222 THE BREEDING OF ANIMALS 



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 procre- 

 ating their kind. Close inter-breeding, 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 inter- 

 breeding is injurious." 



Darwin's conclusions are based upon a very large 

 number of observations. Experienced breeders who are 

 accurate observers, such as Sir J. Sebright, 1 Andrew 

 Knight and Herman von Nathusius, all agree as to the 

 certain injury which always follows long-continued in- 

 breeding. 



Darwin's investigations led him to believe that while 

 many species of plants and animals are hermaphroditic 

 and hence self-fertilizing, and these might be presumed 

 perpetually to fertilize themselves, yet he failed to find 

 a single species in which nature had provided structures 

 which insured self-fertilization. On the other hand, he 

 found innumerable instances in which nature had pro- 

 vided special structures for the sole apparent purpose of 

 insuring cross-fertilization and thus preventing perpetual 

 in-breeding. 



1 Sebright, "The Art of Improving the Breed," 1809. 



