286 
ORIGIN OF LIFE IN AMERICA 
theory, largely based on the supposed drowned valleys of the 
Antillean area, that the two American continents were united 
with one another by means of a West Indian land bridge 
during the Pleistocene Period. He thought this land must 
have had the enormous altitude of from 8,000 to 12,000 feet. 
Against this view it may be urged from a general faunistic 
study that such a land bridge is out of the question, 
because the Antillean fauna would be quite different from 
what it actually is. A very small part of southern Florida 
may have been still joined, by way of the Bahamas, to Cuba 
and Anguilla in Pliocene times. But southern Florida must 
then have been separated from North America by a marine 
channel. Nor could there have been any junction of the 
greater Antillean land with South America, by means of the 
Lesser Antilles, at any time during the Tertiary Era. In all 
probability the West Indies, while still partly united with one 
another, had some kind of land connection with Central 
America, w r hile the latter was cut off by the sea from both 
North and South America. Hence the mammalian supply came 
mostly from the west. Still, it may be asked, how did this 
fauna reach Central America ? Almost all the Antillean mam¬ 
mals, both recent and extinct, can be traced to a southern 
ancestry, except Castoroides, of which we only know that it 
appeared in the north along with a host of southern invaders. 
I believe the cause of the mystery connected with the occa¬ 
sional entrance during various periods in Tertiary times of 
South American forms into North America is now buried 
beneath the waves far out in the Pacific Ocean (see Fig. 14). 
I have frequently suggested the theory that a large land 
surface, mainly tenanted by a South American stock, lay 
westward of Central America. A good deal of the next 
chapter will be devoted to its consideration. Meanwhile, 
I can only state my conviction that the animals alluded to, 
and many others, had their origin on that western Pacific 
land whence they streamed into the neighbouring continents 
whenever physical conditions offered an opportunity for 
doing so. 
The main conclusion which this brief study of the mam¬ 
malian fauna of the Antilles has revealed is that the larger 
islands were formerly united with one another, and with the 
