IN-BREEDING 221 



itary qualities. We have seen how in-breeding may be 

 used to intensify and fix the other desirable characters 

 of a breed, and incidentally greatly increase their pre- 

 potency. If an animal is possessed of the quality of 

 fertility to an unusual degree, why may not in-breeding 

 be employed to increase fertility as well as to improve 

 the qualities of speed in horses or of early maturity 

 in meat-producing animals ? Let us answer the question 

 by an examination of the available data. Is in-breeding 

 per se specifically injurious to the fertility of plants and 

 animals? If in-breeding is injurious at all, how serious 

 is the injury and how far can the breeder take advantage 

 of the known good results without sacrificing the impor- 

 tant quality of fertility ? The data available for answer- 

 ing these questions are to be found in the practical experi- 

 ence of breeders and the results from carefully planned 

 experiments where all other factors have been eliminated 

 excepting only the factor of in-breeding. 



207. Darwin's researches. The greatest single con- 

 tribution to the subject of in-breeding was made by Dar- 

 win. Recognizing the advantages of close in-breeding 

 in fixing desirable characters and admitting that these 

 advantages may outweigh possible injury, he brings 

 forward an array of examples of the injurious effects of 

 in-breeding which are convincing. His conclusions are 

 best stated in his own words : 



1 " That any evil directly follows from the closest 

 inter-breeding has been denied by many persons ; but 

 rarely by any practical breeder; and never, as far as I 

 know, by one who has largely bred animals which prop- 

 agate their kind quickly. Many physiologists attribute 



1 Darwin, "Animals and Plants under Domestication," vol. 

 II, p. 94. 



