Now when Cuvier began to examine closely the Organic Remains of former times, to which his attention was called by the bones dug up in the gypsum quarries of Montmartre, near Paris, about the close of the last century, he brought with him to the task a very large acquaintance with the various forms of life that, in the present age, prevail throughout the world. And he was greatly struck with the marked difference between those living animals with which he had been long familiar, and those with which he now became acquainted for the first time. The more he extended his researches, the more manifest did this difference appear; until at last it became quite clear that the great bulk of the animals whose remains are preserved in the Crust of the Earth, have no representatives now living on its surface. Nevertheless, he observed that, though the Species no longer exists, it often happens that we have still other Species of the same Genus; or if the Genus, too, be extinct, we have other Genera of the same Class. Here, then, is the first great truth at which Cuvier arrived, and which has been since confirmed by extensive observations:--that the animals which formerly dwelt on this Earth of ours, were, for the most part, widely different from those by which it is now inhabited: and yet there is a well-defined likeness between them; that both have been created on a plan so strictly uniform, that the one and the other naturally find their place in the same system of classification. As the science of Palæontology progressed, and new facts were day by day accumulated, another truth, not less important, was gradually but certainly developed. In the distribution of Fossil Remains through the various strata of the Earth, there is a certain order observed, a certain regular law of succession, which cannot have been the mere result of chance, and which it is the business of science to unravel and explain. The facts are these. If we follow a particular set of strata _in a horizontal direction_, we find that the same fossils continue to prevail over hundreds of square miles, nay, often over a space as large as Europe, though beyond certain limits this uniformity of Fossil Remains will gradually be observed to disappear. But when we penetrate _in a vertical direction_ through the strata, the forms of animal and vegetable life that we meet with are constantly changing. After a few hundred yards at the most, we find ourselves in the midst of a group of fossils, altogether different from those which we have passed in the beds above: and so on, as we proceed downward, _each particular set of strata is found to have an assemblage of fossils peculiar to itself_.[82]