Page:Memoirs of the Geological Survey of Great Britain, Volume 1.djvu/350

Rh now inhabiting the British Islands. The zoological works of Fleming, Jenyns, Yarrell, Bell, and W. Thompson, have enumerated the species and treated of the distribution of our indigenous animals, those of Smith, Hooker, Lindley, Babington, Henslow, and especially Watson, have done the same service for our native flora; but the history of the formation, if I may so say, of that fauna and flora, remains to be investigated. This essay is offered as a contribution towards such a history.

There are three modes in which an isolated area may become peopled by animals and plants.

1st. By special creation within that area.

2nd. By transport to it

3rd. By migration before isolation.

The first of these modes, if it operated at all within the limited area under examination, had but very little influence in determining the vegetation and animal population of the British Isles, since, with a very few, mostiy doubtful, exceptions, the terrestrial animals and flowering plants within their area are identical with continental species.

The second mode is not a sufficient one; for, though doubtiess the great mass of cryptogamic plants, a few phanerogamia, and a few terrestrial animals, besides those endowed with powers of flight, may have found their way across the separating waters by the agency of currents, &c., or in the case of plants, their seeds have been conveyed by the winds or birds through the air, yet, after making full allowances for all likely means of transport at present in action, there remains a residue of animals and plants which we cannot suppose to have been transported, since either their bodily characters or certain phenomena presented by their present distribution, prevent our entertaining such an idea. For, when we consider that within this limited area a great number of animals and plants are not universally dispersed, but congregated in such a way as to form distinct regions or provinces, which have remained unchanged as long as we have any record, we can hardly grant transporting powers to have operated so far as to sketch out those regions in the likenesses of distant and distinct lands, and yet not afterwards to have continued their action, so as eventually to have merged them all into an homogeneous whole.

There remains the third mode by which a land may be peopled; namely, by colonization from another or from several neighbouring lands previous to isolation. This, of course, involves the consideration of geological causes, the part played by which in the present disposition of organized beings on the surface of our globe, has never as yet received due consideration. It is through this mode, I believe, the British Islands have chiefly acquired their fauna and flora; but, before showing how, it may be well to point out certain peculiarities presented by their