It results from the above and the immediately preceding statement that specific and generic types enter on the stage in great force, and gradually taper off toward extinction. They should so appear in the geological diagrams made to illustrate the succession of life. This applies even to those forms of life which come in with fewest species and under the most humble guise. What a remarkable swarming, for example, there must have been of Marsupial Mammals in the early Mesozoic; and in the Coal formation the only known Pulmonates, four or five in number, belong to as many generic types. I have already referred to the permanence of certain species in geological time. I may now place this in connection with the law of origination and more or less continuous transmission of varietal forms. I may, perhaps, best illustrate this in connection with a group of species with which I am very familiar, that which came into our seas at the beginning of the Glacial age, and still exists. With regard to their permanence, it can be affirmed that the shells now elevated in Wales to 1,200 and in Canada to 600 feet above the sea, and which lived before the last great revolution of our continents, a period vastly remote as compared with human history, differ in no tittle from their modern successors after thousands or tens of thousands of generations. It can also be affirmed that the more variable species appear under precisely the same varietal forms then as now, though these varieties have changed much in their local distribution. The real import of these statements, which might also be made with regard to other groups well known to palæontologists, is of so great significance that it can be realised only after we have thought of the vast time and numerous changes through which these humble creatures have survived. I may call in evidence here a familiar British and American animal, the common sand clam, _Mya arenaria_, and its relative, _Mya truncata_, which now inhabit together all the northern seas; for the Pacific specimens, from Japan and California, though differently named, are undoubtedly the same. _Mya truncata_ appears in Europe in the older Pliocene, and was followed by _M. arenaria_ a little later. Both shells occur in the Pleistocene of America, and their several varietal forms had then already developed themselves, and remain the same to-day; so that these humble mollusks, littoral in their habits, and subjected to a great variety of conditions, have continued, perhaps for one or two thousand centuries, to construct their shells precisely as at present. Nor are there any indications of a transition between the two species. Similar statements may be made with regard to other mollusks of the Pliocene and Modern periods, and there are even species which extend unchanged from the early Eocene. Nor is it impossible that some modern bivalves of the Brachiopod group may be scarcely modified descendants even of Palæozoic species.