Toward the end of the tertiary period there was an upheaval of land between this old South American island and North America, near what is now the Isthmus of Panama, thereby making a bridge across which the teeming animal life of the northern continent had access to this queer southern continent. There followed an inrush of huge, or swift, or formidable creatures which had attained their development in the fierce competition of the arctogaeal realm. Elephants, camels, horses, tapirs, swine, sabre-toothed tigers, big cats, wolves, bears, deer, crowded into South America, warring each against the other incomers and against the old long-existing forms. A riot of life followed. Not only was the character of the South American fauna totally changed by the invasion of these creatures from the north, which soon swarmed over the continent, but it was also changed through the development wrought in the old inhabitants by the severe competition to which they were exposed. Many of the smaller or less capable types died out. Others developed enormous bulk or complete armor protection, and thereby saved themselves from the new beasts. In consequence, South America soon became populated with various new species of mastodons, sabre-toothed tigers, camels, horses, deer, cats, wolves, hooved creatures of strange shapes and some of them of giant size, all of these being descended from the immigrant types; and side by side with them there grew up large autochthonous ungulates, giant ground sloths wellnigh as large as elephants, and armoured creatures as bulky as an ox but structurally of the armadillo or ant-eater type; and some of these latter not only held their own, but actually in their turn wandered north over the isthmus and invaded North America. A fauna as varied as that of Africa to-day, as abundant
