dent birds, the green woodpecker and the nuthatch; of annual summer migrants, the wood wren and the tree pipit. But we should not have the stock dove,—a resident species in the midland and eastern counties of England;—nor would the melodious nightingale favour us with its presence, so definitively marked is the line of its migration. As to other species, which are found though rarely to the westward,—in Cornwall and Wales,—as the lesser whitethroat, &c., they might then, as a matter of course, be expected as rare visitants; such they possibly may be now, though more unfrequently than they would be in the other instance.
In like manner, the junction of Great Britain throughout its parallels of latitude, with the nearest continental land, would add greatly to the number of British birds, that island being as deficient comparatively in those of the most western European countries, as Ireland is, in comparison with it. The sea lying between the shores of Great Britain and the continent, has the same effect as that extending between the former and Ireland. Were there an island even of equal size to Ireland, situated as far distant to the westward of that country as it is from Great Britain, the diminution of species would be still greater than that actually existing between Ireland and Great Britain, and so on, in an increased ratio, were island after island, about equidistant from each other, placed still farther to the westward.
The falling off would be owing to the principle, that species continue diminishing (each within its different range) the farther we recede from their metropolis, and that the diminution is accelerated by the insular nature of the land, as opposed to its being conterminous or continental.
The preceding remarks apply only to islands like Great Britain and Ireland lying near a continent and deriving their birds
