VERTICAL DISTRIBUTION OF MARINE ANIMAL LIFE The vertical distribution of marine animal life is simi larly shown by a broad division into kahrizetrical zones of various depths. The mussel, cockle, periwinkle, &c., are found within the limit of high and low-water ; the star-fish, sea urchin, and other forms, abound in water 50 feet deep ; while the oyster, various forms of star-fishes, and a host of other marine animals, exist in still deeper water. Until recently it was believed that, at a certain depth, there was a "zero of life "—that at great depths the conditions were such that no living organism could possibly exist. But the Challenger and other expeditions have proved con clusively that animal life is abundant, even at the greatest depths. "The sea-bottom is inhabited by a fauna more rich and varied, and with organisms in many cases even more elaborately and delicately formed, and more exquisitely beau tiful in their soft shades of colouring, and in the rainbow tints of their wonderful phosphorescence, than the fauna of the well-known belt of shallow water, teeming with innumerable invertebrate forms, which fringes the land." The conditions under which these delicate organisms exist are certainly striking. We have already noticed the enormous pressure of water even at moderate depths. But the most delicately frail organisms may, by reason of their tissues being of the same density as the surrounding water, be not incommoded in the least by the pressure, even at the greatest depth, enormous though it must be ; but how are they supported I and whence do they derive their nourish ment I There is certainly, at a comparatively small depth, a total absence of light, and consequently of vegetation. The great distinction between an animal and a vegetable is, that the latter only has the power to convert inorganic matter for its own nutrition, and that the animal is dependent for its subsistence upon this power of the vegetable of converting inorganic into organic matter. The total absence of vegetation, concurrently with the most prolific animal life, in the depths of the sea, most probably suggested the idea that the forami nifera, sponges, &c., found at great depths, have the power of directly converting the inorganic matters dissolved in the waters of the ocean for their own nutrition.' Many of
them certainly do draw their supplies of carbonate of lime or silica for their shells directly from the water in which it is dissolved, and they may probably abstract and convert other inorganic matters in solution in the water. A high authority on the subject supposes that the minute forms which teem in the depths of the sea are supported by absorbing the animal matter, of which an immense quantity must be suspended or dissolved in the water.
The conditions of pressure at great depths, considered in relation to the delicately-formed organisms teeming therein, are indeed extraordinary. The graphic illustration given by Sir C. Wyville Thomson shows the actual amount of pressure better, perhaps, than a mere abstract statement. That at 2,000 fathoms a man would bear on his shoulders a weight equal to 20 locomotive engines, each with a long train loaded with pig iron, is almost incredible; yet millions of delicate marine forms bear, seemingly without the least difficulty, a pressure far exceeding this—and they not only bear it, but it is apparently absolutely necessary to their continued existence, for nearly all the specimens dredged up from the bottom were either dead, or dying, when brought up to the surface. The fresher and lighter surface water may certainly have affected them, but it is more probable that they died because the normal pressure was reduced. Even sharks, dragged up in Setubal Bay from depths of about 500 fathoms, were dead before they reached the surface.' Now it is well known that human beings exist best under ordinary atmospheric pressure—that is, 14i lbs. on the square inch ; but even this pressure, if exerted in any other way, would crush the strongest man to death. And yet, if this pressure be sensibly reduced, as on the higher mountains, mere existence is a matter of difficulty, while if it be still further reduced, death is certain to ensue. Reasoning by analogy, then, we conclude that the enormous pressure of the water under which the marine forms live in depths of the sea, is as neces sary a condition of their existence as the ordinary pressure of the atmosphere is to human beings and land animals generally.