1894. PROCEEDINGS OF THE NATIONAL MUSEUM. 441 



It scarcely seems necessary to enter into any argument to sliow that 

 these tropical forms now found in Florida are not the lineal descend- 

 ants of the Helices and BuUmulus, the Cylindrella and Cerion or other 

 species of the Miocene silex beds of Tampa. The living land and fresh- 

 water mollusks of Florida of tropical origin are abt^olutely identical with 

 forms at present found in Cuba, Jamaica, and the continent, while those 

 of the Tampa beds are all extinct, and we can not for a moment sup- 

 pose that their descendants would be specilically identical with Antil- 

 lean and Mexican forms that had come from another line of descent. 

 I believe that the present species have been colonized but a short time 

 in Florida, and the fact that, although the soil, contour, and climate of 

 the country are quite different from those of tropical America, not a 

 single introduced form has as yet changed specifically, and only one 

 possibly varietally, is strong evidence in this direction. 



It is most likely that tropical land snails have been cast on the shores 

 of the peninsula with the jetsam and flotsam of the sea ever since the 

 Gulf Stream has had its present course, an amply sufficient length of 

 time for the development of species from some of the original wander- 

 ers, as that great ocean river was probably turned into the Gulf of 

 Mexico and past the State of Florida during the latter i^art of the 

 Miocene, when the Isthmus of Panama is believed to have been raised 

 and North and South America were joined together. I wt)uld suggest 

 that the reason why no such new species exist there might be that in 

 all probability any forms that were colonized on the peninsula prior to 

 the time of the Glacial Epoch were destroyed by the change of climate, 

 which swept out of existence, and drove to the southward so much of 

 the animal and vegetable life of North America.* The presence of a 

 great cap of ice coming down to the latitude of 40°, within 10° or 000 

 miles of the northern jiart of the State, would, especially at certain 

 times in winter during the prevalence of northerly winds, be likely 

 to destroy by cold such species as might be landed by the Gulf Stream. 

 Even now, with a much milder climate than this region j)ossessed dur- 

 ing Glacial times, an occasional unusually severe winter almost annihi- 

 lates the tropical fishes of this region, and kills in part, or even entirely, 

 many forms of West Indian vegetation as far ^south as Cape Sable. 

 Several of these Cuban land snails are only met with on the lower 

 keys, an area practically free from Irost. 



We know that a very slight difterence in latitude or climate may 

 often set a bound on the distribution of different forms of animal and 

 vegetable life. Thus, nearly all the immigrant West Indian vegetation 

 now found in Florida is confined to the southern half of the x>eninsula, 

 though there are no apparent reasons so far as soil, food, and moisture 



* It may be claimed that the continual addition of fresh individuals from Cuba by 

 the currents has kept the species already landed in Florida from becoming specili- 

 cally changed. But the same addition of fresh specimens must have occurred in the 

 Bahamas and yet we there tind a number of new species and countless varieties. 



