DISTRIBUTION AND MIGRATIONS 



277 



physi of the Palaearctic region are represented by comparatively 

 few Cat-fishes, all of which belong to Indian genera, and a large 

 number of Carps and Loaches. These must have spread 

 northwards from their original headquarters, penetrating first 

 into temperate Asia and later invading Europe. A fair number 

 of Carps occur in Europe, a few Loaches, and one Cat-fish, 

 the Wels or Glanis {Silurus), found only in the rivers east of 

 the Rhine. In our own islands about twenty-two species of 

 true fresh-water fishes may be recognised, of which fourteen 

 belong to the order Ostariop/iysi. All of these are also found on 

 the European continent, and a num- 

 ber extend eastwards into Asia. The 

 importance of the Pyrenees moun- 

 tains as a barrier is emphasised by 

 the fact that only one of these fishes 

 occurs in the Iberian Peninsula, al- 

 though about half of them have suc- 

 ceeded in penetrating into Italy. 

 Two species, the Burbot {Lota) and 

 the Pike {Esox), occur also in North 

 America. It is of interest to note 

 that all the twenty-two species occur 

 in Yorkshire, and nearly all in the 

 Trent, the Ouse, and in Norfolk, but 

 there are parts of the British Isles 

 where the true fresh-water fauna is 

 a very poor one. In Ireland there are 

 only ten species, and in Britain there 

 is a marked diminution in the num- 

 ber of species from south to north, 

 culminating in a complete absence 

 of true indigenous fresh-water fishes in the northern highlands 

 of Scotland. A similar decrease in number of species is notice- 

 able from east to west, and quite a number of them are absent 

 from Wales west of the Severn system. The reasons for the 

 very dissimilar distribution in the British Isles of certain species 

 with a very wide and essentially similar distribution on the 

 continent of Europe and Asia are to be found in the former 

 connection of our islands with one another and with the main- 

 land. The whole question of the origin and distribution of 

 British fresh-water fishes has been dealt with in full by Dr. Regan 

 in his book on British Fresh-water Fishes, to which reference 

 may be made for further details. He points out that our islands 



Fig. 102. 



Restoration of Pleistocene Geo- 

 graphy of the British Isles, 

 showing the coast-line coin- 

 cident with the contour of 80 

 fathoms. (After Jukes-Browne.) 



