r 



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142 GOOD FKOM CROSSING. C hap. XVII. 



be ffiven of sexual incompatibility between certain mal 



and female domesticated animals, which, nevertheless, are fertile 

 when matched with any other individual of the same kind. 



In the early part of this chapter it was shown that the 

 crossing of distinct forms, whether closely or distantly allied, 

 gives increased size and constitutional vigour, and, except in 

 the case of crossed species, increased fertility, to the offspri] 

 The evidence rests on the universal testimony of breeders (foi 

 it should be observed that I am not here speaking of the evi 

 results of close interbreeding), and is practically exemplified ii 

 the higher value of cross-bred animals for immediate consump 

 tion. The good results of crossing have also been demon 

 strated, in the case of some animals and of numerous . plants 



b 



by actual weight and measurement. Although animals of pure 



blood will obviously be deteriorated by crossing, as far 

 characteristic qualities are concerned, there seems to be no 

 exception to the rule that advantages of the kind just men- 

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

 previous close interbreeding. The rule applies to all animals, 

 even to cattle and sheep, which can long resist breeding in-and- 

 in between the nearest blood-relations. It applies to individuals 

 of the same sub-variety but of distinct families, to varieties or 

 races, to sub-species, as well as to quite distinct species. 



In this latter case, however, whilst size, vigour, precocity, 

 and hardiness are, with rare exceptions, gained, fertility, in a 

 greater or less degree, is lost ; but the gain cannot be exclu- 

 sively attributed to the principle of compensation ; for there is 

 no close parallelism between the increased size and vigour of 

 the offspring and their sterility. Moreover it has been clearly 



proved that mongrels which are perfectly fertile g 



advantages as well as sterile hybrids 

 The evil consequences of long-continued close interbreeding 

 are not so easily recognised as the good effects from crossing, 

 for the deterioration is gradual. Nevertheless it is the general 

 opinion of those who have had most experience, especially with 

 animals which propagate quickly, that evil does inevitably follow 

 sooner or later, but at different rates with different animals. 

 No doubt a false belief may widely prevail like a superstition ; 

 yet it is difficult to suppose that so many acute and original 











