Page:Encyclopædia Britannica, Ninth Edition, v. 3.djvu/38

Rh hill is quite conformable to what we see in other cases, in which a heavier under-strstnm has a definite set towards a si ope; and whilst the existence of i such a westerly set is, ex hypo- thesi, a necessary consequence of the southerly &amp;lt; &amp;lt; movement of the C0 c Arctic under- | soo flow, no other | explanation of 1 it has been sug- * 2 c gested. We now 1 1 4 o c see that the cold |, c Labrador Cur- z 1. &quot; 1 S rent overlies a band of water us &quot; n cold as itself; 2200 and the south- ,400 ward extension a c of this cold band, far beyond that of any definite current - move- Fr(J ment, and its &amp;gt;&amp;lt;s ofikCMf Stream!;

[chart]

entrance into the Gulf of Mexico, through the Florida Channel, at the side of and beneath the outflowing Gulf Stream, are thus accounted for. The remarkable accordance of so many facts of actual observation, in the Atlantic area, with the probabilities deducible from a theory whose soundness can scarcely be disputed, seems now to justify the admission of the general (vertical) oceanic circulation, sustained by opposition of temperature as an accepted doctrine of terrestrial physics.

Distribution of Organic Life. All that will be attempted under this head will be to indicate the general conditions that seem, from recent researches, to have the greatest influence on the distribution of plants and animals through this great oceanic basin. The distribution of marine plants seems mainly deter mined by light, temperature, and depth, a further influence being exerted by the character of the shores. The diminu tion of liyht iu its passage through sea-water is so rapid, that the quantity which penetrates to a depth of 250 or 300 fathoms may be regarded as almost infinitesimal ; and in conformity with this we find a very rapid diminution of Algal life below the depth of 100 fathoms. The upper stratum is occupied for the most part by the larger and coarser forms of the Fucaceoe, or olive-green sea-weeds, whilst the more delicate Ceramiactce, or red sea-weeds, fre quent deeper waters ; and, as it appears from experiments made in aquaria that the latter do not flourish in full light, but grow well in shadow, it may be concluded that their preference for a moderate depth is rather for reduced light and stillness than for depth per se. At a depth of 150 fathoms very few ordinary sea-weeds maintain their ground; and below this we seldom find any Algae, save the Coral lines and Nulliporcs consolidated by calcareous deposit. The distribution of particular types over different parts of the Atlantic area appears to be mainly regulated by tem perature ; and this would seem to be remarkably the case with the floating Dlatomacece, which, though they form green bands in the surface-water of polar seas, have not been encountered in like abundance in the Atlantic, and do not contribute largely, by the subsidence of their siliceous lories, to the composition of its bottom-deposit. Although it is the habit of the larger Algre to grow from a base of attachment (their roots serving no other purpose however, than that of anchorage), the enormous mass of Gulf -weed found in the Sargasso Sea seems quite independent of any such attachment. It was at one time supposed that this originally grew on the Bahama and Florida shores, and was torn thence by the powerful current of the Gulf Stream ; but it seems certain that if such was its original source, the &quot; Gulf-weed &quot; now lives and propagates whilst freely floating on the ocean-surface, having become adapted by various modifications to its present mode of existence. The distribution of the animals that habitually live in that upper stratum of the ocean whose degree of warmth varies with the latitude, seems mainly determined by temperature. Thus the &quot;right whale&quot; of Arctic seas, and its representative in the Antarctic, seems never to enter the inter-tropical area, generally keeping away from even the temperate seas, whilst, on the other hand, the sperm-whale ranges through the parts of the ocean where the &quot; right whales &quot; are never seen. The distribution of fishes seems generally to follow the same rule ; as does also that of floating mollusks. Thus the little Clio (a Pteropod mollusk), which is a principal article of the food of the &quot; right whales &quot; in polar seas, is rarely met with in the Atlantic, where, however, other pteropods, as Hyalcea, present themselves in abundance. On the other hand, the warmer parts of. its area swarm with Salpa-chains, which are not frequent in higher latitudes ; and the few representatives of the Nautiloid Cephalopods, that were so abundant in Cretaceous seas, are now restricted to tropical or sub-tropical areas. And the distribution of the mollusks, echinoderms, and corals, which habitually live on the bottom, seems to be determined, within certain limits at least, by temperature rather than by depth. The bathymetrical range to which animal life of any higher type than the Ehkopodal might extend, was until recently quite unknown; but the researches initiated by Prof. Wyville Thomson and Dr Carpenter in 1868, and since prosecuted by the &quot;Challenger&quot; expedition, have fully established the existence of a varied and abundant fauna in ocean-depths ranging downwards to 2000 fathoms. And these researches have further established that the distribu tion of this fauna is mainly determined by the tempera ture of the sea-bed ; so that whilst in the channel between the north of Scotland and the Faroes there were found at the same depths, and within a few miles of each other, two faunas almost entirely distinct one a boreal and the other a warmer-temperate on sea-beds having respectively the temperatures of 30 and 43, various types to which a low temperature is congenial are traceable continuously along the whole abyssal sea-bed that intervenes between those northern and southern polar areas within which they present themselves at or near the surface. And hence it becomes clear that, since glacial types are even now being embedded in the strata which are in process of formation beneath the equator, no inferences as to terrestrial climate can be drawn from the character of marine deposits. One very remarkable feature which presents itself over a large proportion of the Atlantic basin is the abund ance of the minute Globigerince and other Foraminifera, the accumulation of whose shells, and of their disintegrated remains, is giving rise to a calcareous deposit of unknown thickness, that corresponds in all essential particulars to Chalk. This deposit, in some parts of the North Atlantic, is replaced by an Arctic drift of fine sand, whilst in other parts there is a mixture of arenaceous and of calcareous components, such as is found in certain beds of the Cretaceous formation. Now on the surface of this deposit there have been found so many living types, especially belonging to the groups of Echinoderms, Corals, Siliceous Sponges, and Foraminifera, which closely correspond with types hitherto regarded as characteristic of the Cretaceous, epoch, that the question naturally suggests itself whether 