94 GOOD FROM CROSSING. Chap. XVII. 



effects of close interbreeding may be checked or quite pre- 

 vented by the related individuals being separated for a few- 

 generations and exposed to different conditions of life. This 

 conclusion is now held by many breeders ; for instance Mr. 

 Carr 2 remarks, it is a well-known " fact that a change of soil 

 and climate effects perhaps almost as great a change in the 

 constitution as would result from an infusion of fresh blood." 

 I hope to show in a future work that consanguinity by itself 

 counts for nothing, but acts solely from related organisms gene- 

 rally having a similar constitution, and having been exposed 

 in most cases to similar conditions. 



That any evil directly follows from the closest interbreeding 

 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 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 

 disease, if not actually ill, are fully capable of procreating 

 their kind. Close interbreeding, on the other hand, often 

 induces sterility ; and this indicates something quite distinct 

 from the augmentation of morbid tendencies common 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 related 

 to them in blood ; and that, on the other hand, long-continued 

 close interbreeding is injurious. 



Various general considerations have had much influence in 

 leading me to this conclusion ; but the reader will probably 

 rely more on special facts and opinions. The authority of 

 experienced observers, even when they do not advance the 

 grounds of their belief, is of some little value. Now almost 

 all men who have bred many kinds of animals and have 

 written on the subject, such as Sir J. Sebright, Andrew 



* ' The History of the Rise and Progress of the Killerby, &c. Herds,' p. 41. 



