The fauna of the deep sea—with a few exceptions hitherto only known as fossils—are new and specially modified forms of families and genera inhabiting shallow waters in modern times, and have been driven down to the depths of the ocean by their more powerful rivals in the battle of life, much as the ancient Britons were compelled to withdraw to the barren and inaccessible fastnesses of Wales. Some of their organs have undergone considerable modification in correspondence to the changed conditions of their new habitats. Thus down to 900 fathoms their eyes have generally become enlarged, to make the best of the faint light which may possibly penetrate there. After 1000 fathoms these organs are either still further enlarged or so greatly reduced that in some species they disappear altogether and are replaced by enormously long feelers. The only light at great depths which would enable large eyes to be of any service is the phosphorescence given out by deep-sea animals. We know that at the surface this light is often very powerful, and Sir Wyville Thomson has recorded one occasion on which the sea at night was “a perfect blaze of phosphorescence, so strong that lights and shadows were thrown on the sails and it was easy to read the smallest print.” It is thought possible by several naturalists that certain portions of the sea bottom may be as brilliantly illumined by this sort of light as the streets of a European city after sunset. [Illustration: THE BOTTLE-FISH AND THE PELICAN-FISH.] [Illustration: AT THE BOTTOM OF THE TROPICAL SEA. The large floating object is the phosphorescent, compound, oceanic hydrozoan _Agalma elegans_, a physophore related to the jellyfishes. Its tentacles trail over dead corals,—madrepore, brain-corals, etc.; while the living reef beyond is crowned by branching corals, corallines and seaweeds.] One of the most striking examples of this vertical distribution, which forms layers of animal life, as it were, in the ocean from the abysses to the shallows, is shown by the coral-reefs. The foundations of these polyp-built barriers or islands are laid by the millions of minute individuals of one solid, heavy kind of coral which can flourish only in pretty deep water. When these have reached their highest growth they cease to propagate there, and a second kind comes and colonizes upon the summit of this massive foundation and carries the work a little farther up. Then these die off, and a third kind plants itself upon their remains and carries the structure to the top, near the surface of the sea, where many surface-corals, corallines, and various other limy and flinty plants and animals help to erect a dry reef, upon which land vegetation can find a root-hold, and where, after a while, men may dwell. When these coral-built islands are ring-shaped they are called _atolls_, and are believed to be living crowns about the summits of submerged mountains.