Page:Popular Science Monthly Volume 12.djvu/545

Rh evidently the parent stock of Crocodilians, became extinct with Hyposaurus of the same horizon, leaving the crocodile and gavial, with their more perfect procœlian vertebræ, to contend for the supremacy. In the early Eocene, both of these types were abundant, but some of the crocodiles possessed characters pointing toward the alligators, which do not appear to have been completely differentiated until later.

Nothing is really known to-day of the earlier genealogy of the Pterosauria; but our American forms, without teeth, are clearly the last stage in their development before this peculiar group became extinct. The oldest European form, Dimorphodon, from the Lower Lias, had the entire jaws armed with teeth, and was provided with a long tail. The later genus, Pterodactylus, retained the teeth, but had essentially lost the tail; while Ramphorhynchus had retained the elongated tail, but had lost the teeth from the fore-part of both jaws. In the genus Pteranodon, from the American Cretaceous, the teeth are entirely absent, and the tail is a mere rudiment. In the gradual loss of the teeth and tail, these reptiles followed the same path as birds, and might thus seem to approach them, as many have supposed. This resemblance, however, is only a superficial one, as a study of the more important characters of the Pterodactyls shows that they are an aberrant type of reptiles, totally off the line through which the birds were developed. The announcement made not long since in Europe, and accepted by some American authors, that the Pterosauria, in consequence of certain points in their structure, were essentially birds, is directly disproved by American specimens, far more perfect than those on which the conclusion was based.

It is now generally admitted, by biologists who have made a study of the vertebrates, that birds have come down to us through the Dinosaurs, and the close affinity of the latter with recent struthious birds will hardly be questioned. The case amounts almost to a demonstration, if we compare, with Dinosaurs, their contemporaries, the Mesozoic birds. The classes of Birds and Reptiles, as now living, are separated by a gulf so profound that a few years since it was cited by the opponents of evolution as the most important break in the animal series, and one which that doctrine could not bridge over. Since then, as Huxley has clearly shown, this gap has been virtually filled by the discovery of bird-like reptiles and reptilian birds. Compsognathus and Archæopteryx of the Old World, and Ichthyornis and Hesperornis of the New, are the stepping-stones by which the evolutionist of to-day leads the doubting brother across the shallow remnant of the gulf once thought impassable.