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Show 14~ EFFECT OF THE EXTENSION [Ch.IX. must produce on the condition of others inhabiting the same reO'ions. When the necessary consequences of such extensi9ns ha~e been fully explained, the reader will be prepared to appreciate the important influence which slight modifications in the physical geography of the globe may exert on the condition of organic beings. In the first place it is clear, that when any region is stocked with as great a variety of animals and plants as the productive powers of that region will enable it to support, the addition of any new species, or the pe1·manent numerical increase of one previously established, must always be attended either by the local extermination or the numerical decrease of some other species. 'l~here may undoubtedly be considerable fluctuations from year to year, and the equilibrium may be again restored without any permanent alteration; for in particular seasons a. greater supply of heat, humidity, or other causes may augment the total quantity of vegetable produce, in which case all the animals subsisting on vegetable food, and others which prey on them, may multiply without any one species giving way; but whenever the aggregate quantity of vegetable produce remains unaltered, the progressive increase of one animal or plant implies the decline of another. All agriculturists and gardeners are familiar with the fact, that when weeds intrude themselves into the space appro· priated to cultivated species, the latter are starved in their growth or stifled. If we abandon for a short time a field or garden, a host of indigenous plants, Tho darnel, hemlock, ami rank fumitory, pour in and obtain the mastery, extirpating the exotics, ot• putting an end to the monopoly of some native plants. If we inclose a park, and stock it with as many deer as the herbage will support, we cannot add sheep without lessening the number of the deer; nor can other herbivorous species be subsequently introduced, unless the individuals of each species in the. park become fewer in proportion • Ch. IX.] OF TilE RANGE OP SPECIES. 148 So if there be an island where leopards are the only beasts of prey, and the )ion, tiger, and hyrena afterwards enter, the leopards, if they stand their ground, will be reduced in number. If the locusts then arrive and swarm greatly, it may deprive a large number of phytophagous animals of their food, and thereby cause a famine, not only among them, but among the beasts of prey ;-certain species, perhaps, which had the weakest. footing in the island will thus be annihilated. We have seen how many distinct geographical provinces there are of aquatic and terrestrial species, and how great are the powers of migration conferred on different classes, whereby the inhabitants of one region may be enabled from time to time to invade another, and do actually so migrate and diffuse themselves over new countries. Now, although our knowledge of the history of the animate creation dates from so recent a period, that we can scarcely trace the advance or decline of any animal or plant, except in those cases where the influence of man has intervened, yet we can easily conceive what must happen when some new colony of wild animals or plants enters a region for the first time, and succeeds in establishing itself. Let us consider how great are the devastations committed at certain periods by the Greenland bears, when they are dt·ifted to the shores of Iceland in considerable numbers on the ice. These periodical invasions are formidable even to man ; so that when the bears arrive, the inhabitants collect together, and go in pursuit of them with fire-arms-each native who slays one being rewarded by the king of Denmark. The Danes of old, when they landed in their marauding expeditions upon our coast, hardly excited more alarm ; nor did our islanders muster more promptly for the defence of their lives and property against a common enemy, than the modem Icelanders against these formidable brutes. It frequently happens, says Henderson, that the natives are pursued by the bear when he l1as . been long at sea, and when his natural ferocity has be.en strengthened by the keenness of hunger; if |