Thirdly, This more complete action is also more easily ascertained. It is more easy to demonstrate its effects, because, the number of quadrupeds being limited, and the greater part of their species, at least the large ones, being known, we have more means of determining whether fossil bones belong to them, or to a species that is now lost. As, on the other hand, we are very far from being acquainted with all the testaceous animals and fishes which inhabit the sea, and as we are still probably ignorant of the greater number of those which live in deep water, it is impossible to know with certainty, whether a species which occurs in a fossil state, may not still exist somewhere alive. And hence, we see naturalists persisting in giving the name of pelagic shells, that is to say, shells inhabiting the open sea, to the belemnites, cornua-ammonis, and other testaceous remains, which have hitherto been found only in the older strata; meaning by this, that if they have not yet been discovered in a living state, it is because they inhabit the depths of the sea, far beyond the reach of our nets. _Small probability of discovering New Species of large Quadrupeds._ Naturalists, certainly, have not yet explored all the continents, nor do they even know all the quadrupeds which inhabit the countries that they have explored. New species of this class are discovered from time to time; and those who have not examined with attention all the circumstances belonging to these discoveries, might also imagine that the unknown quadrupeds, whose bones are found in our strata, may remain to this day concealed, in some islands not yet discovered by navigators, or in some of the vast deserts which occupy the middle of Asia, Africa, the two Americas, and New Holland. However, if we carefully examine what kinds of quadrupeds have been recently discovered, and in what circumstances they have been found, we shall see that there is little hope of our ever finding alive those which have hitherto been observed only in a fossil state. Islands of moderate extent, and at a considerable distance from the continents or large islands, possess very few quadrupeds, and these, for the greater part, of diminutive size. When they happen to contain any of the larger species, these must have been carried to them from other countries. Bougainville and Cook found no other large quadrupeds than hogs and dogs in the South Sea Islands; and the largest species of the West India Islands was the _agouti_.