Before the close of the period true reptiles appear, but this race of animals is destined to make a more spectacular advance than the amphibians and will be discussed in connection with Mesozoic life. The amphibians, however, are regarded as being the ancestors of the reptiles as well as the higher quadrupeds which follow them. Although living reptiles are readily distinguished from living amphibians there is a different situation with regard to these primitive forms, for among the fossils it becomes increasingly difficult to separate the two groups as new material is investigated. Invertebrates had their ups and downs during the period. Trilobites became scarce, and brachiopods for a time were the most abundant of the shelled animals but later declined rapidly. In favorable localities the crinoids established a wonderful record for new species before the period had advanced very far. Hundreds of species of Carboniferous invertebrates are known, and in many of the rocks of the period they are the only fossils to be found, for the vertebrates were still unable to venture far from the swampy districts, and much inland territory was too well drained to support either the floras or faunas then existing. In the Colorado area there are both marine and continental formations but the great coal-making forests and their inhabitants were limited to other localities. As a consequence this region is not famous for Carboniferous fossils. PERMIAN HARDSHIPS For a time there was no great change in North America following the opening of the Permian. Then began a series of mountain-making movements and continental uplifts which drained the swamps, lakes, and inland seas. With the passing of the vegetation which had established itself in and around these areas much of the animal life followed. It is probable that a considerable proportion of the marine life survived, much more than is indicated by the fossil record, but the receding seas carried the survivors into territory which is now inaccessible to fossil hunters. After Middle Permian time the climate everywhere seems to have been cold and dry. By the end of the period there had been accomplished more geographical change throughout the world than at any time since the beginning of the Paleozoic era. Traces of the crustal movements which produced new mountain ranges can be followed in Europe, Asia, and North America. The Appalachian region was raised to a great height, possibly in excess of three miles. A major disturbance of this character is known among geologists as a revolution, and to this particular one the name “Appalachian Revolution” has been given.