Every little stream and ditch of running water in our lanes and fields
abounds with a species, which, except for the fresh-water habit, and
a little difference in the shade of colour,--the latter being of a
yellowish, instead of a bluish grey,--is so absolutely identical with
the Locust Screw, that, notwithstanding it is registered under a name
of its own, _Gammarus pulex_, it is difficult to believe that the two
are not of common parentage. Certain facts observed in Sweden give
colour to such a suspicion. The large inland seas of fresh water, Lakes
Wetter and Wener, in the south of that country, are situated on high
ground, and have the surface of their water 300 feet above the level of
the Baltic, whereas the bottom is 120 feet below such level. In these
lakes (which appear to have been lifted up with the gradual uprising of
the country) have been found several genera and species of Crustacea,
three of which are AMPHIPODA, which are affirmed to be identical
with marine ones, viz., _Gammaracanthus loricatus_ (Sabine, Ross,
Kröyer), _Pontoporeia affinis_ (Lindström), and _Gammarus cancelloides_
(Gerstfeldt).
The first is now only known to exist in the Arctic seas, the second in
the Baltic, and the last was found in Lake Baikal, in Central Asia.
It is therefore suggested by Lovèn, that the land was raised so as to
convert these waters from marine bays into inland lakes. These marine
species were retained within the basins, the waters of which have
since been changed, through the agency of springs, into fresh water;
and with the gradual transfer of the water, the habits of the animals
have also changed gradually, and that without any outward alteration of
form. Professor Lovèn thinks that there is sufficient evidence to show
that this change in the condition of these lakes must have taken place
during the great glacier period, at a time when the animals now found
in it (and which are known at this day only to inhabit the extreme
north) could have lived in the same latitude as the south of Sweden.
The evidence of these fresh-water lakes suggests that similar changes
in the relative position of sea and land may have been the cause of our
having fresh-water Crustacea nearly allied to marine species in our
rivers and inland streams.[78]
Crawling about perpendicular faces of rocks, generally at a
considerable distance above high-water mark, we may find a crustacean,
not very remotely allied to these, of rather large size.[79] It is
broad and flat, a form which marks it as belonging to another order,
though in many peculiarities of structure agreeing with its near
neighbours the high-backed Screws. The present species crawls readily
with its fourteen short strongly-hooked feet, with which it clings to
every little roughness of the stone. It swims much less effectively:
indeed, I have never seen it spontaneously in the water; and those
which I have captured on the rocks, and transferred to an aquarium,
have died after a very brief interval. They have little agility under
any circumstances, and trust rather to craft than to power for their
safety. When alarmed, they instantly gather in their feet, and drop
from their hold; and, as I have said that the positions in which they
are found are for the most part perpendicular surfaces, such as cliffs
and sea-walls, their one trick is often successful.