it safe to continue in-breeding. 

 If we limit the application of the term in-breeding to mat- 

 ings between parent and offspring or between brother 

 and sister, then we cannot escape the conclusion that 

 long-continued in-breeding results in decreasing fertility, 

 and probably also weakens the constitution and decreases 

 the size of the offspring. * " Continued in-breeding," 

 says Kraemer, " always must result in weakened con- 

 stitution through its own influence." But such results 

 follow long-continued in-breeding. What are the limits 

 of safety? How long may the domestic animals be 

 closely in-bred without injury? An answer to these 

 questions is only possible when all the conditions are 

 known, including a knowledge of the inherited tendencies 

 of the in-bred animals. But it seems entirely safe to 

 conclude from the evidence available that the almost 

 universal prejudice against the practice of in-breeding 

 is in a large degree unwarranted. Such a prejudice 

 has undoubtedly limited the usefulness of many valuable 

 breeding animals and has caused real economic loss to 

 many breeders. 



221. Selection important. The practice of in-breed- 

 ing will never be successful in the absence of rigorous 

 selection. As the undesirable qualities are transmitted 

 with the same intensity as the good, constant vigilance 

 is required to guard against bringing forward latent char- 

 acters which are less desirable. Particularly animals 



1 Kraemer, "Mitteilung der Deutsches Landwirtschafts 

 Gesellschaft," September, 1913. See also Journal of Heredity, 

 1914, p. 226. 



