[EC] This I have long maintained on grounds connected with Pleistocene fossils, amount of denudation and deposit, &c., and I am glad to see that Prestwich, the best English authority on such subjects, has recently announced similar conclusions, based on independent reasons.
Simultaneously with the return of the old temperate flora, the arctic plants that had overspread the land retreated to mountain-tops, now bared of ice and snow, and back to the polar lands whence they came; and so it happens that, on the White Mountains, the Alps, and the Himalayas, we have insular patches of the same groups of plants that exist around the pole.
These changes need not have required a very long time, for the multiplication and migration of plants are very rapid, especially when aided by the agency of migratory animals. Many parts of the land must, indeed, have been stocked with plants from various sources, and by agencies--as that of the sea--which might at first sight seem adverse to their distribution. The British Islands, for example, have no indigenous plants. Their flora consists mainly of Germanic plants, which must have migrated to Britain in that very late period of the Post-glacial when the space now occupied by the North Sea was mostly dry land. Other portions of it are Scandinavian plants, perhaps survivors of the Glacial age, or carried by migratory birds; and still another element consists of Spanish plants, brought north by spring migrants, and establishing themselves in warm and sheltered spots, just as the arctic plants do on the bleak hill-tops. The Bermudas, altogether recent islands, have one hundred and fifty species of native plants, all of which are West Indian and American, and must have been introduced by the sea-currents or by migratory birds.
And so the earth became fitted for the residence of modern man. Yet it is not so good or Edenic a world as it once was, or as it may yet become, were another revolution to restore a mild climate to the arctic regions, and to send down a new swarm of migratory species to renew the face of the earth and restore it to its pristine fertility of vegetable life.
Thus closes this long history of the succession of plants, reaching from the far back Laurentian to the present day. It has, no doubt, many breaks, and much remains to be discovered. Yet it may lead us to some positive conclusions regarding the laws of the introduction of plants.
