tish species extends
to Ireland, which has no less than five species altogether peculiar to it.
If the species of our own two islands are thus distinct, what reason have
we for believing that they will be otherwise than distinct from those of
Scandinavia? At all events, with the amount of evidence we already possess
of the very restricted ranges of many of our species, we must certainly
hold them to be peculiar till they have been proved to be otherwise.

The great speciality of the Irish fishes is very interesting, because it is
just what we should expect on the theory of evolution. In Ireland the two
main causes of specific change--isolation and altered conditions--are each
more powerful than in Britain. Whatever difficulty continental fishes may
have in passing over to Britain, that difficulty will certainly be
increased by the second sea passage to Ireland; and the latter country has
been longer isolated, for the Irish Sea with its northern and southern
channels is considerably deeper than the German Ocean and the {343} Eastern
half of the English Channel, so that, when the last subsidence occurred,
Ireland would have been an island for some length of time while England and
Scotland still formed part of the continent. Again, whatever differences
have been produced by the exceptional climate of our islands will have been
greater in Ireland, where insular conditions are at a maximum, the
abundance of moisture and the equability of temperature being far more
pronounced than in any other part of Europe.

Among the remarkable instances of limited distribution afforded by these
fishes, we have the Loch Stennis trout confined to the little group of
lakes in the mainland of Orkney, occupying altogether an area of about ten
miles by three; the Welsh charr confined to the Llanberris lakes, about
three miles in length; Gray's charr confined to Lough Melvin, about seven
miles long; while the Loch Killin charr, known only from a small mountain
lake in Inverness-shire, and the vendace, from the equally small lakes at
Loch Maben in Scotland, are two examples of restricted distribution which
can hardly be surpassed.

_Cause of Great Speciality in Fishes._--The reason why fishes alone should
exhibit such remarkable local modifications in lakes and islands is
sufficiently obvious. It is due to the extreme rarity of their transmission
from one lake to another. Just as we found to be the case in Oceanic
Islands, where the means of tra