I recollect right, by the late lamented Edward Forbes--a sufficient
one may be found in one look over a bridge, in any river of the East
of England. There we see various species of Cyprinidae, 'rough' or
'white' fish--roach, dace, chub, bream, and so forth, and with them
their natural attendant and devourer, the pike.

Now these fish belong almost exclusively to the same system of
rivers--those of north-east Europe. They attain their highest
development in the great lakes of Sweden. Westward of the Straits of
Dover they are not indigenous. They may be found in the streams of
south and western England; but in every case, I believe, they have
been introduced either by birds or by men. From some now submerged
'centre of creation' (to use poor Edward Forbes's formula) they must
have spread into the rivers where they are now found; and spread by
fresh water, and not by salt, which would destroy them in a single
tide.

Again, there lingers in the Cam, and a few other rivers of north-
eastern Europe, that curious fish the eel-pout or 'burbot' (Molva
lota). Now he is utterly distinct from any other fresh-water fish of
Europe. His nearest ally is the ling (Molva vulgaris); a deep-sea
fish, even as his ancestors have been. Originally a deep-sea form,
he has found his way up the rivers, even to Cambridge, and there
remains. The rivers by which he came up, the land through which he
passed, ages and ages since, have been all swept away; and he has
never found his way back to his native salt-water, but lives on in a
strange land, degraded in form, dwindling in numbers, and now fast
dying out. The explanation may be strange: but it is the only one
which I can offer to explain the fact--which is itself much more
strange--of the burbot being found in the Fen rivers.