acteristic and peculiar structure had arisen as they handed down by inheritance to all the sauropsida and mammalia; there is the same shoulder girdle and pelvic girdle, the same simple hollow bones in the upper arm and upper leg, the same pair of bones in the forearm and lower leg, the same complicated union of bones in the wrist and ankle, the same typical arrangement of five fingers and five toes. This striking agreement in the assembling of the bony framework in all the higher four-footed vertebrates struck many thoughtful observers more than a hundred years ago; among others it led our greatest poet and thinker, Goethe, to those remarkable observations on the morphology of animals that we may consider the direct precursors of the modern ideas of Darwin.
We can, in fact, show, as a certain sign of the derivation of man from the oldest five-toed or pentadactylate amphibians, the fact that we possess to-day on our hand five fingers and on our foot five toes. Man and most primates (not all) show in this and in other respects that through conservative inheritance they have preserved the original plan of structure much more closely than have the majority of other mammals, especially the ungulates. Among others the one-toed horse on the one side and the two-toed ruminants on the other, are much more modified and specialized than are the primates.
The oldest amphibia of the Carboniferous period, the armored Stegocephali (and especially the remarkable Branchiosauria discovered by Credner), are now quite justly considered by all discriminating zoologists as the undoubted common stem group whence were derived all four-footed animals (Tetrapoda or Quadrupeda), all amphibia and amniota. But what was the origin of this important group itself? To this question also the great advances of palæontology afford a satisfactory answer which harmonizes excellently with the older solutions given by comparative anatomy and ontogeny. Already in Jena, forty-four years ago, the first master of comparative anatomy, Carl Gegenbaur, in a series of classical essays, pointed out that the most important parts in the vertebrate skeleton, particularly the skull and the bones of the limbs, reveal to us to-day, in the succession of classes of living vertebrates, a coherent scale of phyletic steps of development. Apart from the more lowly organized Cyclostomatait is especially the true fishes, and among them again the primitive fishes or Selachians (sharks and rays), which have proved most constant to the original form in the essential relations of their bodily structure. To the Selachians are closely allied the ganoids or enamel fishes, especially the Crossopterygii which take us farther back to the Dipneusta. Among these last the Australian fish Ceratodus has recently become of great interest, its anatomy and ontology having been carefully investigated by Günther and Semon. By this transition group of Dipneusta' or amphibious fishes—that is to say, fishes with lungs, but also with fins, with pentadactylate limbs—is the morphological bridge to the early amphibians
