Page:Popular Science Monthly Volume 41.djvu/840

820 is that lie has finally succeeded in finding among this mass of bones one bone, at least, which bears unmistakable traces of having been connected with a humerus, the head of which must have been as substantial as in cassowaries. He thus considers it proved that the Dinomithidæ, like the kiwis, descended from birds which could fly. The last missing link is thus discovered, and the chief points in the genealogy of birds are thus already settled, while many a gap which still remains will certainly be filled up when the rich materials recently excavated in both Americas have been carefully examined by anatomists.

The same may be also said in regard to mammals, if the recent discoveries in North and South America are taken into account. The earliest traces of mammals have been found, as is known, in the Triassic deposits of Germany, Basutoland, the Cape Colony, and North Carolina; and it is also known, through the previous remarkable works of Professors O. C. Marsh and H. F. Osborn, that the Jurassic deposits of Wyoming have yielded a rich fauna, among which we find the remote ancestors of various orders of the present mammalia. But the most important finds, which throw a new light both on the earlier and the subsequent forms, have been made in that immense area of lacustrine beds which have been deposited in the region of the great salt lakes of Utah, Wyoming, and Colorado, from the end of the Cretaceous period down to the middle parts of the Tertiary epoch. There, and especially in the Eocene "Puerco" and "Wahsatch" beds, as well as in the Eocene "Uinta" formation, a rich fauna of mammals has been unearthed. All those Eocene mammals had something in common in their leading features, and yet they offered a sufficient diversity for being considered as the probable ancestors of nearly all orders of placental mammals. To mention their feet only, they were adapted, in all of them, for walking upon the sole, and were provided with five toes; but it is easy to recognize in the structure of the feet of the different genuses such divergences as necessarily ought to evolve, under certain conditions—on the one side, the plantigrade foot of the bears, and, on the other side, the digitigrade foot of the Ungulata (horses, camels, elephants, and so on), who walk upon the points of their toes; and, again, among