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Show 138 RECIPROCAL INFLUENCE OF [Cb. VIII. in some places, heaped one upon ano~her, to the de~th of four feet, in Russia, Poland, and Lilhuama; and when ln southern Africa they were driven into the sea by a north-west wind, they formed, says Barrow, along the shore, for fift~ miles, a bank three or four feet high •. Dut when we consider that forests are stripped of their foliage, and the earth of its green garment, for thousands of square miles, it may well be supposed that the volume of animal mattet· produced may equal that of great herds of quadrupeds and flights of large birds suddenly precipitated into the sea. . . . The occurrence of such events at certam mtervals, m hot countries, like the severe winters and damp summers returning after a series of years in the temperate zone, affect the proportional numbers of almost all classes of animals and plants, and arc probably fatal to the existence of many which would otherwise thrive there, while, on the contrary, they must be favourable to certain species which, if deprived of such aid, mjo-ht not maintain their ground. ~lthough it may usually be remarked that the extraordinary increase of some one species is immediately followed and checked by the multiplication of another, yet this is not always the case, partly because many species feed in co~mon on the same kinds of food, and partly because many kinds of food are often consumed indifferently by one and the same species. In the former case, where a variety of different animals have precisely the same taste, as, for example, when many insectivorous birds and reptiles devour alike some particular fly or beetle, the unusual numbers of the latter may only cause a slight and almost imperceptible augmentation of each of those species of bird and reptile. In the other instance, where one animal preys on others of almost every class, as, for example, where our English buzzards devour not only small quadrupeds, as rabbits and field-mice, but also birds, frogs, lizards, and insects, the profusion of any one of these last may cause all such general feeders to subsi~t more exclusively upon * Travels in Afl"ica, p. 25(, Kirby a.nd Spence, vol: i., P· 215. Ch. VIII.] AQUATIC AND TERRESTRIAL SPECIES. 139 the species thus in excess, and the balance may thus be restored. The number of species which are nearly omnivorous is considerable ; and although every animal has, perhaps, a predilection for some one description of food rather than another, yet some arc not even confined to one of the great kingdoms of the organic world. '"fhus when tho racoon of the West Indies can neither procure fowls, fish, snails, nor insects, it will attack the sugar-canes, and devour various kinds of grain. The civets, when animal food is scarce, maintain themselves on fruits and roots. Numerous birds, which feed indiscriminately on insects and plants, arc perhaps more instrumental than any other of the terrestrial tribes in preserving a constant equilibrium between the relative numbers of different classes of animals and vegetables. If the insects become very numerous and devour the plants, these birds wiJJ immediately derive a larger portion of their subsistence from insects, just as the Arabians, Syrians, and Hottentots feed on locusts, when the locusts devour their crops. The intimate relation of the inhabitants of the water to those of the land, and the influence exerted by each on the relative number of species, must not be overlooked amongst the complicated causes which determine the existence of animals and plants in certain regions. A large proportion of the amphibious quadrupeds and reptiles prey partly on aquatic plants and animals, and in part on terrestrial; and a deficiency of one kind of prey causes them to have immediate recourse to the other. The voracity of certain insects, as the dragon-fly, for example, is confined to the water during one stage of their transformations, and in their perfect state to the air. Innumerable water-birds both of rivers and seas derive in like manner their food indifferently from either element; so that the abundance or scarcity of prey in one induces them either to forsake or more constantly to haunt the other. Thus an intimate connexion between the state of the animate creation in a lake or |