Edwards foresaw the general
influence of the study of distribution in depth upon the interpretation
of geological phenomena. Forbes connected the two orders of inquiry still
more closely; and in the thoughtful essay "On the connection between the
distribution of the existing Fauna and Flora of the British Isles, and
the geological changes which have affected their area, especially during
the epoch of the Northern drift," to which reference has already been
made, he put forth a most pregnant suggestion.

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 areas, 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
e