of the Birds of Ireland. 267 



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 migrations. As to other species, which are 

 found, though rarely, to the westward, — in Cornwall and 

 Wales, — as the lesser white-throat, &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 Euro- 

 pean 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 dimi- 

 nution 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 thence. There are, however, instances of islands 

 situated sufficiently near large continents to admit of the 

 flight of birds from the latter, and yet deriving comparatively 

 few, or none of their species from them. The most remark- 

 able example is presented by the Galipagos archipelago^ si- 

 tuated under the equatorial line, and which, though only 500 

 to 600 miles westward of the coast of South America, does 

 not contain a land bird from the continent. Even some of 



