These fifteen peculiar fishes differ from each other and 

 from all British and continental species, not in colour 

 only, but in such important structural characters as the 

 number and size of the scales, form and size of tlie fins, 

 and the form or proportions of the head, body, or tail. 

 Some of them, like >S'. killinensis and the Coregoni are in 

 fact, as Dr. Glinther assures me, just as good and distinct 

 species as any other recognised species of fish. It may 

 indeed be objected that, until all the small lakes of 

 Scandinavia are exj^lored, and their fishes compared with 

 ours, we cannot be sure that we have any peculiar species. 

 But this objection has very little weight if we consider 

 how our own species vary from lake to lake and from island 

 to island, so that the Orkney species is not found in 

 Scotland, and only one of the peculiar British species 

 extends to Ireland, which has no less than five species 

 altogether peculiar to it. If the sj^ecies of our own two 

 islands are thus distinct, what reason have we for believing 

 that they will be otherwise than distinct from those of 

 Scandinavia ? At all events, with the amount of evidence 

 we already possess of the very restricted ranges of many of 

 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 

 wdll 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 



