Chap. VIII. WHEN CROSSED. 257 



.it onre ranked l)v ina.sl naturalists as species. For instance, 

 the l)lue and red ijiiiipi'rnel, wliich are considered by most bot- 

 anists as varieties, are said l)y Giirtncr not to be quite fertile 

 when crossed, and he consequently ranks them as undoubted 

 species. If we thus art»-uo in a circle, the fertility of all varie- 

 ties produced under nature will assuredly have to be granted. 



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

 j)roduced, under domestication, we arc still involved in doubt. 

 For when it is stated, for instance, that the German Spitz dog 

 crosses more easily with the fox than do other dogs, or that 

 (v-rtain South American indigenous domestic dogs do not 

 rcadilv unite with European dogs, the explanation which will 

 o^'cur to every one, and jirobably the true one, is, that these 

 dogs are descended from aboriginally distinct species. Never- 

 theless the perfect fertility of so many domestic varieties, dif- 

 fering 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 arc, which, 

 though resembling each other most closely, are utterly sterile 

 wiien intercrossed. Several considerations, how'ever, render 

 the fertility of domestic varieties less remarkable. In the first 

 ])hice, it may be observed that the amount of external dill'er- 

 cnce between two species is no suni guide to their degree of 

 mutual sterility, so that similar differences in the case of vari- 

 eties would be no sure guide. It is almost certain that with 

 species the cause lies exclusively in differences in their sexual 

 constitution. Now the conditions to which domesticated ani- 

 mals and cultivated plants have been subjected, have had so 

 little tendency tow^ard 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 would have becMi in some degree sterile when crossed, 

 become ])(;rfectly fertile together. With plants, so far is cul- 

 tivation from giving a tendency toward sterility between dis- 

 tinct species, that in several well-authenticated cases already 

 alluded to, certain ])lants have been affected in an opposite 

 mann(^r, for they^ have become self-impotent, while still retain- 

 ing the ea|)a(itv of fertilizing and being fertilized by, other 

 species. If the Pallasian doctrine of the elimination of sterility 

 through long-continued domestication be admitted, and it can 

 hardly be rejected, it becomes in the highest degree improl> 



