Page:The Natural History of Ireland vol1.djvu/12

 foregoing causes,—viz., physical geography, mineralogical structure, climate, and absence of old timber. It should be borne in mind that in all the preceding remarks the mere absence or presence of species is considered; consequently, nothing is said of birds from different causes being less frequently met with in Ireland, than in particular parts of England or Scotland. Such points will be fully treated of under the respective species.

Although in their polar and equatorial migrations, the crossing of a sea,—as the Mediterranean for instance,—offers no obstacle to birds, yet is it different when they are spreading latitudinally; either to the east or to the west, in which case the migration of many species terminates at the margin of the sea. Were Ireland therefore geographically joined to Great Britain, some species that are not now found would certainly inhabit it, but the junction would make no difference with respect to others:—resident as well as migratory birds. In that event, we should in the east of Ireland at least, have those species which are found throughout the most western portion of Great Britain in the same parallel of latitude; but not those whose range of distribution does not extend to the most western counties of England and to Wales. The species which Ireland would and would not have, under such circumstances, may be inferred from an examination of the summary appended to the end of each Order of Birds, where the distribution over Great Britain of the species not known as Irish, is pointed out. We should, for example, if the country suited them, have as resi-