CHAPTER XXVI 



INBREEDING AND CROSSBREEDING 



It is the opinion of most experienced animal breeders that 

 close inbreeding should be avoided because it has a tendency 

 to decrease the size, vigor and fecundity of the race in which 

 it is practiced. Many even believe that it leads to the pro- 

 duction of abnormal individuals or monstrosities. On the 

 other hand some of those who have had greatest success in 

 producing new or improved breeds of domesticated animals 

 have practiced the closest kind of inbreeding and attribute 

 their success in part to this fact. 



In human society we find a nearly unanimous condemna- 

 tion of the marriage of near-of-kin. Nearly all peoples, 

 civilized or uncivilized, forbid it. Only exceptionally, as in 

 the case of the royal families of ancient Egj-pt and ancient 

 Peru, has the marriage of brother and sister been sanctioned. 

 The underlying reason in such cases was a belief that the 

 family in question constituted a superior race whose members 

 could find no fit mates outside their own number. There was 

 probably no thought that inbreeding itself was beneficial but 

 only a desire to conserve the superior excellence believed to 

 reside in certain individuals. The same considerations, prob- 

 ably have led to the occasional practice of inbreeding in 

 animal husbandry, viz., the desire to conserve and perpetu- 

 ate the superiority of particular individuals. 



If we inquire into the biological foundation of the idea that 

 inbreeding is harmful, we come upon seemingly conflicting 

 evidence. No generalization can be drawn which is applic- 

 able to all organisms. 



By inbreeding we mean the mating of closely related in- 

 dividuals. As there are different degrees of relationship be- 

 tween individuals, so there are different degrees of inbreeding. 

 The closest possible inbreeding occurs among plants in what 



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