RELATIONS TO OTHER SCIENCES 557 



barrier, evidently a sea, which separated the northern and southern 

 parts of America during the greater part of the Tertiary epoch, and 

 only disappeared to allow the free migration of land animals towards 

 the close of the Pliocene period. The removal of this barrier, which is 

 also indicated by purely geological researches, resulted thus in a 

 mingling of the native South American Tertiary fauna with a host of 

 invaders, whose ancestors flourished on the lands of the northern 

 hemisphere. In other words, the surviving land animals of South 

 America have been derived from two sources - - some from the native 

 stock which evolved in the country itself during the Tertiary epoch, 

 some from the late Pliocene invasion of northern life. Now, the native 

 stock just mentioned is of uncertain origin, but in its prime it in- 

 cluded the New World monkeys, many peculiar rodents, the sloths, 

 anteaters, and armadillos, and numerous remarkable hoofed animals 

 - altogether an assemblage unknown in any other region of the 

 world. Therefore, it seems impossible to escape from the further 

 conclusion that during the greater part of the early Tertiary epoch 

 South America was an isolated land, and its mammals developed 

 independently of those of other continents. On the other hand, it is 

 to be observed that during part of this time there lived in South 

 America several primitive carnivorous animals, perhaps marsupials, 

 which were most strikingly similar to the thylacines and dasyures 

 of the Australian region. There was also a horned land-tortoise, 

 Miolania, essentially identical with one of which species occur in the 

 Pleistocene deposits of Australia and Lord Howe's Island. Finally, 

 there was the familiar mud-fish, Ceratodus, which now survives only 

 in the Queensland rivers. It has therefore been thought that the 

 occurrence of remains of these animals among the Tertiary fossils of 

 South America favors the theory of a former land-connection be- 

 tween that country and Australia. In fact, they are sometimes 

 quoted as helping to confirm the hypothesis of the former existence 

 of a great Antarctic continent, which has broken up into the lands 

 now known as Australia, New Zealand, South Africa, and South Amer- 

 ica. The surviving thylacines of the Australian region, however, are 

 the very slightly altered descendants of a race of small-brained, prim- 

 itive carnivores, which are known to have lived throughout the 

 northern hemisphere, and were probably cosmopolitan at the begin- 

 ning of the Tertiary epoch. The Middle Tertiary carnivores of South 

 America and the modern thylacines of Australia may, therefore, be 

 merely the last survivors of an effete race, which was exterminated 

 early at all points except the two extremes of its once extensive 

 range. Similarly, Miolania is a horned and armored member of a sub- 

 order of tortoises (Pleurodira) , which was probably almost as nearly 

 cosmopolitan at the end of the Mesozoic and beginning of the Tertiary 

 epoch as is the suborder of Cryptodiran tortoises in the existing world. 



