338 TROPICAL NATURE, AND OTHER ESSAYS. 



liighlands of Mexico and Guatemala united to North 

 America. 



Around the Gulf of Mexico and tlie Caribbean Sea 

 there is a wide belt of rather shallow water, and during 

 the alternate elevations and subsidences to which this 

 region has been subjected, the newly raised land would 

 afford a route for the passage of immigrants between 

 North and South America. The great depression of the 

 ocean, believed to have occurred during the Glacial 

 period (caused by the locking-up of the w^ater in the two 

 polar masses of ice), may perhaps have afforded the 

 opportunity for those latest immigrations which gave so 

 striking a character to the North American fauna in 

 Post- Pliocene times. 



Anions: the chanp:es which South America itself has 

 undergone, perhaps the most important has been its 

 separation into a group of large islands. Such a change 

 is clearly indicated by the immense area and low eleva- 

 tion of the great alluvial plains of the Orinoko, Amazon, 

 and La Plata, as well as by certain features in the dis- 

 tribution of the existing Neotropical fauna. A subsi- 

 dence of less than 2,000 feet would convert the highlands 

 of Guiana and Brazil into islands separated by a shallow 

 strait from the chain of the Andes. "When this occurred 

 the balance of the land was probably restored by an 

 elevation of the extensive submerged banks on the east 

 coast of South America, which in South Brazil and 

 Patagonia are several hundred miles wide, embracing 

 the Falkland Islands, and reaching far to the south of 

 Cape Horn. 



Looking, then, at the whole of the evidence at our 

 command, we seem justified in concluding that the past 



