CHAPTER I.. Oceanography.

OCEANOGRAPHY.

One of the most important facts that has been established by modern investigations of the Sea is that there is no region in its vast extent that is entirely devoid of animal life. The surface waters in the Equatorial calms and the ice-cold waters between the ice-bergs of the Arctic regions are densely populated by animals, large and small; the heavy and heated waters of the Mediterranean and Red Seas, and the cold and comparatively fresh waters of the Norwegian fjords, the shallow waters of the coasts and the greatest depths of the ocean-beds all present us with their characteristic forms of living creatures. There is no Azoic region known to us. Wherever we use the trawl or dredge we may expect to find some representatives of the various classes of marine animals. But the seas exhibit so many varying conditions that, as we might have expected, the animals that characterise one region are absent from another; and while, in some places animal life is abundant, in others it is very scarce; just as on land we find the grass-lands and forests teeming with life, and the great deserts and mountain tops inhabited only by a few solitary Lizards, Birds or Insects.

In order that we may fully understand, then, the nature of the problems concerning the distribution of animals through the seas, it is of importance to consider first the conditions under which they must live in the different parts of the ocean. A knowledge of geography is clearly necessary for those who study the distribution of terrestrial and aërial animals, and equally necessary is it for those who wish to learn something about the distribution of the aquatic animals, to consider first of all the rudimentary principles of hydrography. The principal Sea areas of our globe may be roughly divided into two groups: namely, the great oceans--the Atlantic, Pacific and Indian Oceans--and the Inland seas, which are partly enclosed by land, such as the German Ocean, the Mediterranean Sea, and the Red Sea. Taking the areas of the great oceans and the seas together, we find that no less than 141 millions of square miles, or nearly three-quarters of the surface of the globe, are covered with water. Moreover, these great areas are continuous, so that it would be possible for an animal, other conditions being favourable, to pass from any one sea, such as the Black Sea, to any other, such as the Hudson’s Bay, without leaving the water,--an important fact in the consideration of the distribution of marine forms of life.

The depth of the sea varies very considerably in the different parts of the world. In the inland seas the water is comparatively shallow, but in the great oceans it is very deep. In the middle of the North Sea, for example, we should not expect to find a depth much exceeding 250 fathoms, but in the Atlantic or Pacific Oceans we have to pay out more than 2000 fathoms of the sounding-line before the bottom is reached. In some parts of the ocean-basins a few very deep holes or furrows may be found in which the depth exceeds 4000 fathoms, or 24,000 feet. One of these deep holes occurs in the Atlantic Ocean, a little to the North of the Virgin Islands in the West Indies, and there is another in the Pacific Ocean close to the coast of Japan; but the greatest depth that has yet been found is one recently discovered by H.M.S. “Penguin” off the coast of New Zealand of over 5000 fathoms. Apart, however, from the fact that these very great depths are only of local occurrence, the areas of deep water--that is, of more than 2000 fathoms--are so much greater than the areas of shallow water, that when we make a calculation of the average depth of the sea we find it is no less than 2100 fathoms, or 12, 600 feet.

The temperature of the sea is another feature which undoubtedly influences very greatly the character of its Fauna. The main source of the heat of the sea is the sun--for the heat derived from submarine volcanoes must be comparatively so small that we may omit it from consideration. Consequently we find that in the Equatorial regions the surface waters of the ocean are warmer than they are in the Temperate regions. These, again, are warmer than in the Arctic circles. But water is well-known to be a bad conductor of heat, and therefore the direct influence of the sun affects only the most superficial layers. In the Equatorial region of the Pacific Ocean, for example, the surface temperature is sometimes as high as 80° Fahr., at 100 fathoms from the surface it is only 60°, at 400 fathoms only 45°, and at 1000 fathoms only a few degrees above freezing point. On the land the temperature falls as we pass from the coast to the high plateaux and mountains, and we find snow-capped mountains in Central Africa just as in Switzerland or Norway. In the sea the temperature falls as the thermometer is sent deeper from the surface. Just as on the land the snow line of the mountains is reached at high altitudes in the Tropics, at lower altitudes in the Temperate regions, and in the Arctic circle at the level of the sea, so in the sea the cold water that is found 500 fathoms below the surface in the Tropics, reaches a higher level in the Temperate regions, and is at the surface in the Arctic circle.

