HYBRIDISM 261 



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 con- 

 siderations, 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 differ- 

 ences in the case of varieties would be no sure guide. It is cer- 

 tain that with species the cause lies exclusively in differences in 

 their sexual constitution. Now the varying conditions to whicfi 

 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 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 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 elimination 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-continued should likewise induce 

 this tendency; though in certain cases, with species having a 

 peculiar constitution, sterility might occasionally be thus caused. 

 Thus, as I believe, we can understand why, with domesticated 

 animals, varieties have not been produced which are mutually 

 sterile; and why with plants only a few such cases, immediately 

 to be given, have been observed. 



The real difficulty in our present subject is not, as it appears 

 to me, why domestic varieties have not become mutually in- 

 fertile when crossed, but why this has so generally occurred with 

 natural varieties, as soon as they have been permanently modi- 

 fied in a sufficient degree to take rank as species. We are far 

 from precisely knowing the cause; nor is this surprising, seeing 

 how profoundly ignorant we are in regard to the normal and 

 abnormal action of the reproductive system. But we can see that 



