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 ca^uses 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 be 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 



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