It was an evergreen world, a world, apparently, of perpetual summer. No trace is found until the next period of an alternation of summer and winter--no trees that shed their leaves annually, or show annual rings of growth in the wood--and there is little trace of zones of climate as yet. It is true that the sensitive Ammonites differ in the northern and the southern latitudes, but, as Professor Chamberlin says, it is not clear that the difference points to a diversity of climate. We may conclude that the absence of corals higher than the north of England implies a more temperate climate further north, but what Sir A. Geikie calls (with slight exaggeration) "the almost tropical aspect" of Greenland warns us to be cautious. The climate of the mid-Jurassic was very much warmer and more uniform than the climate of the earth to-day. It was an age of great vital expansion. And into this luxuriant world we shall presently find a fresh period of elevation, disturbance, and cold breaking with momentous evolutionary results. Meantime, we may take a closer look at these interesting inhabitants of the Middle Ages of the earth, before they pass away or are driven, in shrunken regiments, into the shelter of the narrowing tropics. The principal change in the aspect of the earth, as the cold, arid plains and slopes of the Triassic slowly yield the moist and warm ow-lying lands of the Jurassic, to consists in the character of the vegetation. It is wholly intermediate in its forms between that of the primitive forests and that of the modern world. The great Cryptogams of the Carboniferous world--the giant Club-mosses and their kindred--have been slain by the long period of cold and drought. Smaller Horsetails (sometimes of a great size, but generally of the modern type) and Club-mosses remain, but are not a conspicuous feature in the landscape. On the other hand, there is as yet--apart from the Conifers--no trace of the familiar trees and flowers and grasses of the later world. The vast majority of the plants are of the cycad type. These--now confined to tropical and subtropical regions--with the surviving ferns, the new Conifers, and certain trees of the ginkgo type, form the characteristic Mesozoic vegetation.