I look at this question from exactly the same point of view as I did nearly thirty years ago, when in a short paper[27] I endeavoured to show that, under favourable circumstances, an individual variation of a species may become the origin of a local variety if it finds itself in an isolated region. Suppose an island had no diurnal butterflies, until one day a fertilized female of a species from the continent was driven thither, found suitable conditions of life, laid its eggs, and became the founder of a colony; the prevention of constant crossing 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. [27] _Ueber den Einfluss der Isolirung auf die Artbildung_, Leipzig, 1872. 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 ichnusa_) of our common little _Vanessa urticæ_ in which the two black spots on the anterior wing exhibited by the original species are wanting. The large tortoise-shell (_Vanessa polychloros_) also occurs there, but it has not varied and still exhibits the black spots. Our little indigenous butterfly (_Pararga megæra_), 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.