PIIEFACE. 
IX 
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 accele- 
rated 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 
