CHAP. XVI.] 
THE BRITISH ISLES. 
323 
our species, we must certainly hold them to be peculiar till they 
have been proved to be otherwise. 
The great speciality of the Irish fishes is very interesting, 
because it is just what we should expect on the theory of 
evolution. In Ireland the two main causes of specific change — 
isolation and altered conditions — are each more powerful than in 
Britain. Whatever difficulty continental fishes may have in 
passing over to Britain, that difficulty will certainly he increased 
by the second sea passage to Ireland ; and the latter country has 
been longer isolated, for the Irish Sea with its northern and 
southern channels is considerably deeper than the German Ocean 
and the eastern half of the English Channel, so that, when the 
last subsidence occurred, Ireland would have been an island for 
some length of time while England and Scotland still formed 
part of the continent. Again, whatever differences have been 
produced by the exceptional climate of our islands will have been 
greater in Ireland, where insular conditions are at a maximum, 
the abundance of moisture and the equability of temperature 
being far more pronounced than in any other part of Europe. 
Among the remarkable instances of limited distribution 
afforded by these fishes, we have the Loch Stennis trout 
•confined to the little group of lakes in the mainland of Orkney, 
occupying altogether an area of about ten miles by three ; the 
Welsh charr confined to the Llanberris lakes, about three miles 
in length ; Gray’s charr confined to Lough Melvin, about seven 
miles long ; while the Lough Killin charr, known only from a 
small mountain lake in Ireland, and the vendace, from the 
equally small lakes at Lochmaben in Scotland, are two examples 
of restricted distribution which can hardly be surpassed. 
Cause of Great Speciality in Fishes— The reason why fishes 
alone should exhibit such remarkable local modifications in lakes 
and islands is sufficiently obvious. It is due to the extreme 
rarity of their transmission from one lake to another. Just as 
we found to be the case in Oceanic Islands, where the means 
of transmission were ample hardly any modification of species 
occurred, while where these means were deficient and individuals 
once transported remained isolated during a long succession of 
ages, their forms and characters became so much changed as to 
