340 THE EVOLUTION THEORY 



useful although not indispensable. But the matter is different in the 

 case of the transformation of a colony upon a geographically isolated 

 region. 'Amiktic ' forms, such as Vanessa ichnusa of Corsica, 

 are hardly likely to be sexually alienated from the parent form ; 

 we have here to do only with the preponderance of a fortuitous 

 and biologically valueless variation and its consequent elevation to 

 the rank of a variety. The new form was not an adaptation, but 

 only a variation, and as it was of no use, it was not in a position 

 to incite any process of selection favouring its advancement. 



But even adaptive transformations on isolated regions from 

 which the parent species is excluded are not likely to develop 

 rapidly any sexual antipathy as regards the parent stock, and 

 I should not be surprised if experiments showed that there is perfect 

 mutual fertility between, for instance, many of the species of 

 AchatineUa on the Sandwich Islands or of Xanina on Celebes, or 

 between the species of thrushes on the different islands of the 

 Galapagos Archipelago, or between these and the ancestral species 

 on the adjacent continent, if that species is still in existence. For 

 there was no reason why sexual antipathy to the parent form 

 should have developed in any of these adaptation forms which 

 have arisen in isolation, and therefore it has probably not been 

 evolved. 



That our view of the mutual sterility between species, as an 

 adaptation to the utility of precise species-limitation, is the correct 

 one is evidenced not only by our domesticated races, but even more 

 clearly by plants, in regard to which it is particularly plain that the 

 sexual relations between two species are adaptational. We have 

 already seen in what a striking way the sensitiveness of the stigma 

 of a flower is regulated in reference to pollen from the same plant, 

 that some species are not fertilizable by their own pollen at all, that 

 others yield very little seed when self-fertiHzation is effected, and 

 that others again are quite fertile — as much so as with the pollen 

 of another plant of the same species. We regarded these gradations 

 of sexual sensitiveness as adaptations to the perfectly or only 

 moderately well-assured visits of insects, or to their entire absence. 

 I wish to cite these cases as well as the heterostylism of some flowers 

 as evidence in support of the conception of the mutual sterility of 

 species which I have just outlined. But this only in passing. The 

 point to which I chiefly wish to direct attention is the mutual 

 fertility of many plant-species. In lower as well as in higher plants 

 fertile hybrids occur not infrequently under natural conditions, and 

 cultivated hybrids, such as the new Medicago 'media, a form made by 



