being specifically lighter than the salt, keeps floating on the surface, and is subject to the same movements; howsoever this may be, the fact is certain. A fit and convenient abode is thus fabricated by the action of the feeble, gelatinous polyps, and a wild and almost boundless waste of waters becomes enlivened by oases which navigators have described as earthly paradises.

A barrier-reef is essentially similar to the atoll or coral-island. It runs parallel with the shores of some larger island or continent, separated, however, from the land by a broad and deep lagoon-channel, and having the outer side as deep and steep as in the lagoon island. Here likewise the skeletons of the zoophytes of which the reef is composed are found on the outer precipitous wall as deep as soundingline can reach.

The third class of coral productions, called by Mr. Darwin "fringing-reefs," differ from the barrier-reefs in having a comparatively small depth of water on the outer side, and a narrower and shallower lagoon between them and the mainland.

These differences in the characters of the wonderful fabrications of the coral-producing polyps are explicable by the following facts in their history. The animals of the Porites and Milleporæ cannot exist at a greater depth than twenty or thirty fathoms; beyond this, the stimuli of light and heat derived from the solar beams become too feeble to excite and maintain their vital powers. On the other hand, their tissues are so delicate, that a brief direct exposure to the sun's rays kills them; and unless they are constantly immersed in water or beaten by the surf, they cannot

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live. Thus, in whatever situation the skeleton of a Madrepore or Millepore may be found, it is certain that it must have grown within thirty fathoms of the surface of the ocean. When it coats the summit of the lofty mountain of Tahiti, where Mr. Stutchbury found a regular stratum of semi-fossil coral at 5000 and 7000 feet above the level of the sea, it must have been lifted up by the elevation of the rock on which it was originally deposited. If it is brought up from the depth of 200 or 300 fathoms, as at Cardoo Atoll, or Keeling Atoll, it must have been dragged down to that depth by a gradual subsidence of the foundation on which the living Madrepore once flourished. It is by these movements of upheaval and subsidence of the earth's crust that Mr. Darwin explains the different forms which the coral reefs assume. Elizabeth Island, which is eighty feet high, is entirely composed of coral rock. The coral animals, thus progressively lifted up above their element, are compelled to carry on their operations more and more remote from the former theatre of their constructive energies, but cannot extend deeper than their allotted thirty fathoms; the direction of their submarine masonry is therefore centrifugal and descending. Where the land that supports them is, on the contrary, in a state of submergence, they are compelled to build their edifices progressively higher, and in a narrower circuit; in other words, their growth is centripetal and ascending; the terms 'ascending' and 'descending,' of course, only applying in this case to the relation of the coral builders to the unstable land, not to the level of the unchanging sea.

The prodigious extent of the combined and unintermitting labours of these little world-architects must be witnessed in order to be adequately conceived. They have built up a barrier-reef along the shores of New Caledonia for a length of four hundred miles, and another which runs along the north-east coast of Australia 1000 miles in extent. Now, assuming this latter to be only a quarter of a mile in breadth and 150 feet deep, here is a mound compared with which the walls of Babylon, the great wall of China, or the Pyramids of Egypt are but children's toys; and built too amidst the waves of ocean and in defiance of its storms, which sweep away the solid works of man.

"The geologist," says Professor Owen, "in contemplating these stupendous operations, appreciates the conditions and powers by which were deposited in ancient times, and under other atmospheric influences than now characterize our climate, those downs of chalk which give fertility to the south coast, and many other parts of our native island. The remains of corals in these masses, though allied in their general nature, are specifically distinct from the living polyps which are now actively engaged in forming similar fertile deposits on the undulating and half-submerged crust of the earth washed by the Indian and Pacific Oceans. Again, those masses of limestone rocks which form a large part of the older secondary formations, give evidence by their organic remains that they too are due to the labours of polyps, the species of which perished before those that formed the cretaceous strata were created. As the polyps of the secondary epochs have been superseded by the Porites, Millepore, Madre

SUCCESSION OF SPECIES OF CORAL ZOOPHYTES. 149

pore, and other genera of calcareous Anthozoa of the present day, so these, in all probability, are destined to give way in their turn to new forms of essentially analogous Zoophytes, to which in time to come the same great office will be assigned, of clothing with fertile limestone future rising continents."

CHAPTER XVI.

ACTINIE.

"THE works of Nature far exceed what we know, or are able to know of them. Convinced of this truth, and in order to improve my customary walks by the sea-side to some useful purpose, I bestowed particular attention, in the month of November 1771, upon the Sea-Anemones. My first success in these discoveries soon turned these amusements of mine into a long study, much more laborious than that made in a library." Thus writes the venerable Abbé Dicquemare in the Philosophical Transactions for 1773, and doubtless there is many a naturalist of the present day who will sympathetically appreciate the sentiments he expresses.

Certainly among all the beauteous objects upon the sea-beach that woo our admiration, as though in emulation of each other's charms, none can bear comparison with the subject of the present chapter.

"The living flower that, rooted to the rock,

Late from the thinner element,

Shrunk down within its purple stem to sleep,
Now feels the water, and again

Awakening, blossoms out

All its green anther-necks."