Page:Journal of the Royal Geographical Society of London, Volume 1 (2nd edition).djvu/245

Rh to rise to the surface in the form of a truncated cone, and then their progress being stopped, they work laterally: so that if several of them were near each other, they would unite and form a shelf similar to that which exists round the margin of some of the lagoons. Some of the lagoons which the Blossom entered were from twenty to thirty-eight fathoms in depth. A fact, however, in contradiction to Captain Beechey's opinion, has been previously mentioned by Chamisso; nor were there wanting surmises of what perhaps this navigator has the credit of proving satisfactorily—that the wall of coral which encircles these islands is so narrow and perfect, that they have been improperly designated as groups or chains of islands, in consequence of the wall being broken by channels into the lagoon; for, on examination, the chain is found continuous under water, and as in all probability it will in time reach the surface, and become dry, the whole of the group may generally be considered as one island.

We may remark that the observations of Captain Beechey, while they correspond with those of the French naturalists in many points, do not favour the hypothesis of former navigators, that the saxigenous polypi raise their habitations gradually from a small base, always spreading more in proportion as the structure grows higher: an hypothesis which is objectionable on many grounds.

There can now be no doubt, from the situation of these coral islands with respect to each other, as they often form rows, their union in several places in large groups, and their total absence in other parts of the same seas, that the polypi have founded their buildings on shoals of the sea, or on the tops of mountains lying under water; but it is quite incorrect to suppose, with Messrs. Quoy and Gaimard, that there are no islands of any extent constantly inhabited by man, which are entirely formed of corals,—or, that, far from raising from the depths of the ocean perpendicular walls, these animals form only layers or crusts of a few fathoms thickness. And it is equally impossible to grant to all these animalcules one common impulse, or, as Captain Flinders has it, an instinctive foresight to shelter their habitations from the impetuosity of the winds, by forming constantly high walls and reefs to the windward: a circumstance, the explanation of which we think might easily be sought in physical causes alone. For if the lateral movements of the polypi, or their natural tendency to horizontal construction, happens to be impeded in any one direction, they will gain vertically what they lose horizontally; and the resistance being equal on the same side, the true horizontal extent will be everywhere the same, and a wall will be formed: while, in an opposite direction, the same circumstances not being in existence, the constructions of the polypi will extend horizontally as well as vertically, and consequently will not rise with the same degree of rapidity as those which are erected to the windward; and hence