442 ^'ES T IXDIAX MOLL VSES—SIMPSOX. 



are concerned Tvby it slionld not extend uortli indefinitely. The tropical 

 laud snails of Mexico come north in the low lands lor the most part 

 only to the northern border of that Kepublic. and many other instances 

 of a like restriction by climate could be given. It is probable that a 

 decrease of a very few degrees in temperature would destroy the Antil- 

 lean land and fresh-water mollusks in Florida. 



It is bebeved by many of our ablest glacialists that the Ice Age 

 lasted down to within from 6.000 to 10,000 years of the present, and the 

 period which has elapsed since its close would probably be too short to 

 allow for any considerable variation in mollusks. The Bahamas being 

 protected on the north and west by the Gulf Stream, and lying gener- 

 ally in a lower latitude, no doubt enjoyed during the Glacial Epoch a 

 milder climate than Florida, and have been peopled longer with immi- 

 grant forms: a sufficient time to allow for the development of numer- 

 ous varieties and species, but no groups or genera. 



I think there need be no difficulty in accounting for the presence of 

 tropical land and fresh-water mollusks in Florida by means of the trans- 

 porting agency of the sea. The Gulf Stream sweeps up past northern 

 South and Central America, part of it eddying around in the Gulf of 

 Mexico. A branch of it, however, flows along the north shore of 

 Cuba, and by the shoal in latitude 24° is thrown in close to the lower 

 chain of Florida Keys. Alexander Agassiz says:* 



The curve of the Florida reef along the Gulf Stream is ilue in great measure, as 

 Huut shows, to a counter current along the reef running westward. This current 

 is known to all navigators, and though ill defined at Cape Florida becomes stronger 

 and wider as it goes west. It has a width of at least 10 miles at Key West and 20 

 miles at Tortugas. This is clearly shown by the mass of surface animals driven along 

 upon this westerly current by the southeasterly winds. The tides set strongly across 

 the reefs and through the channels between the keys, the tiood running north and 

 the ebb south. 



^lollusks washed down with trees, bamboos, or masses of drift from 

 the northern shore of Cuba would be swept along by the strong cur- 

 rent of the Gulf Stream to the eastward and northward, and many of 

 them, carried by the southeast winds into this counter current, might 

 be landed by the intiowing tide among the lower keys within a few 

 days after leaving their native island. Species from Honduras might 

 at long intervals be drifted by westerly winds across to the eastern 

 part of the Gulf Stream, and so be carried around and landed in the 

 way I have described; or they might possibly sometimes survive a 

 passage around the eddy in the gulf. The fact that there are more forms 

 from Cuba found in Florida than there are from Middle America, and 

 that only a single very doubtful South American species is known in 

 that state,t illustrates the comparative difficulty which these wanderers 



''Three cruises of the Blake, i. p. 57. 



i Bullnuilus dormaui, \V. G. Binney, is thought to he the same as B. maculatus, Lea 

 of Cartagena, Colombia, but this is not certain. B. muliilineatus, Say, formerly 

 believed to be an immigrant from South America, is now known to be found in Cen- 

 tral America and Yucatan. 



