the rivers which now run into the German Ocean, from the Humber on
the west to the Elbe on the east, discharged themselves into the sea
between Scotland and Norway, after wandering through a vast lowland,
covered with countless herds of mammoth, rhinoceros, gigantic ox,
and other mammals now extinct; while the birds, as far as we know,
the insects, the fresh-water fish, and even, as my friend Mr. Brady
has proved, the Entomostraca of the rivers, were the same in what is
now Holland as in what is now our Eastern counties. I could dwell
long on this matter. I could talk long about how certain species of
Lepidoptera--moths and butterflies--like Papilio Machaon and P.
Podalirius, swarm through France, reach up to the British Channel,
and have not crossed it, with the exception of one colony of Machaon
in the Cambridgeshire fens. I could talk long about a similar
phenomenon in the case of our migratory and singing birds; how many
exquisite species--notably those two glorious songsters, the Orphean
Warbler and Hippolais, which delight our ears everywhere on the
other side of the Channel--follow our nightingales, blackcaps, and
warblers northward every spring almost to the Straits of Dover, but
dare not cross, simply because they have been, as it were, created
since the gulf was opened, and have never learnt from their parents
how to fly over it.

In the case of fishes, again, I might say much on the curious fact
that the Cyprinidae, or white fish--carp, etc.--and their natural
enemy, the pike, are indigenous, I believe, only to the rivers,
English or continental, on the eastern side of the Straits of Dover;
while the rivers on the western side were originally tenanted, like
our Hampshire streams, as now, almost entirely by trout, their only
Cyprinoid being the minnow--if it, too, be not an interloper; and I
might ask you to consider the bearing of this curious fact on the