nsmission were ample hardly any modification
of species occurred, while where these means were deficient and individuals
once transported remained isolated during a long succession of ages, their
forms and characters became so much changed as to 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 only
one appears to be common to the British and Irish lakes. {344}

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 migration, 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 o