Page:Popular Science Monthly Volume 17.djvu/388

374 would retire south or up the mountains of the tropics when the cold moderated and the warm season again commenced. Then another season of glacial cold begins in the north. The plants which had before been left on the mountains would stand a good chance of being driven by the increasing cold to the plains, and still farther south, even perhaps across the equator into the southern hemisphere, and when the cold again decreased would retire to the fastnesses of the mountains.

If we accept this view of the influence of glacial periods on the vegetable kingdom, we shall see that many of the apparently anomalous cases of distribution mentioned in the first part of this article are explained. We can easily see why some of the inhabitants of the temperate and arctic zones of the northern are represented in corresponding zones in the southern hemisphere; it is easily explained why identical species of mosses are found in Lapland, on mountains of Jamaica, and the Peak of Teneriffe; why plants of the Pyrenees are identical with those of the Arctic regions; why species are found on the White Mountains of New Hampshire and in Greenland, but not in the intervening region, and why species are found in northern Europe and America, in Chili and New Zealand. This theory of alternate hot and cold periods is as yet the only one by means of which these cases can be explained.

Such, then, are some of the natural methods for the distribution of plants; the air, the water, beasts, birds, and fishes, as we have shown, all perform their several offices; but there is still another method of transport of which nothing has been said, and this is the part which man plays in the grand work. This is by no means insignificant, and can be shown in many ways. Out of the 2,582 species given in Gray's "Manual of Botany", there are 305 introduced species, and, of these, 278, all but 27, were imported from Europe. Nothing shows more strikingly man's influence than this fact, which is further corroborated by the assertion that the greater part of the plants naturalized at the Cape of Good Hope and in Australia are of European origin. With the great increase of facilities for travel, on land and on sea, with the extension of commerce to all quarters of the globe, and with the settlement and consequent clearing off of formerly unoccupied lands, we find both the fauna and flora of many countries greatly modified. There can be no more striking example of this influence of mankind then that shown in the Island of St. Helena. "When St. Helena was discovered, about the year 1506," says Lyell, "it was entirely covered with forests, the trees drooping over the tremendous precipices that overhang the sea. Now, says Dr. Hooker, all is changed; fully five sixths of the island is entirely barren, and by far the greater part of the vegetation which exists, whether herbs, shrubs, or trees, consists of introduced European, American, African, and Australian plants, which propagated themselves with such rapidity that the native plants could not compete with them.