Page:EB1911 - Volume 28.djvu/1041

Rh For the distribution of invertebrate animals generally, the reader may be referred to the articles dealing with the various groups of that assemblage. An exception must, however, be made with regard to that group of spiders known as the Mygalomorphae (which includes the trap-door spiders) on account of the remarkable general similarity presented by its distribution to that of mammals. According to Mr R. I. Pocock, the distribution of this group justifies the mapping of the world into the following zoological regions:—(1) The Holarctic, including Europe north of the southern mountain-chains, North Asia, and North America north of about the 45th parallel of latitude. (2) The Mediterranean, including South Europe, Africa north of the Sahara and the desert-regions of south-western Asia. (3) The Sonoran, comprising the United States of America south of about the 45th parallel and the plateau of Mexico. (4) The Ethiopian, embracing Africa south of the Sahara, South Arabia and Madagascar. The last-mentioned island ranks merely as a subregion of the Ethiopian. (5) The Oriental, including India, Ceylon, Burma, Siam, and all the Indo- and Austro-Malayan Islands to Australia; "Wallace's line" being non-existent so far as spiders are concerned. (6) The Australian, containing Australia and New Zealand; the latter being worthy of recognition as a subregion. (7) The Neotropical, including Central America, apart from the Mexican plateau, the West Indies and South America.

These spiders furnish, moreover, strong evidence in favour of a former union between Africa and South America, and of a connexion between the Afro-Mascarene and Austro-Zealandian continents on the one hand and Austro-Zealandia and the southern extremity of South America on the other. As regards the "regions," apart from the greater divisions, or "realms, the distribution of these spiders accords with remarkable closeness to that of mammals, if we except the more intimate connexion indicated between the faunas of Ethiopian Africa and Madagascar.

The fact that the generally accepted scheme of division of the land-surface of the globe into zoological regions is based almost entirely upon the present distribution of mammals and birds has already been emphasized. It is perhaps only fair to quote the views of Dr A. E. Ortmann (who has devoted much study to the distribution of animals), although they by no means wholly commend themselves to the present writer:—

"(1) Any division of the earth's surface into zoo-geographical regions which starts exclusively from the present distribution of animals, without considering its origin, must be unsatisfactory, since always only certain cases can be taken in, while others remain outside of this scheme. (2) Considering the geological development of the distribution of animals, we must pronounce it impossible to create any scheme whatever that covers all cases. (3) Under these circumstances, it is incorrect to regard the creation of a scheme of animal distribution as an important feature or purpose of zöo-geographical research."

Dr Ortmann adds in a later paragraph, "the chief aim of zöo-geographical study consists—as in any other branch of biology—in the demonstration of its geological development."

II.

That the fauna of the ocean, apart from the influence of temperature, would be much more uniform, and therefore less susceptible of being divided into zoological provinces, or regions, will be apparent from a glance at the map of the world on Mercator's projection, in which the fact that the three great oceans the Pacific, the Atlantic and the Indian are in free communication with one another in the southern hemisphere is clearly brought out. There is, however, more than this; for there is evidence that during the early part of the Tertiary period the Pacific and the Atlantic were not separated by the isthmus of Darien; while there is a probability that the Mediterranean was at one time in communication with the Red Sea, and that other connexions of a like nature have existed.

In addition to this general community of the marine fauna of the world, there is the further important fact that such faunas may be divided into three main, and for the most part perfectly distinct, groups: namely, the littoral, or shallow water, fauna, the abyssal, or deep-sea, fauna, and the pelagic, or surface, fauna. Of these three the first alone is really susceptible of division into more or less ill-defined zoological regions, the other two being practically uniform in character. Moreover,

