302 FERTILITY OF VARIBTIE8 



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 produced, 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 tlie perfect fertility of so many do- 

 mestic races, differing widely from each other in appear- 

 ance, 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 do- 

 mestic 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 differences in their sexual consti- 

 tution. Now the varying conditions to which domesticated 

 animals and cultivated plants have been subjected, have 

 had so little tendency toward modifying the reproductive 

 system in a manner leading to mutual sterility, that we 

 have good grounds for admitting the directly opposite doc- 

 trine of Pallas, namely, that such conditions generally 

 eliminate this tendency; so that the domesticated descend- 

 ants of species, which in their natural state probably would 

 have been in some degree sterile when crossed, become per- 

 fectly fertile together. With plants, so far is cultivation 

 from giving a tendency toward 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, while still 

 retaining the capacity of fertilizing, and being fertilized 

 by, other species. If the Pallasian doctrine of the elimi- 

 nation of sterility through long-continued domestication be 

 admitted, and it can hardly be rejected, it becomes in the 

 highest degree improbable that similar conditions long-cou- 



