124 GOOD FROM CROSSINa. Chap. XVIL 



freely tlian the individuals of tlie same species breed together. 

 Cases will, also, be given of sexual incompatibility between 

 certain male and female domesticated animals, which, never- 

 theless, are fertile when matched with any other individual 

 of the same kind. 



In the early part of this chapter it was sho\\Ti that the 

 crossing of individuals belonging to distinct families of the 

 same race, or to different races or species, gives increased size 

 and constitutional vigour to the offspring, and, except in the 

 case of crossed species, increased fertility. The evidence 

 rests on the universal testimony of breeders (for it should be 

 observed that I am not here speaking of the evil results of 

 close interbreeding), and is practically exemplified in the 

 higher value of cross-bred animals for immediate consump- 

 tion. The good results of crossing have also been demon- 

 strated with some animals and with numerous plants, by 

 actual weight and measurement. Although animals of pure 

 blood will obviously be deteriorated by crossing, as far as 

 their characteristic qualities are concerned, there seems to be 

 no exception to the rule that advantages of the kind just 

 mentioned are thus gained, even when there has not been any 

 previous close interbreeding ; and the rule applies to such 

 animals as cattle and sheep, which can long resist breeding 

 in-and-in between the nearest blood-relations. 



In the case of crossed species, although size, vigour, j^re- 

 cocity, and hardiness are, with rare exceptions, gained, fer- 

 tility, in a greater or less degree, is lost ; but the gain in the 

 above respects can hardly be attributed to the principle of 

 compensation ; for there is no close parallelism between the- 

 increased size and vigour of hybrid offspring and their 

 sterility. Moreover, it has been clearly proved that mongrels 

 which are perfectly fertile gain these same advantages as well 

 as sterile hybrids. 



With the higher animals no special adaptations for ensuring 

 occasional crosses between distinct families seem to exist. 

 The eagerness of the males, leading to severe competition 

 between them, is sufficient ; for even with gregarious animals, 

 the old and dominant males will be dispossessed after a time 

 and it would be a mere chance if a closely related member 



