Vlll 
PKEFACE. 
foregoing causes,— viz., physical geography, mineralogical struc- 
ture, climate, and absence of old timber. It should be borne in 
mind that in all the preceding remarks the mere absence or pre- 
sence 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 junc- 
tion 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 circum- 
stances, may be inferred from an examination of the summary ap- 
pended 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- 
* A paper on birds seen crossing the Mediterranean in spring, by the author, 
will be fully given in the appendix to the last volume ; but it was considered de- 
sirable to notice each, bird included there when treating of its species. This was 
done that a person might be enabled on referring to any species to have the whole 
information respecting it before him at once. The author, however, feels that a 
reader of the matter continuously, may accuse him of too frequent allusion to the 
subject. 
