174 



SUMMARY OF THE 



Chap. xix. 



much more numerous than the other, the former will If 

 mately absorb the latter. Should both varieties exist in nea -1 

 equal numbers, it is probable that a considerable period would 

 elapse before the acquirement of a uniform character • and th 



pre- 



character ultimately acquired would largely depend 

 potency of transmission, and on the conditions of life ; for'tk 

 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 discrimination, some decree of un- 

 methodical selection would likewise 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 has often been supposed, the character of the 



offspring in future times ; 

 has had a considerable effect. 



although in 



some cases it probably 



Domestication, as a general rule, increases the prolificness 

 of animals and plants. It eliminates the tendency to sterility 



hich is common to 

 nature and crossed. On this 



first 



from a state of 



ad 



h 



dir 



evidence; but 



of dogs, cattle, pigs, &c 



tainly descended from aboriginally distinct stocks, and 



races are 



fully fertile together, or at least incom- 



these 



parably more fertile than most species when crossed, we may 

 with much confidence accept this conclusion. 



Abundant evidence has been given that crossing adds to the 



and fertility of the offspring. This holds good when 



close interbreeding. It applies to 

 the individuals of the same variety but belonging to different 

 families, to distinct varieties, sub-species, and partially even to 

 species. In the latter case, though size is often gained, fertility 

 is lost ; but the increased size, vigour, and hardiness of many 



has been 



previous 



© 



hybrids cannot be accounted for solely on the principle of 

 pensation from the inaction of the reproductive system. 



Certain 



plants, both of pure and hybrid origin, though perfectly healthy 

 have become self-impotent, apparently from the unnatural con 

 ditions to which they have been exposed ; and such plants, a: 

 well as others in their normal state, can be stimulated to fer 



