CH. Ill] DISPERSAL OF MOLLUSCA. BY INSECTS. 143 



attached." The great water beetle Dytiscus has also been 

 proved to aid in the dispersal of fresh-water Mollusca, 

 while the small Notonecta, the " water boatman," belongs 

 to the same category. These insects are powerful fliers 

 and could certainly get from pond to pond even when 

 weighted with a small bivalve. It is bivalves that they 

 have been proved to convey, not univalves. Birds, newts 

 and toads have also been shown to act as the means of 

 dispersal of Lamellibranchs. The fact that these can and 

 do snap at and hold tight to any object that is thrust 

 between the two valves of the shell ensures their dispersal 

 in this way. 



The above facts enable us to understand the peopling 

 of isolated ponds. The distances to be traversed are so 

 small, that the means of dispersal appear to be quite 

 adequate. We now come to the more difficult question of 

 crossing arms of the sea or, it may be, wide tracts of 

 ocean. We must consider also the terrestrial as well as 

 the aquatic species. That both classes of Mollusca do 

 cross even the widest seas is shown by their occurrence in 

 oceanic islands; that this occurs but rarely is shown by 

 the peculiarities of oceanic island Mollusca as compared 

 with those of the nearest mainland, whence they have 

 been presumably colonised. It is noteworthy however 

 that those who know most about the group invoke a 

 former land connection where possible. Thus the Rev. 

 A. H. Cooke in considering the rich fauna of land Mollusca 

 of the Philippine Islands 1 , so varied upon the different 



1 P.Z.S. 1892, p. 447. 



