MODES OF MIGRATION. 359 



their germs, may often make distant passive journeys througli 

 the air. The seeds of many plants are provided with light 

 feathery processes, which act as parachutes and facilitate their 

 flight in the air, and prevent their falling. Spiders make 

 journeys of many miles through the air on their fine fila- 

 ments, their so-called gossamer threads. Young frogs are 

 frequently raised by whirlwinds into the air by thousands, 

 and fall down in a distant part as a " shower of frogs." Storms 

 may carry birds and insects across half the earth's circum- 

 ference. They drop in the United States, having risen in 

 England. Starting from California, they only come to rest 

 in China. But, again, many other organisms may make the 

 journey from one continent to another together with the 

 birds and insects. Of course all parasites, the number of 

 which is legion, fleas, lice, mites, moulds, etc., migrate with 

 the organisms upon which they live. In the earth which 

 often remains sticking to the claws of birds there are also 

 small animals and plants or their germs. Thus the volun- 

 tary or involuntary migration of a single larger organism 

 may carry a whole small flora and fauna from one paii} of 

 the earth to another. 



Besides the means of transport here mentioned, there 

 are many others which explain the distribution of animal 

 and vegetable species over the large tracts of the earth's 

 surface, and especially the general distribution of the so- 

 called cosmopolitan species. But these alone would not 

 nearly be sufficient to explain all chorological facts. How 

 is it, for example, that many inhabitants of fresh water 

 live in various rivers or lakes far away and quite apart from 

 one another ? How is it that many inhabitants of moun- 

 tains, which cannot exist in plains, are found upon entirely 



