242 
ORIGIN OF LIFE IN AMERICA 
during which the sea spread across Central America. The 
general opinion seems to be that the submergence of Central 
America was due to extensive subsidence in the Caribbean 
area and likewise in the Gulf of Mexico. The Mediterranean 
character of the West Indian marine fauna, moreover, implies 
the probability of a free migration from the one area to the 
other along some ancient shore-line. A land bridge joining 
North and South America along the chain of the Lesser 
Antilles, alluded to by Professor Gregory, may possibly have 
existed; but it must, I think, already have been destroyed 
at the time of the submergence of the Isthmus of Panama. 
And yet I concur with Professor Gregory in thinking that 
when the latter was submerged there need not necessarily 
have been free communication between the Atlantic and the 
Pacific Oceans. We must remember that all efforts have 
hitherto failed to discover any traces of Tertiary sediments on 
the sea-board between southern Mexico and Panama. This 
seems to imply that land lay to the west of Central America, 
and that the Pacific Ocean was formerly situated further 
westward than at present. What would appear as connections 
between the two oceans may have been merely shallow bays in 
the land referred to. Certain peninsulas would then have 
projected eastward from this old Pacific land towards those 
parts of Central America that were then in existence (see 
Fig. 16). Without giving further evidence, these theories 
may appear somewhat visionary, but as the subject will be 
more fully discussed later on (p. 408), I need not enlarge 
upon it at present. 
A comparison of the marine faunas of the two shores of 
Central America does not yield such satisfactory results in 
establishing the geological age of the submergence, because 
we have as yet little idea of the length of time during which' 
animals may retain their specific characters. The evidence 
derived from the first appearance in North America of dis¬ 
tinctly South American mammals would seem to> give us a 
better clue as to the date of the formation of the present 
Central American land bridge. 
This appeal to the past dispersals of mammals in recon¬ 
structing former conditions of land and water has been utilised 
in several of the previous chapters, and in this case many bio- 
