OCR Text |
Show 76 ISLAND LIFE. [PART I. 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 distribution. The Dispersal of Land 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 cosmopolitan, 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 now here 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 dia.phragm 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.1 Owing to their exceedingly slow motion, their powers of voluntary dispersal, even on land, are very limited, and this will explain the extreme restriction of their range in many cases. Great Antiquity of Land_-Shells.-The clue to the almost universal distribution of the several families and of many genera, is to be found, however, in their immense antiquity. In the Pliocene and Miocene formations most of the land-shells are either identical with living species or closely allied to them, 1 Mr. Darwin found that the large ]felix pomatia lived after immersion in sea-water for twenty days. It is hardly likely that this is tile 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 water in calm weather, the distance might extend to a thousand miles or more. The eggs of fresh-water mollusca are known to attach themselves to the feet of aquatic birds, and this is supposed to account for their very wide diffusion. CHAP. V.] DISPEHSAL OF ANIMALS AND PLANTS. 77 while even in the Eocene almost all are of livino- o-enera and 0 0 ' one British Eocene fossil still lives in Texas. Strange to say, no true land-shells have been discovered in the Secondary formations, but they must certainly have abounded, for in the far more ancient Palreozoic coal measures of Nova Scotia two species belonging to the living genera Pupa and Zonites have been found in considerable abundance. Land-shells have therefore survived all the revolutions the earth has undergone since Palreozoic times. They have been able to spread slowly but surely into every land that has ever been connected with a continent, while the rare chances of transfer across the ocean, to which we have referred as possible, have again and again occurred during the almost unimaginable ages of their existence. The remotest and most solitary of the islands of the mid-ocean have thus become stocked with them, though the variety of species and genera bears a direct relation to the facilities of transfer, and the shell fauna is never very rich and varied, except in countries which have at one time or other been united to some continental land. Causes favouring the abundance of Land-Shells.-The abundance and variety of land-shells is also, more than that of any other class of animals, dependent on the nature of the surface and the absence of enemies, and where these conditions are favourable their forms are wonderfully luxuriant. The first condition is the presence of lime in the soil, and a broken surface of country with much rugged rock offering crevices for concealment and by bernation. The second is a limited bird and mammalian fauna, in which such species as are especially shell-eaters shall be rare or absent. Both these conditions are found in certain large islands, and pre-eminently in the Antilles, which possess more species of land-shells than any single continent. If we take the whole globe, more species of land-shells are found on the islands than on the continents-a state of things to which no approach is made in any other group of animals whatever, but which is perhaps explained by the considerations now suggested. The D'ispersal of Plants.-The ways in which plants are dispersed over the earth, and the special facilities they often possess |