158 SUMMAKT OF THE Chap. XIX. 



much more numerous than the other, the former will ulti- 

 mately absorb the latter. Should both varieties exist in 

 nearly equal numbers, it is probable that a considerable 

 period would elapse before the acquirement of a uniform 

 character; and the character ultimately acquired would 

 largely depend on prepotency of transmission and on the con- 

 ditions of life; for the nature of these conditions would 

 generally favour one variety more than another, so that a kind 

 of natural selection would come into play. Unless the crossed 

 offspring were slaughtered by man without the least discri 

 mination, some degree of unmethodical selection would like- 

 wise come into action. From these several considerations 

 we may infer, that when two or more closely allied species 

 first came into the possession of the same tribe, their crossing 

 will not have influenced, in so great a degree as Ins often 

 been supposed, the character of the offspring in future times ; 

 although in some cases it probably has had a considerable 

 effect. 



Domestication, as a general rule, increases the prolificness 

 of animals and plan Is. It eliminates the tendency to sterility 

 which is common to species when first taken from a state of 

 nature and crossed. On this latter head we have no direct 

 evidence : but as our races of dogs, cattle, pig-. &<?., are almost 

 certainly descended from aboriginally distinct stocks, and as 

 these races are now fully fertile together, or at least incom- 

 parably more fertile than most species when crossed, we may 

 with entire confidence accept this conclusion. 



Abundant evidence has been given that crossing adds to 

 the size, vigour, and fertility of the offspring. This holds 

 good when there has been no previous close interbreeding. 

 It applies to the individuals of the same variety but belonging 

 to different families, to distinct varieties, sub-species, and 

 even to species. In the latter case, though size is gained, 

 fertility is lost ; but the increased size, vigour, and hardiness 

 of many hybrids cannot be accounted for solely on the 

 principle of compensation from the inaction of the reproduc- 

 tive system. Certain plants whilst growing under their 

 natural conditions, others when cultivated, and others of 

 hybrid origin, are completely self-impotent, though per- 



