If we consider that similar bird-republics are to be found on almost every rocky coast or surf-beaten cliff of the northern seas, we must needs be astonished at the inexhaustible prodigality of Nature, which covers desolate rocks with such a profusion of life. The vast number of sea-birds is the more surprising, as many species, such as the guillemot, the auk, the fulmar, and the puffin, lay but one single egg on the naked rock, and often in so precarious a situation, that it is almost inconceivable how breeding can take place. When the birds are surprised and suddenly fly off, many of the eggs tumble down into the surf. Sea-eagles, falcons, and raptorial gulls destroy a great number, and pounce upon the young; thousands fall a prey to the rigours of an Arctic winter; the spring-tides sweeping over low shores, often carry away whole generations at once, and many a maritime population lives entirely upon the sea-fowl that breed upon the sterile soil. And yet, in spite of so many enemies and persecutions, their numbers remain undiminished, nor has their importance ever ceased in the domestic economy of the rude islanders of the north. [Illustration: Auk.] [Illustration: Sea-Fowl Shooting.] But however valuable the eggs and the oil, the feathers and the flesh of the hyperborean bird-republics may be to man, they are far from equalling in importance the guano producing sea-fowl of the tropical seas. This inestimable manure, which has become so indispensable to the British agriculturist, is found scattered over numerous localities in the intertropical regions. It abounds on many of the rocky islets of the Red Sea, where the life-teeming waters afford sustenance to innumerable sea-gulls, cormorants, and pelicans; but its most widely celebrated stores cover the small Chincha Islands, not far from Pisco, about a hundred miles to the south of Callao, where they form enormous layers 50 or 60 feet deep. The upper strata are of a greyish-brown colour, which lower down becomes darker; and in the inferior strata the colour is a rusty red, as if tinged by oxide of iron. The guano becomes progressively more and more compact from the surface downwards, a circumstance naturally accounted for by the gradual deposit of the strata and the increasing superincumbent weight. As is universally known, guano is formed of the excrements of different kinds of marine birds; but the species which Tschudi, the celebrated Peruvian traveller, more particularly enumerates are--_Larus modestus_ (Tschudi), _Rhynchops nigra_ (Linn.), _Plotus anhinga_ (Linn.), _Pelecanus thayus_ (Mol.), _Phalacrocorax Gaimardii_ and _albigula_ (Tsch.), and chiefly the _Sula variegata_ (Tsch.).