CREATION BY EVOLUTION
Miocene epoch of the Tertiary period Central America and the Isthmus of Panama were raised above the sea, and North and South America were thus connected. This uplift made possible the migration of mammals in both directions, the northern forms going south, the southern forms going north. The earliest northern mammal yet discovered in South America is a raccoon-like carnivore, found in the upper Miocene of Catamarca, the Andean province of Argentine; and the first known Neotropical creature to arrive in North America was a ground sloth, found in the Miocene rocks of Oregon. From this beginning, the proportion of mammals common to both continents steadily increased, reaching a maximum in the Pleistocene, before the beginning of the great extinction already mentioned. In that extinction both continents lost a large proportion of their mammals. In particular, the southern invaders of North America, which had been so abundant in the Pleistocene, nearly all disappeared, leaving only the Canada porcupine as a remnant of that invasion.
In marked contrast to this failure of the Neotropical animals to establish themselves permanently in North America was the success of the northern immigrants in maintaining their foothold in South America. True, many of them, such as the mastodons, horses, short-faced bears, and sabre-tooth tigers, eventually died out, but they died out also in North America, having been among the many victims of the great Pleistocene extermination. However, large numbers are still there and constitute the second group of Neotropical mammals enumerated above. Some of these immigrants, notably the deer and the beasts of prey, have been so long in their southern home that they have undergone considerable modification and must be placed in genera different from those that include their relatives in North America. Among these
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