78 



ISLAND LIFE 



PART I 



carried into a new country they cannot live. Many 

 can only feed in the larva state on one species of plant ; 

 others are bound up with certain groups of animals on 

 whom they are more or less parasitic. Climatal influences 

 have a great effect on their delicate bodies ; while, however 

 well a species may be adapted to cope with its enemies in 

 one locality, it may be quite unable to guard itself against 

 those which elsewhere attack it. From this peculiar 

 combination of characters it happens, that among insects 

 are to be found examples of the widest and most erratic 

 dispersal and also of the extremest restriction to limited 

 areas ; and it is only by bearing these considerations in 

 mind that we can find a satisfactory explanation of the 

 many anomalies we meet with in studying their distribu- 

 tion. 



The Dispersal of Zand Mollusca, — The only other group 

 of animals we need now refer to is that of the air-breathing 

 mollusca, commonly called land-shells. These are almost 

 as ubiquitous as insects, though far less numerous ; and 

 their wide distribution is by no means so easy to explain. 

 The genera have usually a very wide, and often a cosmo- 

 politan range, while the species are rather restricted, and 

 sometimes wonderfully so. Not only do single islands, 

 however small, often possess peculiar species of land-shells, 

 but sometimes single mountains or valleys, or even a 

 particular mountain side, possess species or varieties found 

 nowhere else upon the globe. It is pretty certain that 

 they have no means of passing over the sea but such as are 

 very rare and exceptional. Some which possess an 

 operculum, or which close the mouth of the shell with a 

 diaphragm of secreted mucus, may float across narrow 

 arms of the sea, especially when protected in the crevices 

 of logs of timber ; while in the young state when attached 

 to leaves or twigs they may be carried long distances by 

 hurricanes.^ Owing to their exceedingly slow motion, 



^ Mr. Darwin found that the large Helix pomatia lived after immersion 

 in sea- water for twenty days. It is hardly likely that this is the extreme 

 limit of their powers of endurance, but even this would allow of their being 

 floated many hundred miles at a stretch, and if we suppose the shell to be 

 partially protected in the crevice of a log of wood, and to be thus out of 



