[Illustration: Plate 13.--A Slab of Very Early Paleozoic (Cambrian) Rock, Covered with Some of the Oldest Known Definitely Determinable Animal Remains. These creatures lived in a sea which overspread the site of the Rocky Mountains of southern Canada fully 25,000,000 years ago. Most of the fossils are trilobites (including some very small ones) and other related crustacean forms (lighter portions). (_After C. D. Wolcott, Smithsonian Institution, Washington, D. C._)] [Illustration: Plate 14.--(_a_) Photographs of Small Slabs of Ordovician Strata Full of Fossils. These slabs are actual bits of sea bottom at least 20,000,000 years old. The left picture shows "stone-lily" stems, so-called "sea mosses," brachiopods. Right picture shows various species of brachiopods. (_Photo by the author._)] [Illustration: Plate 14.--(_b_) An Outcrop of Middle Ordovician Stratified Limestone in Northern New York. This ledge is full of fossils similar to those above. The material was deposited on the floor of the Ordovician sea which overspread much of the continent. (_Photo by the author._)] The introduction of the higher flowering plants (Angiosperms) "was, perhaps, the most important and far-reaching event in the whole history of vegetation, not only because they almost immediately became dominant, but also because of their influence upon the animal life of the succeeding periods. Hardly had flowers appeared, before a great horde of insects, which fed upon their honey or pollen, seem to have sprung into existence. The nutritious grasses and the various nuts, seeds, and fruits afforded a better food for noncarnivores than ever before in the history of the world. It was to be expected, therefore, that some new type of animal life would be developed to take advantage of this superior food supply. As we shall see in the discussion of the Tertiary (next chapter), the mammals, which kept a subordinate position throughout the Mesozoic, rapidly took on bulk and variety and acquired possession of the earth as soon as they became adapted to this new food, quickly supplanting the great reptiles of the Mesozoic." (Cleland.)