Before we attempt to describe the conditions which prevail at great depths of the ocean, a few words should be said as to the part played by cable-laying in the investigation of the subaqueous crust of the earth. This part, though undoubtedly important, is sometimes exaggerated; and we have seen how large an array of facts has been accumulated by expeditions made mainly in the interest of pure science. The laying of the Atlantic cable was preceded, in 1856, by a careful survey of a submerged plateau, extending from the British Isles to Newfoundland, by Lieutenant Berryman of the Arctic. He brought back samples of the bottom from thirty-four stations between Valentia and St. John’s. In the following year Captain Pullen, of H.M.S. Cyclops, surveyed a parallel line slightly to the north. His specimens were examined by Huxley, and from them he derived the Bathybius, a primeval slime which was thought to occur widely spread over the sea-bottom. The interest in this ‘Urschleim’ has, however, become merely historic, since John Y. Buchanan, of the Challenger, showed that it is only a gelatinous form of sulphate of lime thrown down from the sea-water by the alcohol used in preserving the organisms found in the deep-sea deposits.
The important generalizations of Dr. Wallich, who was on board H.M.S. Bulldog, which, in 1860, again traversed the Atlantic to survey a route for the cable, largely helped to elucidate the problems of the deep. He noticed that no algæ live at a depth greater than 200 fathoms; he collected animals from great depths, and showed that they utilize in many ways organisms which fall down from the surface of the water; he noted that the conditions are such that, whilst dead animals sink from the surface to the bottom, they do not rise from the bottom to the surface; and he brought evidence forward in support of the view that the deep-sea fauna is directly derived from shallow-water forms. In the same year in which Wallich traversed the Atlantic, the telegraph cable between Sardinia and Bona, on the African coast, snapped. Under the superintendence of Fleeming Jenkin, some forty miles of the cable, part of it from a depth of 1,200 fathoms, was recovered. Numerous animals, sponges, corals, polyzoa, molluscs, and worms were brought to the surface, adhering to the cable. These were examined and reported upon by Professor Allman, and subsequently by Professor A. Milne Edwards; and, as the former reports, we ‘must therefore regard this observation of Mr. Fleeming Jenkin as having afforded the first absolute proof of the existence of highly organized animals living at a depth of upwards of 1,000 fathoms.’ The investigation of the animals thus brought to the surface revealed another fact of great interest, namely, that some of the specimens were identical with forms hitherto known only as fossils. It was thus demonstrated that species hitherto regarded as extinct are still living at great depths of the ocean.
During the first half of the last century an exaggerated idea of the depth of the sea prevailed, due in a large measure to the defective sounding apparatus of the time. Thus Captain Durham, in 1852, recorded a depth of 7,730 fathoms in the South Atlantic, and Lieutenant Parker mentions one of 8,212 fathoms—depths which the Challenger and the Gazelle corrected to 2,412 and 2,905 fathoms respectively. The deepest parts of the sea, as revealed by recent research, do not lie, as many have thought, in or near the centres of the great oceans, but in the neighbourhood of, or at no great distance from, the mainland, or in the vicinity of volcanic islands. One of the deepest ‘pockets’ yet found is probably that sounded by the American expedition on board the Tuscarora (1873-1875) east of Japan, when bottom was only reached at a depth of 4,612 fathoms. More recently, soundings of 5,035 fathoms have been recorded in the Pacific, in the neighbourhood of the Friendly Islands, and south of these again, one of 5,113 fathoms; but the deepest of all lies north of the Carolines, and attains a depth of 5,287 fathoms. It thus appears that there are ‘pockets’ or pits in the sea whose depth below the surface of the water is about equal to the height of the highest mountains taken from the sea-level. Both are insignificant in comparison with the mass of the globe; and it is sometimes said that, were the seas gathered up, and the earth shrunk to the size of an orange, the mountain ranges and abysmal depths would not be more striking than are the small elevations and intervening depressions on the skin of the fruit.
