DISPEESAL OF ANIMALS 323 



animals is shown in a very interesting manner in the case of the 

 Mediterranean and the Ked Sea. In each of these there is a 

 surface current constantly flowing in from the open ocean and 

 bringing in vast numbers of individuals, both larval and adult, 

 which never find their way out again. Hence these almost 

 enclosed seas form a kind of trap for marine animals and we 

 accordingly find them to be inhabited by an exceptionally rich 

 and varied fauna. 



The range of marine species, though sometimes very wide, is 

 usually more or less strictly limited, so that the shores of every 

 continent have their own characteristic fauna and flora. This is 

 no doubt partly accounted for by differences in climatic conditions, 

 food supply and so forth, but it is mainly due to the fact that, 

 in spite of the facilities for travel afforded by ocean currents, 

 the dangers incidental to a long voyage from one continent to 

 another are rarely surmounted, at any rate by shore-dwelling 

 organisms. We know that many such forms flourish quite as well 

 in some other part of the world as in their original home if they 

 can once overcome the initial difficulty of migration. Thus 

 the artificial introduction of the American oyster into British 

 seas has accidentally brought with it the introduction of the 

 remarkable limpet-like Crepidula, which attaches itself to the 

 oyster shells and runs riot over the oyster beds on the Essex 

 coast. That such occurrences may occasionally take place in a 

 state of nature, and a species thereby be enabled to extend its 

 area of distribution, there can be no reasonable doubt, for even 

 American turtles have occasionally been carried by the Gulf 

 Stream to the shores of Great Britain. 



Amongst the higher forms of non-aquatic animals, and 

 especially birds and mammals, their own powers of locomotion 

 constitute the most important means of dispersal. Even these f 

 however, are frequently transported for long distances by those 

 external agencies which are chiefly responsible for the dispersal 

 of less highly organized forms. 



Leaving out of account, for the moment, the action of man, 

 which has brought about immense changes in the geographical 

 distribution of the existing fauna, the chief agents to be noticed 

 in this connection are wind and water. 



It is to the action of the wind that large numbers of winged 

 animals insects, birds and bats owe the wide distribution which 

 they enjo}^. All- actively flying land animals are liable to be 



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