For instance, when we look at the butterflies of a much larger region than our little island–namely, at those of a great continent like Africa or South America–we find that there are species which show gradations. Thus at a series of points, A, B, C, D, separated by some hundreds of miles from each other, we find a corresponding series of butterflies which are apparently closely similar species of one genus, differing by a few spots of colour, or darker and lighter tint, much as our Large White, Garden White, and Green-veined White differ. But when the butterflies are caught which occur at points intermediate between A and B, B and C, C and D, we find intermediate varieties, and, in fact, if we get a very large number from intermediate regions, we can, in some instances, arrange them in line so that they constitute a graduated series of forms, each being scarcely distinguishable from the one before or the one behind it, yet differing clearly from one a dozen places away. In such cases there is often evidence to show that the variety found at A breeds with that found at B, that of B with that of C, of C with D, so that they form an inter-breeding group, though perhaps the varieties at D will not pair with those at A, or even with those at B. Then sometimes we find in such a series, otherwise complete, a gap. Let us suppose it is between the butterflies of B and C. We find the series of gradations nearly complete, but some natural condition–such as the encroachment of the sea, or the slow elevation of a mountain range, or the climatic destruction of the necessary food-plant–has "wiped out" a few forms somewhere between those of B and C. They no longer exist. The series is no longer connected by inter-breeding forms; those occurring from A to B and some distance beyond are one "species" varying in the direction of the series C to D, but abruptly broken off from the latter. The series C to D is also a "species" with graduated varieties, but distinct; it is cut off from the lot once in continuity with it by the destruction of the intermediate forms inhabiting an intermediate area. Thus the one species becomes two, and these may again break up, and, having become thus disconnected and stabilized, they may spread over one another's territory–fly side by side and yet remain distinct forms which do not pair together–although originally they were varieties spreading from a common centre, where the ancestral species lived and multiplied.