But it is not with these exceptional abysses that we have to do; they are as rare and as widely scattered as great mountain-ranges on land. It is with the deep sea, as opposed to shoal water and the surface layers, that this article is concerned; but the depth at which the sea becomes ‘deep’ is to some extent a matter of opinion. Numerous attempts, headed by that of Edward Forbes, have been made to divide the sea into zones or strata; and, just as the geological strata are characterized by peculiar species, so, in the main, the various deep-sea zones have their peculiar fauna. These zones, however, are not universally recognized; and their limits, like those of the zoogeographical regions on land, whilst serving for some groups of animals, break down altogether as regards others. There are, however, two fairly definite regions in the sea; and the limit between them is the very one for our purpose. This limit separates the surface waters, which are permeable by the light of the sun and in which owing to this life-giving light, algæ and vegetable organisms can live, from the deeper waters which the sun’s rays cannot reach, and in which no plant can live. The regions pass imperceptibly into one another; there is no sudden transition. The conditions of life gradually change, and the precise level at which vegetable life becomes impossible varies with differing conditions. With strong sunlight and a smooth sea, the rays penetrate further than if the light be weak and the waters troubled.
Speaking generally, we may place the dividing-line between the surface layer and the deep sea at 300 fathoms. Below this no light or heat from the sun penetrates; and it is the absence of these factors that gives rise to most of the peculiarities of the deep sea. It is a commonplace, which every schoolboy now knows, that all animal life is ultimately dependent on the food-stuffs stored up by green plants; and that the power which such plants possess of fixing the carbonic acid of the surrounding medium, and building it up into more complex food-stuffs, depends upon the presence of their green colouring matter (chlorophyll), and is exercised only in the presence of sunlight. But, as we have pointed out, ‘the sun’s perpendicular rays’ do not ‘illumine the depths of the sea’; they hardly penetrate 300 fathoms. This absence of sunlight below a certain limit, and the consequent failure of vegetable life, gave rise at one time to the belief that the abysses of the ocean were uninhabited and uninhabitable; but, as we have already seen, this view has long been given up.
The inhabitants of the deep sea cannot, any more than other creatures, be self-supporting. They prey on one another, it is true; but this must have a limit, or very soon there would be nothing left to prey upon. Like the inhabitants of great cities, the denizens of the deep must have an outside food-supply, and this they must ultimately derive from the surface layer.
The careful investigation of life in the sea has shown that not only the surface layer, but all the intermediate zones teem with life. Nowhere is there a layer of water in which animals are not found. But, as we have seen, the algæ upon which the life of marine animals ultimately depends, live only in the upper waters; below 100 fathoms they begin to be rare, and below 200 fathoms they are absent. Thus it is evident that those animals which live in the surface layers have, like an agricultural population, their food-supply at hand, while those that live in the depths must, like dwellers in towns, obtain it from afar. Many of the inhabitants of what may be termed the middle regions are active swimmers, and these undoubtedly from time to time visit the more densely peopled upper strata. They also visit the depths and afford an indefinite food-supply to the deep-sea dwellers.
But probably by far the larger part of the food consumed by abysmal creatures consists of the dead bodies of animals which sink down like manna from above. The surface layers of the ocean teem with animal and vegetable life. Every yachtsman must at times have noticed that the sea is thick as a purée with jelly-fish, or with those little transparent, torpedo-shaped creatures, the Sagitta. What he will not have noticed, unless he be a microscopist, is that at almost all times the surface is crowded with minute organisms, foraminifera, radiolaria, diatoms. These exist in quite incalculable numbers, and reproduce their kind with astounding rapidity. They are always dying, and their bodies sink downwards like a gentle rain.[1] In such numbers do they fall, that large areas of the ocean bed are covered with a thick deposit of their shells. In the shallower waters the foraminifera, with their calcareous shells, prevail, but over the deeper abysses of the ocean they take so long in falling that the calcareous shells are dissolved in the water, which contains a considerable proportion of carbonic acid gas, and their place is taken by the siliceous skeletons of the radiolarians and diatoms. Thus there is a ceaseless falling of organisms from above, and it must be from these that the dwellers of the deep ultimately obtain their food. As Mr. Kipling in his ‘Seven Seas,’ says of the deep-sea cables:
‘The wrecks dissolve above us; their dust drops down from afar—
Down to the dark, to the utter dark, where the blind white sea-snakes are.’
In trying to realize the state of things at the bottom of the deep sea, it is of importance to recognize that there is a wonderful uniformity of physical conditions là-bas. Climate plays no part in the life of the depths; storms do not ruffle their inhabitants; these recognize no alternation of day or night; seasons are unknown to them; they experience no change of temperature. Although the abysmal depths of the polar regions might be expected to be far colder than those of the tropics, the difference only amounts to a degree or so—a difference which would not be perceptible to us without instruments of precision. The following data show how uniform temperature is at the bottom of the sea.