The geographical distribution of animals, with a study of the relations ...

32 DISPERSAL AND MIGRATION. [part i.times attach themselves to the feet of birds walking or restingon the ground, and as many of the waders often go far inland,this may have been one of the methods of distributing speciesof land shells ;for it must always be remembered that nature canafford to wait, and that if but once in a thousand years a singlebird should convey two or three minute snails to a distant island,thisis all that is required for us to find that island well stockedwith a great and varied population of land shells.Means of Dispersal of Insects a?id the Barriers luhich Limittheir Range.—Winged insects, as a whole, have perhaps morevaried means of dispersal over the globe than any other highlyorganised animals. Many of them can fly immense distances,and the more delicate ones are liable to be carried by stormsand hurricanes over a wide expanse of ocean. They are oftenmet with far out at sea. Hawk-moths frequently fly on boardships as they approach the shores of tropical countries, and theyhave sometimes been captured more than 250 miles from thenearest land. Dragon-flies came on board the Adventure frigatewhen fifty miles off the coast of South America. A southerlywind brought flies in myriads to Admiral Smyth's ship in theMediterranean when he was 100 miles distant from the coast ofAfrica. A large Indian beetle (Chrysochroa ocellata) was quiterecently caught alive in the Bay of Bengal by Captain Payne ofthe barque William Mansoon, 273 miles from the nearest land.Darwin caught a locust 370 miles from land; and in 1844swarms of locusts several miles in extent, and as thick as theflakes in a heavy snowstorm, visited Madeira.come with perfect safety more than 300 miles ;These must haveand as they continuedflying over the island for a long time, they could evidentlyhave travelled to a much greater distance, Numbers of livingbeetles belonging to seven genera, some aquatic and some terrestrial,were caught by Mr. Darwin in the open sea, seventeenmiles from the coast of South America, and they did not seeminjured by the salt water.Almost all the accidental causes thatlead to the dispersal of the higher animals would be still morefavourable for insects. Floating trees could carry hundreds ofinsects for one bird or mammal ;and so many of the larvre, eggs,