It is to be observed with reference to the age of these beds, that as the Later Cretaceous and Eocene flora of Europe and America migrated from the north, the plants found in the beds of that age in the temperate latitudes may really be somewhat older in the Arctic regions, a fact which produces some uncertainty as to their actual age. The warmth required for the growth of luxuriant forests near the Pole might be secured by a different distribution of land and water, and of the oceanic currents, but the requirements of plants as to light seem more difficult to meet, and it has been doubted whether species similar to those which are accustomed in modern times to regular alternations of day and night could submit to the long Arctic winter darkness. It is known, however, that in conservatories in Northern Russia plants supplied with heat and moisture can endure in winter great deprivation of light, and at Disco, in Greenland, roses and fuchsias flourish as house plants.[76] These facts show that if there were sufficient light and heat in summer, a great number of the plants of temperate latitudes could endure extreme cold and much deprivation of light in winter. It may be well here to inform the reader that some confusion as to the succession of the Cretaceous and Tertiary floras in America has arisen from the fact that the plants which are evidently Eocene in Greenland and America have been until lately incorrectly regarded as Miocene in Europe. In the Western States, the Dakota group of Lesquereux is overlain by 2000 feet of Cretaceous beds, containing the marine shells characteristic of that age, but no plants. But in Vancouver’s Island these same Upper Cretaceous beds contain an abundant flora, which some botanists have called Tertiary for the reason already mentioned. Above the 2000 feet of marine beds overlying the Dakota group is the Lower Lignite group of Lesquereux, holding many fossil plants, including Palms and other evidences of a warmer climate than that of the Cretaceous, and which constitute a Lower Eocene flora corresponding in some respects to that of Europe. This is succeeded by an Upper Lignite group, also Eocene, but representing a more temperate climate, and therefore resembling more nearly the Cretaceous flora. This is nearly identical with the so-called Miocene of Greenland, Alaska, and Mackenzie River, which the facts collected by the Canadian geologists have shown to be really Eocene.[77] But the Canadian reports containing these facts are comparatively little known in Europe, hence incorrect ideas as to the succession of these floras have been handed from one writer to another.