INFLUENCE OF ISOLATION ON THE FORMATION OF SPECIES 281 



area to distant parts of the earth, as, for instance, when tlie English 

 humble-bees were imported into Xew Zealand witli a view to 

 securing the fertilization of the clover; but such colonies also 

 occur in thousands of cases 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 

 away across the sea, to find, if fortune favours them, a new home 

 on some remote oceanic island ; fresh-Avater 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 land and sea, and finally 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, thouo-h 

 more rarely, be borne in passive migration far beyond limits which 

 are apparently 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 volcanoes in the midst of 

 the Pacific Ocean: and yet they possess a rich fauna of terrestrial 

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

 chance importation of individual snails carried by stra^^ed land-birds. 

 Charles Darwin was the first who attempted to investigate the 

 problem of the colonization of oceanic islands by animal inhabitants, 

 and the chapter in The Origin of Species which deals with the 

 geographical distribution of animals and plants still forms the basis 

 of all the investigations directed towards this point. We learn from 

 tliese that many land-animals, of which one would not expect it on 

 a po'iorl grounds, may be carried away by chance over the ocean, 

 either, as in the case of butterflies and otlier flying insects, and of 

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

 being concealed — either as eggs or as fully-formed animals — 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 (Daphnida3), which are contained in large numbers in 

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

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

 other unicellulars, and to the much more higlily organized Kotifers as 

 well. In all these cases, and in many others, it may happen occasioiiall}' 

 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 



