INTRODUCTION
THE PRIMITIVE SKELETON OF REPTILES
That the reptiles were evolved from the Amphibia, and more specifically from that order known as the Temnospondyli, seems now assured. The earliest as also the most primitive reptiles that we know belong to the order called the Cotylosauria. With the exception of Eosauravus from the middle Pennsylvanian of Ohio, of which, unfortunately, the skull is unknown, our knowledge of them goes no further back than the late Carboniferous and early Permian. At that time there was a considerable diversity of known forms, belonging to at least four well-differentiated groups and twenty or more families; from which we may very properly conclude that their earliest ancestors, the beginning of their stock, lived much earlier, certainly at the beginning of the Upper Carboniferous, and very probably in Lower Carboniferous times. We therefore never can expect to find in the rocks of the Permian any real connecting link between the two classes.
Both the reptiles and the amphibians had changed in this interval, an interval perhaps of millions of years, retaining in varying degrees their ancestral characters, while losing or adding others in various ways. The reptiles, by the acquirement of a new mode of life, the loss of gills in their youth and entire emancipation from the water, became more progressive than the amphibians, and their evolution was more rapid. Characters that are common to many amphibians became more and more rare among the reptiles, and the amphibians, handicapped by inherited habits, were restricted more and more to subordinate rôles, and only a few of the more progressive continued to develop. They, for the most part, lost those characters and adaptations that brought them into immediate competition with the reptiles, and by the close of Triassic times had become restricted to habits and habitats no longer invaded by them. The modern toads, frogs, salamanders, and blindworms differ far more from the higher amphibians of Paleozoic times than did the latter from their contemporary reptiles.
