Species-colonies of this kind may be due to the agency of man, who has spread many of his domesticated animals and plants widely over the earth, but who has also intentionally or unintentionally forced many wild animals and plants from their original area to distant parts of the earth, as, for instance, when the English humble-bees were imported into New Zealand with 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-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 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, though 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 strayed 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 these that many land-animals, of which one would not expect it on _a priori_ grounds, may be carried away by chance over the ocean, either, as in the case of butterflies and other 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 (Daphnidæ), which are contained in large numbers in 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, and to the much more highly organized Rotifers as well. In all these cases, and 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