408 OEIGIN OF LIFE IN AMEEICA 



Eather earlier arrived in North America the mastodons, 

 and no doubt by the same Pacific land connection. They in- 

 vaded the continent from' Asia and remained until Pleistocene 

 times, being thus ca-existent with the early races of man in 

 America. That they travelled beyond North America, pene- 

 trating far into the southern continent, has been clearly de- 

 monstrated. A large number of apparently distinct forms have 

 been described from South America, most of them, as we 

 should expect, from the west coast, though it is very doubtful 

 if more than a few species lived there. Dr. Nordenskiold 

 thinks that Mastodon chilensis, b/L. bolivianus and M. andium 

 all belong to one species. In North America, as in Europe, 

 both the trilophodont and tetralophodont types of mastodon 

 have been discovered, that is to say, animals which possessed 

 intermediate molar teeth with either three or four ridges. 

 In the South American Mastodon andium, at any rate, the 

 molars are in the transition stage between the trilophodont 

 and tetralophodont types. These extinct elephants made their 

 first appearance in Argentina in the Lower Pampean deposits. 

 Since they had thus penetrated so far south in early Pliocene 

 times they must have left North America before Central 

 America had come into existence. They could only have 

 wandered southward along the southern continuation of the 

 Pacific land bridge and have entered the South American 

 continent from the west during the time the bridge was joined 

 to the latter. Dr. Nordenskiold * axgues that, having no more 

 efficient competitors in South America, the mastodon probably 

 lived longer there than in North America. 



Hitherto the theory has been quite generally accepted that 

 the invasions of animals from North America to the southern 

 continent, during later Tertiary and Pleistocene times, took 

 place across the narrow isthmus of Central America as soon 

 as its formation was completed. Some doubts have latterly 

 been raised as to the nature of the barrier which prevented 

 the interchange of the two faunas in the earlier portion of 

 the Tertiary Era. Yet until the discovery of gravigrade sloth 

 remains in the Miocene Mascall beds of Oregon was announced 

 by Professor Sinclair, the assumption seemed perfectly justi- 



* Nordenskiold E., " Saugetier-fossilien des Tarija Tals," pp. 14 — 25. 



