Winged insects are perhaps, of all, most admirably adapted for the special conditions found in one locality, and the barriers against their permanent displacement are numerous. Thus many insects require for their subsistence succulent vegetable food during the entire year, which, of course, confines them to tropical regions; some are dependent on mountain-vegetation; some subsist on water-plants; and yet others, as the Lepidoptera, in the larva state, are limited to a single species of plant. Insects have enemies in every stage of their existence; foes are at hand ready to destroy not only the perfect form, but the pupa, the larva, and the egg; and any one of these enemies may prove so formidable, in a country otherwise well adapted to them, as to render their survival impossible. But, on the other hand, most varied means of dispersal carry insects from their natural habitats to distant regions. They are often met far from land, carried thence by storm or hurricane. Hawk-moths are sometimes captured hundreds of miles from shore, having taken passage on ships which neared tropical countries, and Mr. Darwin narrates that he caught in the open sea, seventeen miles from the coast of South America, beetles, some aquatic and some terrestrial, belonging to seven genera, and they seemed uninjured by the salt-water. Insects, in their undeveloped states, make their abodes in solid timber, which, transported by winds and waves, may carry its undeveloped, winged freight great distances. Tropical insects are not unfrequently captured in the London docks, where they have been carried in furniture or foreign timber. Insects are very tenacious of life, and nearly all can exist for a long time without food. Some beetles bear immersion in strong spirit for hours, and are not destroyed by water almost at the boiling-point. These facts enable us to understand how not only by means of its delicate wings, but by winds, waves, volcanic dust, and a thousand other agencies, insects may be carried to remote regions.
Mollusca, which are less highly organized than insects, have, of course, limited appliances for journeying, and their dispersal and distribution may involve long periods of time. Fresh-water mollusks are very widely distributed, and this is accounted for by Mr. Darwin by the fact that ponds and marshes are frequented by wading and swimming birds. These carry away with them the seeds of plants and the eggs of mollusks. True land-shells are exceedingly sensitive to saltwater, and yet they are found all over the globe. Experiments on their power to resist sea-water show that a membranous diaphragm, which they sometimes form over the mouth of the shell, enables them to survive many days' immersion in it. They may lie dormant for a long time, some having lived between two and three years shut up in boxes; and one snail, from the Egyptian Desert, was found to be alive after having been glued for four years to a tablet in the British Museum. These facts render it quite possible that they may cross the
