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Show 80 AGENCY OF BIRDS [Ch.V, Some of tl1e order of the Passeres, says Ekmarck *, devour the seeds of plants in great quantities, which they eject again in very distant places, without destroying its faculty of ~egetation ; thus a flight of larks will fill the cleanest field With a great quantity of various kinds of plants, as the melilot trefoil (Medicago Zupulina), and others whose seeds are so heavy that the wind is not able to scatter them to any distance. ln like manner the blackbird and missel-thrush, when they devour ' . berries in too great quantities, are known to consign them to the earth undigested in their excrement t. Pulpy fruits serve quadrupeds and birds as food, while theit· seeds, often hard and indigestible, pass uninjured through the intestines, and are deposited far from their original place of growth in a condition peculiarly fit for vegetation!· So well are our farmers, in some parts of England, aware of this fact, that when they desire to raise a quick-set hedge in the shortest possible time, they feed turkeys with the haws of the common white-thorn (Crata!gus oxyacantha), and then sow the stones which are ejected in their excrement, whereby they gain an entire· year in tl~e growth of the plant§. Birds when they pluck cherries, sloes, and haws, fly away with them to some convenient place, and when they have devoured the fruit drop the stone into the ground. Captain Cook, in his account of the volcanic isla"Qd of Tanna, one of the New Hebrides, which he visited in his second voyage, makes the following interesting observation. '' Mr. Forster, in his botanical excursion this day, shot a pigeon, in the craw of which was a wild nutmeg. He took some pains to find the tree on this island, but his endeavours were without success II·" It is easy, therefore, to perceive, that birds in their migrations to great distances, and even across seas, may transport seeds to new isles and continents. • Amren. Acad., vol. iv., Essay 75, § 8. t Wilcke, Amren. A cad., vol. vi. § 22. ! Smith's Introd. to Phys. and Syst. Botany, p. 304, 1807. § This information was communicated to me by Profossor Henslow, of Camb1·idge. 11 Dook iii., ch. 4. Ch. V.] IN DIFFUSING PLANTS. 81 The sudden deaths to which great numbers of frugivorous birds are annually exposed, must not be omitted as auxiliary to the transportation of seeds to new habitations. When the sea retires from the shore, and leaves fruits and seeds on the beach, or in the mud of estuaries, it might, by the returning tide, wash them away again, or destroy them by long immersion; but when they are gathered by land birds which frequent the sea-side, or by waders and water-fowl, they are often borne inland, and if the bird to whose crop they have been consigned is killed, they may be left to grow up far from the sea. Let such an accident happen but once in a century, or a thousand years, it will be sufficient to spread many of the plants from one continent to another; for, in estimating the activity of these causes, we must not consider whether they act slowly in relation to the period of our observation, but in reference to the duration of species in general. Let us trace the operation of this cause in connexion with others. A tempestuous wind bears the seeds of a plant many miles through the air, and then delivers them to the ocean; the oceanic current drifts them to a distant continent; by the fall of the tide they become the food of numerous birds, and one of these is seized by a hawk or eagle, which, soaring across hiJl and dale to a place of retreat, leaves, after devouring its prey, the unpalatable seeds to spring up and flourish in a new soil. The machinery before adverted to is so capable of dissemjnating seeds over almost unbounded spaces, that were we more intimately acquainted with the economy of nature, we might probably explain all the instances which occur of the aberration of plants to great distances from their native countries. The real difficulty which must present itself to every one who contemplates the present geographical distribution of species, is the small number of exceptions to the rule of the nonintermixture of different groups of plants. Why have they not, supposing them to have been ever so distinct originally, become more blended and confounded together in the lapse of ages ? VoL. II. G |