these three faunas are for the most part perfectly welldefined; the pelagic being very sharply sundered from the abyssal, although there may in certain instances be a tendency for the littoral to merge locally into the abyssal. As regards the sharp demarcation between the pelagic and the abyssal faunas, an idea was formerly current that whales, which are essentially pelagic animals, when "sounding," descended to abyssal depths in the ocean. A moment's reflection will show the absurdity of such a supposition; for no surface-dwelling animal could possibly support the enormous pressure existing at great depths, which would crush in the body-cavities. Evidence of this is afforded by the fact that when fishes are brought to the surface from great depths their bodies are practically broken to pieces by the removal of the normal pressure, while their scales start from the skin and the eyes from their sockets. The absolute darkness prevailing at great depths would be another bar to pelagic animals descending to the ocean abysses. We may accordingly regard the pelagic and the abyssal faunas as perfectly distinct and widely sundered from one another; as widely sundered in the case of some species as are beings living in three-dimension space from these (if such there be) inhabiting space of four or more dimensions.

Modern research shows that invertebrates, and probably also fishes, live at the greatest depths that have yet been reached by the dredge, and the inference from this is that they occur everywhere. The general results of these explorations is indeed to show that the fauna of the ocean depths is much the same all the world over, and that identical species occur at points sundered as widely as possible from one another. The ocean floor, as has been well remarked, with its uniformity in the matter of temperature, food, station and general conditions of life, contains, in fact, no effectual barriers to the almost indefinite dispersal of species.

The following general conclusions with regard to the deep-sea fauna were arrived at after working out the material and evidence obtained during the cruise of H.M.S. "Challenger":—(1) Animal-life is present at all depths on the ocean floor. (2) At extreme depths life is not nearly so abundant as at moderate depths; but since representatives of all classes of marine invertebrates are met with at all depths, this poverty probably depends more upon certain causes affecting the composition of the bottom deposits, and the degree to which the bottom-water is provided with chemical substances necessary for animals, rather than upon conditions immediately associated with depth. (3) There is reason to believe that the fauna of "blue water" is chiefly restricted to two belts; one at or near the surface, and the other near the bottom, the intervening zone being more or less completely devoid of inhabitants. From the surface-zone a continual ram of organic debris is falling to the bottom, which, however, in the case of the greatest depths may be completely dissolved in descensu. (4) Although all the chief groups of invertebrates are represented in the abyssal fauna, their relative proportions are unequal; molluscs, crabs and annelids being, as a rule, scarce, while echinoderms and sponges predominate. (5) Depths below 500 fathoms are inhabited by a practically uniform fauna, the genera being usually cosmopolitan, although the species may differ, and be represented by allied forms in widely sundered areas. (6) The abyssal fauna, so far as invertebrates are concerned, is of an archaic type as compared with shallow-water faunas. (7) The most characteristic and archaic abyssal types seen to be most abundant and to attain their maximum dimensions in the southern ocean. (8) In general character the abyssal fauna approximates to that of shallower water in polar latitudes, doubtless owing to the fact that the conditions of temperature, on which the distribution of marine animal life mainly depends, are nearly the same.

In reference to the abundance of sponges in the deep-sea fauna, it may be mentioned that the calcareous group is absent, and that among the siliceous section, the Hexactinellidae, of which the Venus flower-basket (Euplectella) and glass-rope sponge (Hyalonema) are familiar representatives, are exceedingly abundant; this group being likewise of great geological antiquity. Corals are sparse and belong for the most part to the Turbinolidae. Echinoderms are represented by all the existing ordinal groups; some of the crinoids, or stone-lilies, belong to the family Apiocrinidae, which attained its maximum development during the Jurassic epoch; and somewhat similar relationships are exhibited by certain of the brittle-stars (Ophiuroidea). Very noteworthy is the great development of the sea-cucumber group (Holothuroidea), and likewise the bizarre forms assumed by some of its abyssal representatives. Molluscs, however, are poorly represented, and it is not improbable that cephalopods (nautilus and cuttlefish group) are wanting. Bivalves of the genera Leda and Arca have, however, been obtained from a depth of 16,000 ft. Lamp-shells (Brachiopoda) are likewise scarce. 