In certain parts of the sea bottom in the immediate vicinity of the
British Islands, as in the Clyde district, among the Hebrides, in the
Moray Firth, and in the German Ocean, there are depressed areae, forming
a kind of submarine valleys, the centres of which are from 80 to 100
fathoms, or more, deep. These depressions are inhabited by assemblages
of marine animals, which differ from those found over the adjacent and
shallower region, and resemble those which are met with much farther
north, on the Norwegian coast. Forbes called these Scandinavian
detachments "Northern outliers."
How did these isolated patches of a northern population get into these
deep places? To explain the mystery, Forbes called to mind the fact
that, in the epoch which immediately preceded the present, the climate
was much colder (whence the name of "glacial epoch" applied to it); and
that the shells which are found fossil, or sub-fossil, in deposits
of that age are precisely such as are now to be met with only in the
Scandinavian, or still more Arctic, regions. Undoubtedly, during the
glacial epoch, the general population of our seas had, universally, the
northern aspect which is now presented only by the "northern outliers";
just as the vegetation of the land, down to the sea-level, had the
northern character which is, at present, exhibited only by the plants
which live on the tops of our mountains. But, as the glacial epoch
passed away, and the present climatal conditions were developed, the
northern plants were able to maintain themselves only on the bleak
heights, on which southern forms could not compete with them. And,
in like manner, Forbes suggested that, after the glacial epoch, the
northern animals then inhabiting the sea became restricted to the deeps
in which they could hold their own against invaders from the south,
better fitted than they to flourish in the warmer waters of the
shallows. Thus depth in the sea corresponded in its effect upon
distribution to height on the land.