There is however one important point of difference between the distribution of these low temperatures on the land and in the oceans, in that they are broken in the former, and continuous in the latter. If we were to imagine an aquatic animal that could only live in temperatures below 35° Fahr., it would be able to travel below the surface from one pole to the other, or from one ocean to another; but it would be impossible for a terrestrial animal, exhibiting the same peculiarity, to leave the Arctic circle or the Alpine region without traversing lands where the temperature is higher than that which is necessary for its existence. It might be supposed from what has just been said that the temperature of the water at the bottom is constant for the same number of fathoms of depth. This is not, however, the case. The temperature of the sea-bottom of the great ocean-beds is approximately the same, varying from 28° F. in the Atlantic to 35° F. in the Pacific; but in places where main basins occur, surrounded on all sides by shallower ridges, the temperature of the bottom of the basin is the same as that of the lowest ridge. For instance, the temperature of the bottom of the Sulu Sea, lying between Borneo and the Philippines, is 40° F. at a depth of over 2000 fathoms. Again, the temperature of the Red Sea is as high as 70° F., although depths of 1200 fathoms occur in its central portions; and this is the same as the temperature at the Straits of Babel Mandeb, which are 200 fathoms deep, and form the only outlet to the open ocean. These facts probably cause considerable modification in the character of the animals inhabiting such enclosed basins, but further investigations are needed before we can arrive at any very definite conclusions in the matter.

Another important element that must be taken into consideration in studying the environment of marine animals, is the quantity and character of the salts held in solution by the sea-water. In the first place we must remember that the sea-water normally contains a far greater percentage of salts in solution than the water of rivers and lakes, and this causes it to be very much heavier. If a tumbler be half filled with sea-water, upon which some fresh water is slowly and carefully poured, there will be for some time very little mixture of the two fluids, the heavier sea-water remaining at the bottom, and the lighter fresh water at the surface. Now the density of the sea-water, or in other words the amount of salts in solution, is not the same over the whole world, and the differences that may be observed in this respect are due, in most cases, to the simple physical principle just enunciated. If we could imagine a river pouring its waters into a perfectly calm, tideless sea, we should be able to trace the fresh river water far away from the coast, for it would simply float on the heavier sea-water without mixing with it to any appreciable extent. In most cases, however, the tidal-waves, rushing up and down the river estuaries, stir up the fresh and salt water together, and cause a very considerable mixture, so that the water becomes either distinctly salt or brackish. Where very large quantities of fresh water are poured into the ocean, as, for example, at the mouth of the Amazon or the Mississippi, the surface water remains so fresh that the salt taste can hardly be appreciated at a distance of some miles from the coast. This fact sufficiently indicates the influence of great rivers upon the density or saltness of the sea-water in their neighbourhood, and the reader will be prepared for the statement that many inland seas, such as the Black Sea, are appreciably less salt than the great oceans.

Again, the ocean water itself is not of the same density in all latitudes. In regions where there is a copious rainfall and the sea is not frequently disturbed by severe storms, the rain takes some time to mix with the heavier salt water on which it falls, and consequently there may always be discovered in these localities a thin stratum of comparatively fresh water on the surface of the ocean. In some inland seas where there is considerable evaporation and a slight rainfall, as for example, the Red Sea and the Mediterranean, the sea-water reaches an even higher degree of concentration than it does in the open ocean. The following table will serve to illustrate these facts:--

Density of rain-water, 1·00.

Density of the Black Sea surface, below 1·025.

Density of the Atlantic Ocean surface (west of the Canaries), 1·0275.

Density of the Mediterranean Sea, over 1·028.

Density of the Red Sea, 1·030.

Density of the bottom water of the Atlantic, 1·029 (west of the Canaries).

