Cuvier imagined that the whole history of the earth’s crust, since the time when living creatures had first appeared on the surface, must be divided into a number of perfectly distinct periods, or divisions of time, and that the individual periods must have been separated from one another by peculiar revolutions of an unknown nature (cataclysms, or catastrophes). Each revolution was followed by the utter annihilation of the till then existing animals and plants, and after its termination a completely new creation of organic forms took place. A new world of animals and plants, absolutely and specifically distinct from those of the preceding historical periods, was called into existence at once, and now again peopled the globe for thousands of years, till it again perished suddenly in the crash of a new revolution. About the nature and causes of these revolutions, Cuvier expressly said that no idea could be formed, and that the present active forces in nature were not sufficient for their explanation. Cuvier points out four active causes as the natural forces, or mechanical agents, at present constantly but slowly at work in changing the earth’s surface: first, _rain_, which washes down the steep mountain slopes and heaps up débris at their foot; secondly, _flowing waters_, which carry away this débris and deposit it as mud in stagnant waters; thirdly, the sea, whose breakers gnaw at the steep _sea_ coasts, and throw up “dunes” on the flat sea margins; finally and fourthly, _volcanos_, which break through and heave up the strata of the earth’s hardened crust, and pile up and scatter about the products of their eruptions. Whilst Cuvier recognizes the constant slow transformation of the present surface of the earth by these four mighty causes, he asserts at the same time that they would not have sufficed to effect the revolutions of the remote ages, and that the anatomical structure of the earth’s surface cannot be explained by the necessary action of those mechanical agents: the great and marvellous revolutions of the whole earth’s surface must, according to him, have been rather the effects of very peculiar causes, completely unknown to us; the usual thread of development was broken by them, and the course of nature altered.