Chap. VIII. FERTILITY OF MONGRELS. 267 



farmers and gardeners in their frequent exchanges of 

 seed, tubers, &c, from one soil or climate to another, 

 and back again. During the convalescence of animals, 

 we plainly see that great benefit is derived from almost 

 any change in the habits of life. Again, both with 

 plants and animals, there is abundant evidence, that a 

 cross between very distinct individuals of the same spe- 

 cies, that is between members of different strains or 

 sub-breeds, gives vigour and fertility to the offspring. 

 I believe, indeed, from the facts alluded to in our fourth 

 chapter, that a certain amount of crossing is indispens- 

 able even with hermaphrodites; and that close inter- 

 breeding continued during several generations between 

 the nearest relations, especially if these be kept under 

 the same conditions of life, always induces weakness 

 and sterility in the progeny. 



Hence it seems that, on the one hand, slight changes 

 in the conditions of life benefit all organic beings, and 

 on the other hand, that slight crosses, that is crosses 

 between the males and females of the same species 

 which have varied and become slightly different, give 

 vigour and fertility to the offspring. But we have 

 seen that greater changes, or changes of a particular 

 nature, often render organic beings in some degree 

 sterile ; and that greater crosses, that is crosses between 

 males and females which have become widely or spe- 

 cifically different, produce hybrids which are generally 

 sterile in some degree. I cannot persuade myself that 

 this parallelism is an accident or an illusion. Both 

 series of facts seem to be connected together by some 

 common but unknown bond, which is essentially related 

 to the principle of life. 



Fertility of Varieties when crossed, and of their Mongrel 

 offspring. — It may be urged, as a most forcible argu- 



n2 