That the rate of movement of the water influences very largely the character of the animals that live in it, is a fact that it is not necessary to discuss fully in this place; but as it is undoubtedly one of the factors which must be taken into consideration in discussing the character and possible origin of the Fauna of any particular region, a brief survey must be given of some of the principal causes of the movements of the water and the characters of the tides and currents which are manifest in the sea. Twice every twenty-four hours the water of the sea rises and falls. This movement is due to the attracting influences of the sun and moon, and is, as is well-known, greater when the moon is full and when it is new than at the intermediate times. If the distribution of land and water on the surface of our globe were different, and a free waterway occurred round the world, right in the Equatorial band we should probably find a double tidal-wave rushing round the earth every twenty-four hours. As it is, however, the great tidal-wave is checked by the continents, and as it approaches the coasts is retarded and diminished in force. In Archipelagoes and along broken coast lines the tidal-waves produce true surface currents, which frequently run with great rapidity and exert considerable corroding action upon the rocks. In many estuaries and bays the tide rushes in with such force that the water is heaped up to a great height against the land. At the entrance to the Bay of Fundy, for example, the rise at spring-tides is no less than 70 feet, and at the Cardiff docks the difference of level between high and low spring-tides is 42 feet. The tumultuous ebb and flow of such masses of water along the coast is fatal to some forms of animal life and favourable to others, and so to some extent it modifies the character of the Fauna.

In addition to the surface currents of the coast, produced by the tidal-waves, there are also the true ocean-currents, which must be briefly considered. They are caused by the winds which blow constantly in a definite direction across the oceans. The prevailing winds not only raise the sea into waves, but drive the superficial layers of the water over the subjacent layers in one direction. In studying a map of the great ocean-currents, we notice a well-marked one lying to the North of the Equator in both the Atlantic and Pacific. This flows from East to West and follows very closely the lines of the prevailing winds in that region. Similarly in the temperate regions of the Southern Hemisphere there is an ocean-current, flowing however in this case in the opposite direction--from West to East--and so corresponding with the trade-winds of that part of the world. The well-known Gulf-stream of the North Atlantic, although modified in some respects by other more complicated causes, also follows for part of its course the general direction of the prevailing winds.

The currents just described are surface currents only, and do not affect to any great extent the mass of the subjacent waters in the ocean-basins. It is difficult to estimate the depth to which their influence reaches, but it is not probable that it extends more than 200 fathoms below the surface. In addition to these, there is also a series of slow currents in the deep waters flowing in definite directions. In the Tropical regions the waters are constantly being heated by the sun, and passed away by the trade-winds to the North and South and ultimately towards the poles. In their long and complicated journey they are gradually cooled down until, in the regions of the ice-bergs, they reach a temperature just above the freezing point of sea-water. Here the water, being colder and therefore heavier than that of the other regions of the world, sinks to the bottom, and gradually returns in a deep-seated mass towards the Equator, where, welling up from the bottom, it replaces the heated layers of the surface. It is almost impossible to determine with accuracy the rapidity and exact direction of these deep-sea-currents. It is extremely probable that they are immensely modified by the irregularities of the bottom and the outline of the coast banks, but their exact topography must remain for the present one of the secrets of the abyss that are not revealed to us. All that can be said is, that the warm surface water which passes from the Tropics towards the North and South is replaced by deep-seated Polar currents, which account for the extremely cold water that is found at great depths in the ocean-basins, and also for some of the peculiarities of the marine Fauna, which will be referred to later on.

The character of the sea-bottom in various parts of the world must be referred to before passing on, for there can be little doubt of the important effect it has upon the Fauna. In the neighbourhood of continents the bottom of the sea varies very considerably. The great rivers bring with them in suspension the products of the wear and tear of mountains and valleys; the coast line, washed by the continuous ebb and flow of the tides, contributes some of its substance to the formation of the sea-bottom; and the countless millions of animals and plants of the shallow waters leave their skeletons and shells as they die to form an integral part of the floor of the ocean. Thus the sea-bottom in the neighbourhood of the land is formed partly by terrigenous deposits, varying of course in character with the geological nature of the land itself, and partly by the animal and vegetable deposits of the coast. In some cases the deposits brought by the rivers can be traced in the sea-bottom for a very great distance from the coast. The characteristic mud of the Congo river can be traced 600 miles from its mouth, and it is said that the Arabian Sea and the Bay of Bengal are carpeted for 1000 miles by the mud brought down by the Ganges and the Indus. Leaving out of consideration, however, for the moment the exceptional cases of such large rivers as these, we may say that the influence of the land deposits upon the character of the sea-bottom extends to a distance seawards of about 250 miles. If we had a complete and careful survey of all the coast lines, it would be possible for us to draw a line round the great continents marking the limit of the deposits of river and coast mud. This line has been called the mud-line by Mr John Murray, and, as he has clearly pointed out, it is characterised by an abundant and extremely interesting Fauna.

