Chap. XVII. EVIL FROM INTERBREEDING. 115 



remain within a confined area. It bears in an important 

 manner on the influence of free intercrossing, in obliterating 

 individual differences, and thus giving uniformity of character 

 to the individuals of the same race or species ; for if additional 

 vigour and fertility be thus gained, the crossed offspring will 

 multiply and prevail, and the ultimate result will be far greater 

 than otherwise would have occurred. Lastly, the question is 

 of high interest, as bearing on mankind. Hence I shall discuss 

 this subject at full length. As the facts which prove the evil 

 effects of close interbreeding are more copious, though less 

 decisive, than those on the good effects of crossing, I shall, under 

 each group of beings, begin with the former. 



There is no difficulty in defining what is meant by a cross ; 

 but this is by no means easy in regard to " breeding in and in " 

 or " too close interbreeding," because, as we shall see, different 

 species of animals are differently affected by the same degree of 

 interbreeding. The pairing of a father and daughter, or mother 

 and son, or brothers 1 and sisters, if carried on during several 

 generations, is the closest possible form of interbreeding. But 

 some good judges, for instance Sir J. Sebright, believe that the 

 pairing of a brother and sister is closer than that of parents 

 and children ; for when the father is matched with his daughter 

 he crosses, as is said, with only half his own blood. The con- 

 sequences of close interbreeding carried on for too long a time, 

 are, as is generally believed, loss of size, constitutional vigour, 

 and fertility, sometimes accompanied by a tendency to mal- 

 formation. Manifest evil does not usually follow from pairino- 

 the nearest relations for two, three, or even four genera- 

 tions ; but several causes interfere with our detecting the evil 

 — such as the deterioration being very gradual, and the diffi- 

 culty of distinguishing between such direct evil and the inevit- 

 able augmentation of any morbid tendencies which may be 

 latent or apparent in the related parents. On the other hand, 

 the benefit from a cross, even when there has not been any very 

 close interbreeding, is almost invariably at once conspicuous. 

 There is reason to believe, and this was the opinion of that most 

 experienced observer Sir J. Sebright, 1 that the evil effects of 

 close interbreeding may be checked by the related individuals 



1 ' The Art of Improving the Breed, &c.,' 1809, p. 16. 



I 2 



