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Show 144 EFFECT OF TilE l~XTENSTON [Ch. IX. unarmed, it is frequently by stratagem only that they make their escape * · Let us cast our thoughts back to the period when the first polar bears reached Iceland, befor: it was colo~ized by the Norwegians in 874 ;-we may imagme the breakmg up of an immense barrier of ice, like that which, in 1816 and the following year, disappeared from the cast coast of Greenland, which it had surrounded for four centuries. By the aid of such means of transportation, a great number of these quadrupeds might effect a landing at the same time, and the havoc which they would make among the species previous! y settled in the island would be terrific. The deer, foxes, seals, and even birds, on which these animals sometimes prey, would be soon thinned down. But this would be a part only, and probably an insignificant portion, of the aggregate amount of change brought about by the new invader. The plants on which the deer fed being less consumed in consequence 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 mcrease 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 ~he region ; and the changes caused indirectly might ramify throu()'h all classes of the living creation, and be almost end- o less. * Journal of a Residence in Icell.\nd, p. 276. Ch. IX.] OF THE RANGE OF SPECIES. 145 An actual illustration of what we have here only proposed hypot~etically, is in some degree afforded by the selection of small Islan~s by ~he eider duck for its residence during the season of mcubation ; its nests 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 labour in formin()' artificial islands b . b ' y separatmg from the main-land certain promontories J. oined to I. t by narrow isthmuses. This insular position is' neces-s~ ry to guard against the destruction of the eggs and young birds, by ~oxes, dogs, and other animals. One year, says Hooker*, It happened that, in the small island of Vidoc 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 bringin()' another fox. to the island, and fastening it by a string nea~ the haunt of the former, by which he was allured within shot of the hunter. ~t .is usual}~ th~ first appearance of an animal or plant, in a regwn to wluch 1t was previously a stranger, that gives rise to ~he chie~ alteration; since, after a time, an equilibrium is ag~m estabhshed. But it must require ages before such a new adJustment ~f. the relative forces of so many conflicting agents can be defimttvely settl~d. 'l'he causes in .simultaneous action are so numerous, that they admit of an almost infinite number of combinations ; and it is necessary that all these should have occurred once before the total amount of chan()'e capable of flowing from any new disturbing force, can be esti~ated. Thus, for example, suppose that once in two centuries a frost of unusual intensity, or a volcanic eruption of immense violence accom~anied by floods from the melting of glaciers, should occur m Iceland ; or an epidemic disease, fatal to the larger number of individuals of some one species, and not affectin()' others,-these, and a variety of other contingencies, all of which VoL, II. • Tour iu Iceland, vol. i., p. 64, second edition. L |