524 



ISLAND LIFE. 



[I'ART II. 



bring about what we term distinct species or even distinct genera, 

 — so these lake fishes have become modified because the means 

 by which they are enabled to migrate so rarely occur. It is quite 

 in accordance with this view that some of the smaller lakes 

 contain no fishes, because none have ever been conveyed to them. 

 Others contain several ; and some fishes which have peculiarities 

 of constitution or habits which render their transmission somewhat 

 less difficult occur in several lakes over a wide area of country, 

 though none appear to be common to the British and Irish lakes. 



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 sometimes 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 connect- 

 ing 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 



