DISTRIBUTION OF ANIMALS. 311 DISTRIBUTION OF ANIMALS. form ri'fuges for animiils elscwheiv known only in subarctic rt-jrions, wliieh were left on their lofty and now isolated homes from the time when senii-glaeial conditions prevailed over the whole district. DisTuim iio.x WiTiiix THE Sea. Related to climatic inlhiences on land are those conditions in the sea which set invisil)le bounds to the spread of most marine animals, even when ap- parently at full liberty to go anywliere. A few species of bi<^ whales, sharks, and predatory fishes are known in all parts of the oceanic V orld. and even individuals are often very far- ranging. Some otliers are spread throughout all the nortlicrn seas, or all the southern, as tlie case may be; but of tropical marine animals very few are common to both the Atlantic and the" Pacific. On the contrary, the distribution of marine animals of every sort exliil)its local re- striction as fully as does that of land animals. Jlarine animals may be divided into tliree classes as relates to the present tlieme. namely the Lit- toral Fauna, Pelagic Fauna, and Abyssal Fauna. To the first l)elongs the crowded life of the shore region, where the rocks and forests of kelp, coral reefs, and natural bottom from liigli-water mark down to 100 fathoms or so of depth, are the home of a vast number and vajiety of fixed and mov- ing creatures of the lower orders, and a host of higher arthropods, fishes, and cetaceans which live among or upon them. These vary with the conditions. They are most abundant in the tropics and decrease toward the poles. Drifting sand and nuid are nearly barren, but rocky shores are populous. Currents of cold water affect a coast un/avorably, while warm currents bring and sustain many species. Thus physical and climatic conditions influence the amount and dis- tribution of shore life beneath the water much as they do that above it. The "pelagic' fauna includes those animals which habitually dwell upon or near the surface of the open ocean, visiting all parts of it. They may he able to swim actively and so move at will here or there, and such form the class 'nek- ton,' or, like the jellyfisiies and salps. may only float and be drifted at)OUt by winds and currents (the class 'plankton'). Even here, however, cer- tain species and groups are to be gathered only in certain parts of the sea : and their range seems to be limited mainly by factors of temperature. Thus the fauna of the Gulf Stream is distinct from that of the Atlantic for a considerable dis- tance north of Florida. B.MiivMKrmr Distribitiox. A new element enters into the question of the distribution of life in the sea. namely, variation in depth. This is comparable, in reverse, to hypsometric distribu- tion, or that according to height above sea-level. Layers of animal life, as it were, may be ob- served from the shore line to the greatest ex- plored depths. Afost of the creatures to be foiinil between tide-marks are absent or rapidly decrease below a few fathoms, wliile many rarely a|)proach the shore, but are numerous on bot- toms covered by 100 to .300 fathoms of water. Another zone belongs mainly or exclusively be- low that; and the globigerina ooze (q.v.), cover- ing the ocean bottom with grayish mud in most parts of the world, from 400 fathoms down to about 2000 fathoms, has a distinct fauna of its own. Below 2.500 fathoms the sea-bottom is formed of red clav, in which shells are absent, having ajjparently been dissolved during their descent to the greater dei)th. Kven hero, how- ever, is found an abyssal fauna chielly of fishes, "often of a very grotesque ai(|iearance." This bathymetric distribution, from the 100- fatliom line down, depends upon tlie factors of temperature and density. The former would be intlueneed by the ocean currents, and the animal life in the path of an inllu.x of water from the polar regions would be dill'erent from that in the path of a warm current. The abyssal fauna is one luibituatcd to such a degree of cold as would instantly kill mucli of the littoral or surface fauna. In addition to tliis jjowerful localizing influence, that of density, increasing with depth of water, is supreme as limiting the u[>ward and downward range of animals liabiluated to a certain stratum — that is, to a certain average degree of water-density. Surface animals would be smothered at a mile of depth; and those brought by dredges from the abysses are usually found to be burst to pieces by the expansion of the air in their cavities and tissues. Thus vertical as well as horizontal limits are set in the sea. Too little is known of the abyssal fauna to say whether its memliers are world- wide or restricted to local areas; but, as the con- ditions in the deep ocean-basins are nearly uni- form and undisturbed, it is probable that all the life is widely distributed. See Deep-Sea Ex- ploration. Fresh-Water Life. The fresh-water fauna ])resents certain characteristic features, and is divisible into 'Huviatile' forms, inhabiting streams and rivers, and 'lacustrine' forms, inhaliiting large lakes, where, as in the sea, the life is di- visible into 'littoral,' 'pelagic," and 'deep-water.' Dlspersiox. The slow or rapid spread of a species from its point of origination will depend upon its powers of locomotion and its adaptabil- ity to new circumstances. In the numunalia this is a matter of walking and swinuning. Some are excellent swimmers, and the great spread of the tiger throughout the Orient is mainly due to his natatorial ability. The faculty of flight has made bats more nearly cosmopolitan than is any other order of mammals. The wings of birds and insects give them a superior means of dispersal; flightless birds are. and always have been, much more circumscribed than the lliers. Reptiles and amphibians are poorly provided with means of locomotion, and are often very sluggish, while fresh-water fishes, except anadromous ones, usu- ally dwell in confined waters. .s for the mol- lusks, lower orders of insects, worms, and small sedentary animals of the shores, their principal, and often sole, resotirce is in the spread of their eggs or free-swimming larva-. Mechanical aids, however, render important assistance in the dis- persal of certain species, as is the case with so many plants. Land animals, large or small, fre- quently float across spaces of sea on ice cakes or driftwood, and some islands have undoubtedly been colonized in this way; yet it is a remote chance to trust, for unless a pair or a pregnant female were thus transported, no gain would result. Parasitic animals are carried about by their hosts. Small Crustacea and mollusks may be carried great distances by wind, or by adher- ing to the feet of l)irds. Infusoria, the eggs of rotifers, and other microscopic forms may be transported in the dried condition by the wind. Darwin brought forward much curious informa-
