JOURNEYINGS AND DISPERSAL OF ANIMALS. S 77 



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 re- 

 quire 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 

 t hence 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 undevel- 

 oped 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 tmderstand 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 re- 

 mote regions. 



Mollusca, which are less highly organized than insects, have, of 

 course, limited appliances for journeying, and their dispersal and dis- 

 tribution 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 swim- 

 ming birds. These carry away with them the seeds of plants and the 

 eggs of mollusks. True land-shells are exceedingly sensitive to salt- 

 water, 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 Mu- 

 seum. These facts render it quite possible that they may cross the 

 vol. x. 37 



