Chap. XIX. HYBKIDISM. 173 



generally eliminate this tendency ; so that the domesticated 

 descendants of species, which in their natural state would 

 have been in some degree sterile when crossed, become 

 perfectly fertile together. AVith plants, so far is cultivation 

 from giving a tendency towards mutual sterility, that in 

 several well - authenticated cases, already often alluded to, 

 certain species have been affected in a very different manner, 

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

 capacity of fertilising, and being fertilised by, distinct 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 circumstances should commonly both induce and 

 eliminate the same tendency ; though in certain cases, with 

 species having a peculiar constitution, sterility might occa- 

 sionally be thus induced. Thus, as I believe, we can under- 

 stand why with domesticated animals varieties have not been 

 produced which are mutually sterile ; and why with plants 

 only a few such cases have been observed, namely, by Gartner, 

 with certain varieties of maize and verbascum, by other ex- 

 perimentalists with varieties of the gourd and melon, and by 

 Kolreuter with one kind of tobacco. 



With respect to varieties which have originated in a state 

 of nature, it is almost hopeless to expect to prove by direct 

 evidence that they have been rendered mutually sterile ; for if 

 even a trace of sterility could be detected, such varieties would 

 at once be raised by almost every naturalist to the rank of 

 distinct species. If, for instance, Gartner's statement were 

 fully confirmed, that the blue and red flowered forms of the 

 pimpernel (^AnagalUs arvensis) are sterile when crossed, I pre- 

 sume that all the botanists who now maintain on various 

 grounds that these two forms are merely fleeting varieties, 

 would at once admit that they were specifically distinct. 



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 modified in a 

 sufficient and permanent degree to take rank as species. We 

 are far from precisely knowing the cause ; but we can see 



