The only other recent change of great importance which can be adduced to illustrate our present subject, is that which has taken place between North and South America. The living marine shells of the opposite coasts of the isthmus of Panama, as well as the corals and fishes, are generally of distinct species, but some are identical and many are closely allied; the West Indian fossil shells and corals of the Miocene period, however, are found to be largely identical with those of the Pacific coast. The fishes of the Atlantic and Pacific shores of America are as a rule very distinct; but Dr. Günther has recently shown that a considerable number of species inhabiting the seas on opposite sides of the isthmus are absolutely identical. These facts certainly indicate, that during the Miocene epoch a broad channel separated North and South America; and it seems probable that a series of elevations and subsidences have taken place uniting and separating them at different epochs; the most recent submersion having lasted but a short time, and thus, while allowing the passage of abundance of locomotive fishes, not admitting of much change in the comparatively stationary mollusca. _The Glacial Epoch as affecting the Distribution of Animals._--The remarkable refrigeration of climate in the northern hemisphere within the epoch, of existing species, to which the term Glacial epoch is applied, together with the changes of level that accompanied and perhaps assisted to produce it, has been one of the chief agents in determining many of the details of the existing distribution of animals in temperate zones. A comparison of the effects produced by existing glaciers with certain superficial phenomena in the temperate parts of Europe and North America, renders it certain that between the Newer Pliocene and the Recent epochs, a large portion of the northern hemisphere must have been covered with a sheet of ice several thousand feet thick, like that which now envelopes the interior of Greenland. Much further south the mountains were covered with perpetual snow, and sent glaciers down every valley; and all the {41}great valleys on the southern side of the Alps poured down streams of ice which stretched far out into the plains of Northern Italy, and have left their débris in the form of huge mountainous moraines, in some cases more than a thousand feet high. In Canada and New Hampshire the marks of moving ice are found on the tops of mountains from 3,000 to 5,000 feet high; and the whole surface of the country around and to the north of the great lakes is scored by glaciers. Wherever the land was submerged during a part of this cold period, a deposit called boulder-clay, or glacial-drift has been formed. This is a mass of sand, clay, or gravel, full of angular or rounded stones of all sizes, up to huge blocks as large as a cottage; and especially characterized by these stones being distributed confusedly through it, the