ce of the
lessened numbers of that herbivorous species, would soon supply more
food to several insects, and probably to some terrestrial testacea, so
that the latter would gain ground. The increase of these would furnish
other insects and birds with food, so that the numbers of these last
would be augmented. The diminution of the seals would afford a respite
to some fish which they had persecuted; and these fish, in their turn,
would then multiply and press upon their peculiar prey. Many
water-fowls, the eggs and young of which are devoured by foxes, would
increase when the foxes were thinned down by the bears; and the fish on
which the water-fowls subsisted would then, in their turn, be less
numerous. Thus the numerical proportions of a great number of the
inhabitants, both of the land and sea, might be permanently altered by
the settling of one new species in the region; and the changes caused
indirectly would ramify through all classes of the living creation, and
be almost endless.

An actual illustration of what we have here only proposed
hypothetically, is in some degree afforded by the selection of small
islands by the eider duck for its residence during the season of
incubation, its nest being seldom if ever found on the shores of the
main land, or even of a large island. The Icelanders are so well aware
of this, that they have expended a great deal of labor in forming
artificial islands, by separating from the main land certain
promontories, joined to it by narrow isthmuses. This insular position is
necessary to guard against the destruction of the eggs and young birds,
by foxes, dogs, and other animals. One year, says Hooker, it happened
that, in the small island of Vidoe, adjoining the coast of Iceland, a
fox got over _upon the ice_, and caused great alarm, as an immense
number of ducks were then sitting on their eggs or young ones. It was
long before he was taken, which was at last, however, effected by
bringing another fox to the island, and fastening it by a string near
the haunt of the former, by which he was allured within shot of the
hunter.[966]

_The first appearance of a new species causes the chief
disturbance._--It is usually the first appearance of an animal or plant,
in a region to which it was previously a stranger, that gives rise to
the chief alteration; since, after a time, an equilibrium is again
established. But it must require ages before such a new adjustment of
the relative forces of s