INFLUENCE OF ISOLATION ON THE FORMATION OP SPECIES 281 



area to dist:mt pju'ts ol" tlio earth, as, for in.staiK'e, when the English 

 hiiiuble-beos -woi-o imported into New Zealand -with a \iew to 

 securing the IVrtilization of the clover ; bnt such colonies also 

 occur in thousands of eases independently of man's agency, and 

 the means by which they are brought about are very diverse. Little 

 singing-birds are sometimes driven astray by storms, and carried far 

 aAvaj' across the sea, to tind, if fortune fa\ours them, a new home 

 on some remote oceanic island ; fresh- water snails, which have just 

 emerged from the egg, creep on to the broad, webbed feet or among 

 the plumage of a wild duck or some other migratory bird, and are 

 carried by it far over laud and sea, and tinallj' deposited in a distant 

 marsh or lake. This must happen not infrequently, as is evidenced by 

 the wide distribution of our Central European fresh-water snails 

 towards the north and south. But terrestrial snails can also, though 

 more rarely, be borne in passi\-e migration far bej'ond limits which 

 are apparent!}' impassable, as is evidenced by the presence of land- 

 snails on remote oceanic islands. 



The Sandwich Islands are more than 4,000 kilometres from the 

 continent of America ; they originated as \-olcanoes in the midst of 

 the Pacitic Ocean ; and j^et they possess a rich fauna of terrestrial 

 snails, the beginnings of which can only have reached them by the 

 chance importation of indi\-idual snails carried by strayed land-birds. 

 Charles Darwin was the tirst who attempted to investigate the 

 problem of the colonization of oceanic islands by animal inhabitants, 

 and the chapter in The Oriijln- of S/i,riei< which deals with the 

 geographical distribution of animals and plants still forms the basis 

 of all the in\estigations directed towards this point. We learn from 

 these that many laud-animals, of which one would not expect it on 

 (( priori grounds, may be carried away by chance over the ocean, 

 either, as iu the case of butterflies and other flying insects, and of 

 birds and bats, by being driven out of their course by the wind, or bj^ 

 being concealed — either as eggs or as fully-formed auimals — in the 

 clefts of driftwood, where they can resist for a considerable time the 

 usually destructive influence of salt water. Thus eggs of some of the 

 lowest Crustacea (Daphnichv), which are contained in large numbers iu 

 the mud of fresh water, may be transported with some of the mud on 

 the feet of birds, and this may happen also to encysted infusorians and 

 other unicellulars, iuid to the much more highly organized Rotifers as 

 well. In all these cases, jmd in many others, it may happen occasionally 

 that single individuals, or a few at a time, may be carried far afield, 

 and may reach regions from which their fellows of the same species 

 are entirely excluded. If they thrive there they may establish 



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