[Footnote 1:  DARWIN, in his admirable account of the coral formations of the Pacific and Indian oceans, has propounded a theory as to the abundance of fresh water in the atolls and islands on coral reefs, furnished by wells which ebb and flow with the tides.  Assuming it to be impossible to separate salt from sea water by filtration, he suggests that the porous coral rock being permeated by salt water, the rain which falls on the surface must sink to the level of the surrounding sea, “and must accumulate there, displacing an equal bulk of sea water—­and as the portion of the latter in the lower part of the great sponge-like mass rises and falls with the tides, so will the fresh water near the surface.”—­Naturalist’s Journal, ch. xx.  But subsequent experiments have demonstrated that the idea of separating the salt by filtration is not altogether imaginary; as Darwin seems to have then supposed; and Mr. WITT, in a remarkable paper On a peculiar power possessed by Porous Media of removing matters from solution in water, has since succeeded in showing that “water containing considerable quantities of saline matter in solution may, by merely percolating through great masses of porous strata during long periods, be gradually deprived of its salts to such an extent as probably to render even sea-water fresh.”—­Philos.  Mag., 1856.  Divesting the subject therefore of this difficulty, other doubts would appear to suggest themselves as to the applicability of Darwin’s theory to coral formations in general.  For instance, it might be supposed that rain falling on a substance already saturated with moisture, would flow off instead of sinking into it; and that being of less specific gravity than salt water, it would fail to “displace an equal bulk” of the latter.  There are some extraordinary but well attested statements of a thin layer of fresh water being found on the surface of the sea, after heavy rains in the Bay of Bengal. (Journ.  Asiat.  Soc.  Beng. vol. v. p. 239.) Besides, I fancy that in the majority of atolls and coral islands the quantity of rain which so small an area is calculated to intercept would be insufficient of itself to account for the extraordinary abundance of fresh water daily drawn from the wells.  For instance, the superficial extent of each of the Laccadives is but two or three square miles, the surface soil resting on a crust of coral, beneath which is a stratum of sand; and yet on reaching the latter, fresh water flows in such profusion, that wells and large tanks for soaking coco-nut fibre are formed in any place by merely “breaking through the crust and taking out the sand.”—­Madras Journal, vol. xiv.  It is curious that the abundant supply of water in these wells should have attracted the attention of the early navigators, and Cosmas Indicoplenstes, writing in the sixth century, speaks of the numerous small islands off the coast of Taprobane, with abundance of fresh water and coco-nut