DISPEBSAL OF MOLLUSES. 53 



to a much greater distance. "'* In addition to this means of aerial 

 dispersion, the distribution of insects may be to a great extent ef- 

 fected in the condition of eggs, which retain a considerable amount 

 of vitality, and which are not infrequently laid in decaying timber 

 and in the living tissues of various plants. When, therefore, float- 

 ing rafts or mats are apt to be formed, and to be floated out to sea, 

 it is almost certain that with them will be carried out a host of in- 

 sects — whether in the perfect form, as grubs, or as eggs — of different 

 species, a fair proportion of which will, doubtless, have retained 

 their vitality even after a protracted sea-voyage of several thousand 

 miles. It is in this manner that many or most of the tropical forms 

 which periodically make their appearance on the British coast have 

 been transported thither, the current of the Gulf Stream, which 

 trends in a general northeasterly direction, being instrumental in 

 drifting tropical log- wood to the trans- Atlantic temperate shores. 



Dispersal of Mollusks.— The world-wide distribution of the 

 fresh-water and terrestrial MoUusca, and the occurrence of identical 

 or very nearly allied generic forms at opposite quarters of the globe, 

 prove conclusively that the animals of this class are favoured with 

 special instrumentalities by which a broad distribution is eilected. 

 Land-snails of the genus Helix are found in all the continental 

 areas, from the polar regions to the Equator, and from the limit of 

 perpetual snow on mountain summits to the level of the sea ; they 

 are also found in all the oceanic islands, even the most remote, that 

 have thus far been visited. The exact nature of this distribution 

 has not yet been positively determined, and, in fact, there are sev- 

 eral difficulties in the way of accounting for it. It is well known 

 that these animals cannot survive for any length of time the effects 

 of salt water, and this water is almost immediately fatal to the 

 vitality of the eggs. Hence, only under exceptional conditions is 

 it possible to account for a transferrence over a broad expanse of 

 oceanic surface. But it has been ascertained that such forms as are 

 capable of secreting an epiphragm, and therewith closing up the 

 entrance to the shell, are able to resist the injurious effects of salt 

 water for a very considerable period, in some instances as much as 

 two weeks, or more, as has actually been determined experimentally 

 by the immersion of land-shells in the briny medium. In regard to 

 these, therefore, there will be no difficulty in accounting for a broad 

 distribution, since they, and especially the genus Helix of all others, 



