Page:Popular Science Monthly Volume 66.djvu/245

Rh these prototypes connect with the modern mammals. This fauna is found in the Cretaceous and basal Eocene of Europe, North America, and possibly in Patagonian beds of South America (Ameghino), and while giving rise to many dying-out branches, by theory furnished the original spring from which the great radiations of modern mammals flowed. But practically again we await the direct connections and the removal of many difficulties in this theory. In fact, one of the great problems of the present day is to ascertain whether this radiation of Cretaceous mammals actually furnished the stock from which the modern mammals sprang, or whether there was also some other generalized source.

The Tertiary, or Age of Mammals, presents the picture of the dying out of these Cretaceous mammals in competition with the direct ancestors of the modern mammals. I use the word modern advisedly, because even the small horses, tapirs, rhinoceroses, wolves, foxes and other mammals of the early Tertiary are essentially modern in brain development and in the mechanics of the skeleton as compared with the small brained, ill-formed and awkward Cretaceous mammals.

Whatever the origin, two great facts have been established: first, the modern mammals suddenly appear in the Lower Eocene (as distinguished from the basal Eocene, in which the Cretaceous mammals are found), and second, they enjoy a more or less independent evolution and radiation on each of the four great continents. There thus arose the four peculiar or indigenous continental faunæ of South America, of North America, of Europe and Asia or Eurasia and of Africa. Of these South America was by far the most isolated and unique in its animal life. North America and Eurasia were much the closest, and Africa acquired a half way position between isolation and companionship with Eurasia.

South America.—The most surprising result of recent discovery is that the foreign element mingled with the early indigenous South American fauna is not at all North American but Australian. The wonderful variety of eight orders of indigenous rodents, hoofed animals, edentates and other herbivores were preyed upon by carnivores of the marsupial radiation from Australia, which apparently came overland by way of Antarctica. There are possibly here also some South African foreigners. The South American radiation more or less closely imitated that of the northern hemisphere. Late in Tertiary times North America exchanged its animal products with South America, practically to the elimination of the latter.

Eurasia and North America.—Each of these continents contained four orders of mammals in common with South America, namely, the Primates (monkeys), the Insectivores (moles and shrews), the Rodents (porcupines, mice, etc.), and the Edentates (armadillos, etc.). From some early Tertiary source North America, Eurasia and Africa also acquired in common four great orders of mammals which are not found