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, while 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 

 eurface, 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 



