Chap. XVII. EVIL FEOM INTERBREEDING. 93 



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 he 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. I shall 

 therefore 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 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 

 much 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 consequences of close 

 interbreeding carried on for too long a time, are, as is 2;enerallv 

 believed, loss of size, constitutional vigour, and fertility, 

 sometimes accompanied by a tendency to malformation. 

 Manifest evil does not usually follow from pairing the nearest 

 relations for two, three, or even four generations ; but several 

 causes interfere with our detecting the evil — such as the 

 deterioration being very gradual, and the difficulty of dis- 

 tinguishing between such direct evil and the inevitable 

 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 good reason to believe, and this was the ojDinion of 

 that most experienced observer Sir J. Sebright,^ that the evil 



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



