CROSSING AND CLOSE-INTERBREEDING. 231 



species ; that full capacity of the reproductive element 

 requires a full representation therein, of all the forces 

 of all the parts of the species; and, that, when any of 

 the parts fail to contribute their quota, of force, to such 

 reproductive element, the capacity of such is lessened, 

 and impaired, in proportion. 



He says (p. 144, Vol. ii, Animals and Plants, &c) : 



" That evil directly follows from any degree of close- 

 interbreeding, has been denied by many persons ; but, 

 rarely, by any particular 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 ; that this is an active source of mischief, 

 there can be no doubt. It is, unfortunately, too notori- 

 ous, that men, and various domestic animals, endowed 

 with a wretched constitution, and with a strong heredi- 

 tary disposition to disease, if not actually ill, are fully 

 capable of procreating their kind. Close-interbreeding, 

 on the other hand, induces sterility ; and this indicates 

 something quite distinct (/) from the augmentation of 

 morbid tendencies common to both parents. The evi- 

 dence, immediately to be given, convinces me that it 

 is a great law of nature " (" Law of nature " was 

 an excellent good term, before it was ill sorted ; 

 therefore, scientists had need look to it), " that all or- 

 ganic beings profit from an occasional cross with indi- 

 viduals, 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 



