242 ORIGIN OF LIFE IN AMERICA 



during which the S6a 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 Gr.egory 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 betweeoi 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 1 

 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- 



