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 Mio- 

 cene 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 disap- 

 peared, leaving only the Canada porcupine as a remnant of 

 that invasion. 



In marked contrast to this failure of the Neotropical ani- 

 mals 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 mam- 

 mals 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 modi- 

 fication and must be placed in genera different from those 

 that include their relatives in North America. Among these 



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