32 DISPERSAL AND MIGRATION. [part i. 



times attach themselves to the feet of birds walking or resting 

 on the ground, and as many of the waders often go far inland, 

 this may have been one of the methods of distributing species 

 of land shells ; for it must always be remembered that nature can 

 afford to wait, and that if but once in a thousand years a single 

 bird should convey two or three minute snails to a distant island, 

 this is all that is required for us to find that island well stocked 

 with a great and varied population of land shells. 



Means of Dispersal of Insects and the Barriers which Limit 

 their Range. — Winged insects, as a whole, have perhaps more 

 varied means of dispersal over the globe than any other highly 

 organised animals. Many of them can fly immense distances, 

 and the more delicate ones are liable to be carried by storms 

 and hurricanes over a wide expanse of ocean. They are often 

 met with far out at sea. Hawk-moths frequently fly on board 

 ships as they approach the shores of tropical countries, and they 

 have sometimes been captured more than 250 miles from the 

 nearest land. Dragon-flies came on board the Adventure frigate 

 when fifty miles off the coast of South America. A southerly 

 wind brought flies in myriads to Admiral Smyth's ship in the 

 Mediterranean when he was 100 miles distant from the coast of 

 Africa. A large Indian beetle {Chrysochroa ocellata) was quite 

 recently caught alive in the Bay of Bengal by Captain Payne of 

 the barque William Mansoon, 273 miles from the nearest land. 

 Darwin caught a locust 370 miles from land; and in 1844 

 swarms of locusts several miles in extent, and as thick as the 

 flakes in a heavy snowstorm, visited Madeira. These must have 

 come with perfect safety more than 300 miles ; and as they con- 

 tinued flying over the island for a long time, they could evidently 

 have travelled to a much greater distance, Numbers of living 

 beetles belonging to seven genera, some aquatic and some terres- 

 trial, were caught by Mr. Darwin in the open sea, seventeen 

 miles from the coast of South America, and they did not seem 

 injured by the salt water. Almost all the accidental causes that 

 lead to the dispersal of the higher animals would be still more 

 favourable for insects. Floating trees could carry hundreds of 

 insects for one bird or mammal ; and so many of the larvre, eggs, 



