We have as yet no clue to the source of this great and sudden change of climate. Various suggestions have been made,--among others, that formerly the inclination of the earth's axis was greater, or that a submersion of the continents under water might have produced a decided increase of cold; but none of these explanations are satisfactory, and science has yet to find any cause which accounts for all the phenomena connected with it. It seems, however, unquestionable that since the opening of the Tertiary age a cosmic summer and winter have succeeded each other, during which a Tropical heat and an Arctic cold have alternately prevailed over a great portion of the globe. In the so-called drift (a superficial deposit subsequent to the Tertiaries, of the origin of which I shall speak presently) there are found far to the south of their present abode the remains of animals whose home now is in the Arctics or the coldest parts of the Temperate Zones. Among them are the Musk-Ox, the Reindeer, the Walrus, the Seal, and many kinds of Shells characteristic of the Arctic regions. The northernmost part of Norway and Sweden is at this day the southern limit of the Reindeer in Europe; but their fossil remains are found in large quantities in the drift about the neighborhood of Paris, where their presence would, of course, indicate a climate similar to the one now prevailing in Northern Scandinavia. Side by side with the remains of the Reindeer are found those of the European Marmot, whose present home is in the mountains, about six thousand feet above the level of the sea. The occurrence of these animals in the superficial deposits of the plains of Central Europe, one of which is now confined to the high North, and the other to mountain-heights, certainly indicates an entire change of climatic conditions since the time of their existence. European Shells now confined to the Northern Ocean are found as fossils in Italy,--showing, that, while the present Arctic climate prevailed in the Temperate Zone, that of the Temperate Zone extended much farther south to the regions we now call sub-tropical. In America there is abundant evidence of the same kind; throughout the recent marine deposits of the Temperate Zone, covering the low lands above tide-water on this continent, are found fossil Shells whose present home is on the shores of Greenland. It is not only in the Northern hemisphere that these remains occur, but in Africa and in South America, wherever there has been an opportunity for investigation, the drift is found to contain the traces of animals whose presence indicates a climate many degree colder than that now prevailing there.