VERTEBRATES OF WALES AND IRELAND. 325 
Kurope, &c., by inherited instinct. Their ancestors of long ago 
originally took this route because it was over land connecting 
the northern and southern countries which they inhabited re- 
spectively in the warm and cold seasons of the year, till the 
following of this route at each returning time of migration 
became a fixed habit with the whole race. 
Applying this to such a species as the Tree-Pipit (common 
in North Wales, but unknown in Ireland), we might suppose 
that this bird extended its‘range to Britain whilst still part of 
the Kuropean mainland, but after Ireland had become an island. 
When Great Britain became also an island the Tree-Pipit con 
tinued to come to its native district from force of habit, but 
never went on to Ireland, because the instinct to go there had 
not been implanted in the species by oldcustom. Of course, all 
this is theory, but it certainly accounts for much that is puzzling 
in the geographical distribution of birds in these districts. 
I have to thank Capt. G. E. H. Barrett-Hamilton for revising 
the notes on the Mammals of Ireland. 
