256 FERTILITY" OF VARIETIES WHEN CROSSED. [Chap. IX, 



fertile offspring. With some exceptions, presently to be given, I 

 fully admit that this is the rule. But the subject is surrounded by 

 difficulties, for, looking to varieties produced under nature, if two 

 forms hitherto reputed to be varieties be found in any degree sterile 

 together, they are at once ranked by most naturalists as species. 

 For instance, the blue and red pimpernel, which are considered by 

 most botanists as varieties, are said by Gartner to be quite sterile 

 when crossed, and he consequently ranks them as undoubted 

 species. If we thus argue in a circle, the fertility of all varieties 

 produced under nature will assuredly have to be granted. 



If we turn to varieties, produced, or supposed to have been pro- 

 duced, under domestication, we are still involved in some doubt. 

 For when it is stated, for instance, that certain South American 

 indigenous domestic dogs do not readily unite with European dogs, 

 the explanation which will occur to every one, and probably the 

 true one, is that they are descended from aboriginally distinct 

 species. Nevertheless the perfect fertility of so many domestic 

 races, differing widely from each other in appearance, for instance 

 those of the pigeon, or of the cabbage, is a remarkable fact ; more 

 especially when we reflect how many species there are, which, 

 though resembling each other most closely, are utterly sterile when 

 intercrossed. Several considerations, however, render the fertility 

 of domestic varieties less remarkable. In the first place, it may 

 be observed that the amount of external difference between two 

 species is no sure guide to their degree of mutual sterility, so that 

 similar differences in the case of varieties would be no sure guide. 

 It is certain that with species the cause lies exclusively in differ- 

 ences in their sexual constitution. Now the varying conditions 

 to which domesticated animals and cultivated plants have been 

 subjected, have had so little tendency towards modifying the 

 reproductive system in a manner leading to mutual sterility, that 

 we have good grounds for admitting the directly opposite doctrine 

 of Pallas, namely, that such conditions generally eliminate this 

 tendency ; so that the domesticated descendants of species, which in 

 their natural state probably would have been in some degree sterile 

 when crossed, become perfectly fertile together. With plants, so 

 far is cultivation from giving a tendency towards sterility between 

 distinct species, that in several well-authenticated cases already 

 alluded to, certain plants have been affected in an opposite manner, 

 for they have become self-impotent, whilst still retaining the 

 capacity of fertilising, and being fertilised by, other species. If 

 the Pallasian doctrine of the elimination of sterility through long- 

 continued domestication be admitted, and it can hardly be rejected, 



