As to the early Mesozoic flora, I have already suggested that it must have been an invader from the south, for which the intervening Permian age had made way by destroying the Palæozoic flora. This was probably effected by great earth-movements changing geographical conditions. But in the Mesozoic the old conditions to some extent returned, and the Carboniferous plants being extinct, their places were taken by pines, lycopods, and ferns, whose previous home had been in the insular regions of the tropics, and which, as climatal conditions improved, pushed their way to the Arctic circle. But, being derivatives of warm regions, their vitality and capacity for variation were not great, and they only locally and in favourable conditions became great coal producers. The new flora of the Later Cretaceous and the Tertiary, as previously stated, originated in the Arctic, and marched southward. These newer Cretaceous plants presented from the first the generic aspects of modern vegetation, and so enable us much better to gauge their climatal conditions. In general, they do not indicate tropical heat in the far north, but only that of the warm temperate zone; but this in some portions of the period certainly extends to the middle of Greenland, unless, without any evidence, we suppose that the Cretaceous and lower Tertiary plants differed in hardiness of constitution from their modern representatives. They prove, however, considerable oscillations of climate. Gardner, Nathorst and Reid have shown this in Europe, and that it extends from the almost tropical flora of the lower Eocene to the Arctic flora of the Pleistocene. In America, owing, as Grey has suggested, to its great north and south extension, the changes were more regular and gradual. In the warmer periods of the Cretaceous, the flora as far north as 55° was similar to that of Georgia and Northern Florida at the present day, while in the cooler period of the Laramie (Lower Eocene, or more probably Paleocene) it was not unlike that of the Middle States. In the Pleistocene, the flora indicates a boreal temperature in the Glacial age. Thus there are no very extreme contrasts, but the evident fact of a warm temperate or subtropical climate extending very far north at the same times when Greenland had a temperate climate. As I have elsewhere shown,[110] discoveries in various parts of North America are beginning to indicate the precise geographical conditions accompanying the warmer and colder climates. [110] Trans. Royal Society of Canada, 1890-1.