ISLAND LIFE. 
[PART II. 
324 
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 
