408 
ORIGIN OF LIFE JIN AMERICA 
Rather 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 co-existent wjtli 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, M. 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 * argues 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- 
Nordenskiuld E., “ Saugetier-fossilien des Tarija Tals,” pp. Id 
2b. 
