Such are a few of the effects which elevation and subsidence, even on a small scale, would seem (when tested by theory and practice) to produce. It yet remains for us to suggest, that the latter, when carried to its maximum, so as to cause the actual separation by the sea of one district from another, is a contingency of immense significance in regulating the distribution of the Annulose tribes. Their outward contour and aspect we have shown in a previous chapter to be very largely beneath the control of isolation, provided a sufficient _time_ can be granted for the change: but their ultimate absence from any particular place, through the impediment which it offers to their migratory progress, we have not yet touched upon. Let us conceive, therefore, an extensive continent; and, since the insects which at present inhabit our earth must, if the doctrine of specific centres be true, have been originally created in certain definite spots, let us suppose a limited proportion of them to have been first produced upon this tract. Self-dissemination, we will assume, has been going on for centuries: those species which were gifted with quick diffusive powers have become pretty evenly dispersed over its surface; whilst those of naturally slow or sedentary habits have peopled, comparatively, but small areas around the respective localities of their birth. Such may have been the case, at some fixed period, amongst the aboriginal beings of any country which we choose to select as an illustration. But there is another element to be considered. If this region be not insular, it will have received colonists from foci of radiation situated beyond its bounds; and these, therefore, according to their several capabilities for progression, will have, likewise, in parts, overspread, or tenanted, it. Now it is impossible to cite a more simple example than this. But let us endeavour to realize what would be the necessary consequence of the breaking up of such a district as that which we have imagined. If a _general_ sinking should take place, causing its higher points to be alone visible above the ocean, or merely a _partial_ one, so as to admit of the sea encompassing portions of it which would remain unaffected in their altitude; the result practically would be the same,--namely, the constitution of a group of islands out of a once continuous land. Then, as regards the animal population of this tract, the main phaenomena are almost self-evident. Should any of its isolated fragments chance to contain a portion of one of _those limited areas_ which a species of slow progressive powers had succeeded in colonizing, it would of course harbour (provided that the other portion has disappeared) what would now be defined as _endemic_. Numbers of these small areas, or, in other words, of the species which had overspread them, would in all probability be lost for ever; whilst the occurrence of any of the surviving ones in more than a single