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 cther 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 along 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 larve, eggs, 
