THE STORY TOLD BY FOSSIL PLANTS
marked change through early Cretaceous time. The early Cretaceous cycads were essentially the familiar types of later Jurassic time. They were abundant in genera, species, and individuals, and they were quite as dominant an element of the lower Cretaceous floras as they had been of those of late Triassic and Jurassic time. Before the end of the Lower Cretaceous epoch, however, most of these plants had become extinct. In rocks laid down near the end of that epoch we find preserved the first representatives of the flowering plants (the angiosperms—that is, plants having enclosed seeds, ,such as the walnuts, oaks, and maples), and during Upper Cretaceous time these plants gradually became predominant.
Although the seas were widespread in early Mesozoic time there were many large areas of land, but we know nothing about the floras of these areas, which may have been the scene of the evolution of the flowering plants. Certainly during late Cretaceous time they spread continuously southward in Europe, North America, and Asia, and almost everywhere the same forms occur, alike in Bohemia, Alabama, or Sakhalin Island, localities suggesting their northern origin. During Upper Cretaceous time they penetrated far into South America, reaching Argentina, and they even reached Antartica (Graham Land). These Upper Cretaceous floras invariably show a mingling of temperate and tropical types, indicative of a humid warm-temperate climate, and they all contain forms that are to-day largely confined to the Southern Hemisphere. Throughout Upper Cretaceous time new types continued to appear and the stragglers from older floras gradually died out, so that by the dawn of the Tertiary period most of the archaic forms had become extinct.
The flowering plants possess for us a profound interest,
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