INFLUENCE OP ISOLATION ON THE FORMATION OF SPECIES 385 



between this colony and the ancestral continental species would not in 

 itself be any reason why the colony should develop into a variety. But 

 suppose that the foundress of the colony diverged in some unimportant 

 detail of colouring, such as may at any time arise through germinal 

 selection, from the ancestral species ; then this variation would be 

 transmitted to a portion of her progeny, and there would thus be 

 a possibility that a variety should establish itself upon the island 

 which would be the mean of the characters of the surviving progeny. 

 The greater the divergence was in the first progeny of the mother- 

 colonist, and the stronger this variational tendency was, the greater 

 also would be the chance that it would be transmitted further 

 and become a characteristic aberration from the marking of the 

 original species. I then designated this effect of isolation as due to 

 amixia, that is, to the mere prevention of crossing with the members 

 of the same species in the original habitat. 



We have examples of this from the Mediterranean islands, 

 Sardinia and Corsica, which possess in common nine endemic varieties 

 of butterflies, most of which diverge from the species of the continent 

 in a quite inconsiderable degree, though quite definitely and constantly. 

 Thus there flies in these islands a variety {Vanessa ichausu) of our 

 common little Vanessa urticai in which the two black spots on the 

 anterior wing exhibited by the original species are wanting. The 

 large tortoise-shell (Vanessa polycMoros) also occurs there, but it 

 has not varied and still exhibits the black spots. Our little indigenous 

 butterfly [Pararga inegoBra), which is abundant on warm, stony 

 slopes, quarries, and roads, flies about in Sardinia, but as a variety 

 (tigelius), which is distinguished from the original species by the 

 absence of a black curved line on the posterior wings. 



That of two nearly related and similarly marked species, like the 

 large and small tortoise-shell, one should remain unvaried, while the 

 other has become a variety, shows us that amixia alone does not neces- 

 sarily lead to the evolution of varieties in every case. It might of 

 course be objected that one species may have migrated to the islands 

 at a much earlier period than the other, and that it might be a direct 

 effect of the climate which found expression in this way. But we 

 have other similar cases in which one of two species has varied in an 

 isolated region, while the other has not, and in regard to which we 

 can prove definitely that both were isolated at the same time. 



An instance of this kind is to be found in Arctic and Alpine 

 Lepidoptera, which inhabited the plains of Europe during the Glacial 

 period, and subsequently, when the climate became milder again, 

 migrated some to the north into countries within the Arctic zone, and 



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