Page:Popular Science Monthly Volume 30.djvu/615

Rh drew nigh, the ice-cap began to form around the north pole. From the Arctic Ocean, the great sheet of solid glacier descended over Canada and the Eastern States till all New England, New York, and Pennsylvania lay covered with five thousand feet thickness of unbroken crystal. The ice cleared all animal and vegetable life off the face of the earth wherever it rested, and drove before it the old arctic fauna and flora as far south as Maryland, Virginia, and Kentucky. After many minor chances and changes, however, brought about by the recurrent cycles of summer and winter in the northern and southern hemisphere alternately, the world's weather began slowly to improve again. Step by step the ice retreated northward once more, till at last only the comparatively insignificant polar cap remained to bear witness to its sway, with a few casual southern extensions, like the one that still envelops all upper Greenland in its desolating sheet. As it slowly retired, the arctic fauna and flora followed close in its rear, on both sides of the Atlantic, till nowadays the plants and animals which once covered the plains of Europe, Canada, and New England find their last home in Spitzbergen, Nova Zembla, and the extreme northern shores of British America.

They left behind, however, some tokens of their presence to the present day even in the lower latitudes of Europe and America. The Greenland sandwort, among the New Hampshire hills, is just as much a relic of the Glacial Epoch as the striated rocks, the erratic bowlders, and the coarse drift which the Great Ice Age stranded high up the slopes and corries of Mount Washington, where we still find them in our own times. All the other glacial species (including that rare White Mountain butterfly which occurs on the very summit of that one peak, and nowhere else south of Labrador) have struggled on side by side with it, in isolated colonies, from the days when the ice retreated northward to the present moment. But there is a great difference in this respect between Europe and America, as Dr. Asa Gray has well pointed out. With us, in the Old World, great lateral ranges of mountain—Alps, Pyrenees, Dovrefjeld, and Caucasus—still nourish large existing glaciers and snow-fields, the lineal descendants of the universal ice-sheet of the Glacial Epoch. Hence, our European mountain flora, and, to a less extent, our mountain fauna as well, are even now large and flourishing; they retain a marked arctic appearance, and recall all the well-known stunted features of the glacial assemblage of plants and animals. The snowy mountain regions have acted as continuous nurseries for the dwarfed vegetation of the Great Ice Age. In America, on the other hand, you have few hills of any size east of that great backbone of the continent, the Rockies, and none of these hills rise to anything like snow-level. Hence, your mountain flora is, on the whole, but a poor one, and most of the species are the familiar kind which the European botanist already knows well among the Swiss Alps and the Scotch Highlands.