The sea-bottom, then, within the limits of the mud-line, is very largely composed of deposits from the land brought down by the rivers. In some volcanic regions of the world this is, to a great extent, augmented by lava and water-logged pumice, and in other districts by the mud and stones dropped by the melting ice-bergs. The influence of animals and plants upon the formation of the sea-bottom is often very great indeed in shallow water, though it varies considerably in different parts of the world. In the neighbourhood of the British coast, for example, the sea-bottom is, in many places, carpeted with the calcareous Sea-weed, Lithothamnion,--in other places the dredge will come up crammed full of bivalve shells. But such instances as these in which the floor of the sea is covered with animal or vegetable shells, are comparatively rare and of small extent in the neighbourhood of land in the Temperate regions, and in nearly all localities the true terrigenous deposits can be readily obtained by the use of a small meshed dredge. In the warmer regions of the world, however, the sea-bottom in the shallow water is over great areas completely covered by animal and vegetable products. In the West Indies, and in some parts of the Eastern coasts of Tropical America, in the Eastern Archipelago and the coast of East Africa and its islands, Coral-reefs are found. These are entirely built up of the skeletons and shells of animals, and a few Coralline Algæ. In the vicinity of these reefs the floor of the sea is for miles carpeted with the broken-down skeletons of these animals, sometimes in the form of a fine coralline sand, sometimes of large lumps studded with knolls of living Corals, Molluscs, Sea-urchins, and other creatures. We find, therefore, in the warmer regions of the world immense areas of shallow water in which the terrigenous deposits take but a very small part in the formation of the sea-bottom, animal and vegetable life being so vigorous and active as to be able to form enough shells and skeletons to cover every available part of its surface.

Far away from continental lands, and at great depths, the character of the sea-bottom completely changes. At a distance of 100 miles from the coasts of America or Europe, for example, the land deposits have already found their resting-place, and the animal life in the depths of the Atlantic is poor in skeleton-forming genera. However, the surface waters of the ocean teem with creatures of all sorts which, as they die, drop down their skeletons and shells in a gentle shower to form a fine deposit on the bottom. When we get beyond the mud-line, then, and use the dredging or sounding apparatus in depths of 1500 to 2500 fathoms, we find that the bottom is largely composed of the shells of such surface animals as the Pteropods and Globigerinas, and according to the relative abundance of these forms it is called Pteropod Ooze or Globigerina Ooze. In still greater depths than these the character of the bottom again changes, and we find a deposit which is commonly known as the Red clay. The explanation of this change of character depends upon the fact that sea-water exercises a slightly solvent action upon carbonate of lime, and the shells of the Globigerinas and other forms are, in seas of a depth of over 2500 fathoms, dissolved before they can reach the bottom. The only shells that can survive this long journey are the siliceous shells of the Diatoms and Radiolarians, and in those parts of the ocean where these organisms live in abundance their empty shells form an important percentage of the composition of the Red clay. Over a very considerable area of the Pacific Ocean, however, the Red clay contains only a minute proportion of these shells, and its composition has given rise to a good deal of discussion among the authorities. It may be considered to be a conglomerate of the horny fragments of dead surface-living animals, of volcanic and meteoric dust, and of small pieces of water-logged pumice-stone.

In some of the very deep holes of the Pacific Ocean the mud is almost entirely composed of Radiolarian shells, and is then called “Radiolarian ooze”: and in the Southern Sea a mud called Diatom ooze has been found, which consists principally of the siliceous shells of these minute algæ. Notwithstanding these varieties of the mud, and others that space does not allow me to refer to at length, we may suppose that if the floor of any one of the great oceans were exposed it would have the appearance to a traveller of a vast desert-like expanse, without a stone, a rock, or a cliff to vary the monotony of the scene. At one time it was supposed to be an absolute plain, without any important change of level from the mud-line of one continental coast to the other; but the result of modern submarine explorations has been to prove that in all the great ocean-basins, hills and ridges, as well as troughs and deep holes occur, which break the monotony of the generally smooth and level character of the bottom.