344 ISLAND LIFE part ii 



The manner in which fishes are enabled to migrate from 

 lake to lake is unknown, but many suggestions have been 

 made. It is a fact that whirlwinds and waterspouts some- 

 times carry living fish in considerable numbers and drop 

 them on the land. Here is one mode which might certainly 

 have acted now and then in the course of thousands of 

 years, and the eggs of fishes may have been carried with 

 even greater ease. Again we may well suppose that some 

 of these fish have once inhabited the streams that enter or 

 flow out of the lakes as well as the lakes themselves ; and 

 this opens a wide field for conjecture as to modes of migra- 

 tion, because we know that rivers have sometimes changed 

 their courses to such an extent as to form a union with 

 distinct river basins. This has been effected either by 

 floods rising over low watersheds, by elevations of the land 

 changing lines of drainage, or by ice blocking up valleys 

 and compelling the streams to flow over watersheds to find 

 an outlet. This is known to have occurred during the 

 glacial epoch, and is especially manifest in the case of the 

 Parallel Roads of Glenroy, and it probably affords the true 

 solution of many of the cases in which existing species of 

 fish inhabit distinct river basins whether in streams or 

 lakes. If a fish thus wandered out of one river-basin into 

 another, it might then retire up the streams to some of the 

 lakes, where alone it might find conditions favourable to 

 it. By a combination of the modes of migration here 

 indicated it is not difficult to understand how so many 

 species are now common to the lakes of Wales, Cumberland, 

 and Scotland, while others less able to adapt themselves 

 to different conditions have survived only in one or two 

 lakes in a single district ; or these last may have been 

 originally identical with other forms, but have become 

 modified by the particular conditions of the lake in which 

 they have found themselves isolated. 



Peculiar British Insects. — We now come to the class of 

 insects, and here we have much more difficulty in deter- 

 mining what are the actual facts, because new species are 

 still being yearly discovered and considerable portions of 

 Europe are but imperfectly explored. It often happens 

 that an insect is discovered in our islands, and for some 



