Now, a genus widely diffused, and capable of enduring great differences in the temperature and other conditions of the ocean, would probably suffer least from any great physical changes. If all the sea at one locality were converted into land, the genus would be driven into other districts, and thrive as abundantly as ever; or, even supposing that it should become locally extinct, it would still be abundantly represented in other oceans of the globe. In the course of many ages, after many such slow revolutions in the configuration of land and sea, the genus might perhaps become greatly reduced in numbers, until at length some final elevation of the sea-bed, or other change, might cause its total extinction. In the _rhynconella_, we perhaps see one of these genera in its last stage. Any great change in northern latitudes would probably destroy the arctic species, and a similar change around New Zealand might gradually extinguish the southern one. Looking, then, from this point of view into the past history of life upon our planet, we see that such extinctions have often taken place. At first, many of these widely-diffused genera were created. They were represented by a large number of species as well as individuals, and ranged over all the oceans of the globe; but in tracing out their history, we mark one species after another passing away. Some of them lived for but a comparatively short period; others came in with the beginning and saw out the end of an entire geological system; but of all these early species there is not now a single one extant, though some of the genera still inhabit our seas. It is plain, therefore, that but for the operation of another principle, all the genera, too, would ere this have become extinct, for the whole can contain no more than the sum of its parts; and if these parts are destroyed the whole must perish simultaneously. As the species of certain genera died out, however, their places were from time to time filled up with new ones, yet the rate of increase became ever less and less than the rate of decrease, so that the numbers of such genera grew fewer with every successive period, and have reached their minimum in existing seas. There are instances, however, in which this ratio was reversed, the list of added species continually outnumbering that of the extinct, till the genus reached its maximum, when it either continued at that stage till the present day, or began slowly to decline.