1894. PROCEEDINGS OF THE NATIONAL MUSEUM. 425 



plants and animals are carried across more or less wide spaces of ocean 

 from shore to shore in various ways on the surface of the sea. In trop- 

 ical countries especially, heavy rains swell the mountain streams to 

 torrents, which tear up trees and masses of earth held together with a 

 network of roots, and bear them swiftly to the ocean. These are often 

 peopled with laud snails, or carry their eggs, and in their course to the 

 sea may frequently plow up nuid from the bottom which contains fresh- 

 water moUusks. Or fresh-water snails may inhabit the surface and 

 crevices of such trees, that become stranded on their way down, or lay 

 their eggs on them, the whole to be carried onward at the next flood. 

 In many places, notably the northern shores of South America, the sea 

 is constantly encroaching- on the land, and large areas of forest bound 

 together by matted roots and tangled vines are being undermined and 

 swept away. 



Of course if the distance from land to laud is short, and there are 

 winds that blow, or tides that run in the right direction, the probability 

 of su(;h mollusks being safely carried across is much greater than if 

 they have far to travel, have head winds, or contrary currents. Darwin 

 has shown that some of the land snails will live for considerable periods 

 of time in sea water, and many of the fresh-water species will remain 

 alive for some time in the air under favorable circumstances, and others 

 are found living in water more or less brackish. One who has noticed 

 much of the drift in the West Indian region — trees and bamboos, often 

 carrying masses of earth and stones — can easily believe that they might 

 bear witli them snails for quite a voyage without wetting them to any 

 considerable extent.* I think I shall be able to show that most of such 

 migrations in and about the West Indies have been accomplished 

 under favorable circumstances, that long stretches of unprotected sea, 

 head winds, and contrary currents have generally proven fatal to dis- 

 persal by the ocean. 



Third: — The agency of man. It is hardly necessary to mention the 

 well-known instances Avhere man is known to have been the means of 

 carrying from one country to anotlier different animals or plants. 

 Many mollusks are known to have been transported through his agency. 

 Helix similaris is a native of eastern Asia, but is now naturalized in 

 most of the warmer parts of the world. Helix aspersa and laciea, com- 

 mon edible snails of Europe, are colonized in a number of foreign coun- 



*The liollow stems of dead bamboos are a favorite retreat for many species of ter- 

 restrial and arboreal snails. Tbese gigantic tufts of grass are particularly abundant 

 along the streams in tropical countries, and are often washed out in time of floods, 

 and scattered along the banks. Between rainy seasons the upper joints become 

 more or less broken and decayed, and peopled with mollusks, and another freshet 

 sweeps them with their living cargo into the sea. The lower joints, being more 

 solid are perhaps air-tight, and serve to buoy up the whole mass ; the roots weighted 

 with earth and stones depress the lower end of the clump, and throw the ujiper and 

 inhabited end out of water. I have seen just such floating rafts, and nothing could 

 be more admirably contrived for transporting land snails safely across the sea. 



