Kraemer: Effects of Inbreeding 



227 



to deepen his knowledge of all branches 

 of .stock-breeding . 



In the seventeenth chapter Darwin 

 discusses "The Good Effects of Crossing 

 and the Evil Effects of Close Inter- 

 breeding." He states his own observa- 

 tions and cites a long list of English 

 authorities, both theoretical and prac- 

 tical. In following his outline of the 

 case, 1 shall also cite some of the older 

 French and German breeders. 



Since the time of von Lehndorff, we 

 have become more exact in defining the 

 closeness of inbreeding. Darwin himself 

 complains that a definition of close 

 breeding is difficult because the same 

 degree of consanguinity in different 

 species of animals is reached in different 

 manners, yet today we hold gladly to 

 the idea of "free generations" and the 

 "line of ancestors," and apply it to all 

 kinds of animals. Such an accurate 

 measure is highly desirable here, and 

 according to it the pairing of father 

 with daughter or of mother with son is 

 the closest degree of consanguineous 

 breeding. Not improperly, however, 

 does Darwin himself note on this point 

 that as Sebright has remarked, the 

 pairing of brother and sister is really 

 the closest degree of family mating, 

 because here like blood is mated to like, 

 while father in mating with daughter 

 unites with only half his own blood. If 

 evil results occur from inbreeding, they 

 must, therefore, be the most serious 

 when siblings or members of the same 

 fraternity (full brother and sister) are 

 paired. 



But can any evil results be proved 

 from inbreeding itself? "That any evil 

 directly follows from the closest inbreed- 

 ing has been denied by many persons," 

 says Darwin, "but rarely by any 

 practical breeder; and never, so far as I 

 know, by anyone who has largely bred 

 animals which propagate their kind 

 quickly. Many physiologists attribute 

 the evil exclusively to the combination 

 and consequent increase of morbid 

 tendencies common to both parents; 

 and that this is an active source of 

 mischief there can be no doubt. It is 

 unfortunately too notorious that men 

 and various domestic animals endowed 

 with a wretched constitution, and with 



a strong hereditary disposition to dis- 

 ease, if not actually ill, are fully capable 

 of procreating their kind. Close inter- 

 breeding, on the other hand, often 

 induces sterility; and this indicates 

 something quite distinct from the aug- 

 mentation of morbid tendencies com- 

 mon to both parents. The evidence 

 immediately to be given convinces me 

 that it is a great law of nature, that all 

 organic beings profit from an occasional 

 cross with individuals not closely re- 

 lated to them in blood; and that, on the 

 other hand, long-continued close inter- 

 breeding is injurious." 



PROOFS NOT ALL CONVINCING. 



When the proofs that Darwin gives, 

 many of which have already found their 

 way into our textbooks, are examined 

 most carefully, it must be admitted that 

 it is sometimes very difficult clearly to 

 recognize the evils of inbreeding sup- 

 posed to be shown by them. The results 

 are often only too general in their 

 nature, and admit of being strongly 

 influenced by the constitutional sound- 

 ness or unsoundness of the animals con- 

 cerned. And finally, a considerable 

 decrease in strength of constitution 

 may be accompanied with so much of 

 advantage to the breeder that he will 

 completely overlook it in many 

 instances. Whenever, therefore, the 

 statements of breeders are subjective, 

 they offer no proof, in the biological 

 acceptation of the word; and often 

 inbreeding may be credited with such 

 and such results, when these results are 

 due solely to breeding for high per- 

 formance without regard to the consti- 

 tution and vitality of the animals. As 

 Darwin strikingly showed, an actual 

 increase in vigor is clearly enough evi- 

 dent when cross-breeding is practiced. 

 If this is undisputed, the conclusion 

 ought to be well-established, that con- 

 tinuous line-breeding must finally call 

 forth the opposite result. 



Sebright, Knight, Youatt and Low 

 have unanimously borne witness that 

 continued close interbreeding is impos- 

 sible. Prinsep also found that despite 

 the best feeding, the growth of the 

 animals steadily decreased, and John 

 Sinclair expressly testifies that Bake- 



