Page:Popular Science Monthly Volume 17.djvu/387

Rh locusts' dung received from South Africa, Mr. Darwin extracted and raised seeds of seven grass-plants, which belonged to two species of two genera. These locusts are sometimes found as far as three hundred and seventy miles from land; and an account is given of a cloud which hovered round the Island of Madeira for three days, and then disappeared without alighting. Such a cloud as this would undoubtedly be capable of introducing the seeds of foreign plants into insulated localities. The immense number of grasshoppers which have devastated the plains of Kansas and Nebraska would in the same way be the means of introducing seeds of foreign plants.

There is still another method which has been at times used by Nature for the distribution of plants, and that is by means of the alternation of hot and cold epochs, commonly known as glacial periods. Now, it has been demonstrated beyond all doubt that at one period of the earth's history the Arctic regions were much warmer than they are at present; this is proved by the occurrence in the geological formations of these high northern latitudes of plants in a fossilized state, which were utterly incapable of existing in any latitude where the climate was colder than it is now in our temperate regions. Reasoning from analogy and our knowledge of the present distribution of Arctic plants, it would not be improbable that the plants inhabiting the lands of the pole were the same on all longitudes of the Arctic Circle. Let us, then, suppose the glacial period to commence in these warm lands. Each plant, following to a greater or less degree the longitudinal line on which it grew, would be slowly but steadily driven by the increasing cold to take refuge in warmer and more southern stations. The cold, in the course of years following them slowly up, would compel them to keep continuing their journey southward until such time as the maximum of cold had been reached. Then, if, as it is reasonable to suppose, many of these plants had migrated on the longitudinal line upon which they had lived directly southward, we would find that the plants, which at the Arctic Circle, or beyond, had lived in close proximity to each other, would be separated when they reached the temperate zone by hundreds of miles.

In the general journey southward, the plants of the mountains would descend to the plains and mingle with those of the far north. Then the climate commences to moderate; and, as the mountains of ice and snow retire to their original homes in the north, many of the plants would keep company with the cold and return, but many others, encountering mountains in their paths, would find the climate cold enough for their growth, and would be left there in isolation while their nearest relatives would be separated from them by hundreds of miles of country.

Now would come the cold period of the southern hemisphere and drive the plants inhabiting the country there northward, and these again