The New International Encyclopædia/Distribution of Animals

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DISTRIBUTION OF ANIMALS, or Zoögeography. Knowledge of the geographical distribution of animals, and their faunal relations past and present, constitutes a very important part of the science of zoölogy. An animal's environment exerts a formative influence upon its character and structure. Knowledge of its habitat and that of its species, together with such changes as the area and range may have undergone, is therefore essential to a full understanding of the natural history of any kind of animal or animals.

Conditions Controlling Habitat and Range. Some animals are fitted to dwell wholly upon land, others in water. The aquatic animals are, further, divisible into those of salt and those of fresh waters. The former have the oceans of the world open to them, yet few, if any, range through all seas; furthermore, the inhabitants of the surface of the sea differ from those of its bottom or deeper parts. Similarly, animals able to mount into the air and to fly abroad with apparent freedom, are rarely cosmopolitan, and whole groups of these, as well as innumerable species and individuals, reside only within limited areas or along narrow lines of migratory travel. Fresh-water forms are likely to be restricted to separate lakes and river systems. Of the terrestrial animals none are now cosmopolitan, except those which have been carried by man to all parts of the world. (The introduced fauna of a region must always be distinguished from its indigenous fauna.) On the contrary, each of the present grand divisions of the earth has a characteristic fauna of its own, and often a whole family, order, or even subclass, belongs to one continent alone. Coincident with these restrictions and diversities, likenesses between widely separated lands are manifest; and representatives of a single group may be found in regions far distant from one another. To set forth these facts, to endeavor to account for them, and to develop their significance, is the province of zoögeography.

Limits of Dispersion: The Sea as a Barrier. Each kind of animal must have had a point of origin, whence it spread as it increased. Theoretically, the expansion of a species would proceed equally in all directions, but actually this expansion has encountered barriers and restrictions that have confined and shaped habitats within certain areas, in some instances extremely wide, in others surprisingly narrow. What constitutes these barriers and restrictions? That depends in each case on the physical surroundings as related to the needs and abilities of the animal in question. It is plain that a species of fish that originated in or somehow became restricted to an inland water — Lake Baikal, for example — could never spread beyond its shores save by accidental transplantation; while an able sea-going fish may wander indefinitely, so far as mere room is concerned. Among land animals space for expansion, then, is of prime importance. Here the firmest bounds are set by the sea. Any considerable breadth of water, and especially of salt water, is uncrossable except by rare and extraordinary accident; and even then a pregnant female or a pair must be landed on the further shore in order to start a colony, which must, furthermore, find favorable surroundings in order to survive in the new locality. This accounts for the fact that mammals and terrestrial reptiles and amphibians are absent from oceanic islands. Continuity of land, then, is necessary to the spread of a terrestrial species; and when species are found in regions now widely separated, as Europe and North America, it can usually be shown that such regions were formerly connected by lands now submerged. Conversely, the characteristics of the present fauna of such comparable regions assist the geologist to determine when the connection was finally broken.

Insular Faunas. The faunas of most islands near continents date back to the time when these islands were a part of the adjacent mainland, or were separated from it only by narrow straits. The explanation of the broad and conspicuous disparity that exists between the faunas of such large islands as Australia and Madagascar, and those of Asia and Africa, respectively, is accounted for by the evidently very ancient date at which they became dissevered. “It is evident,” remark Flower and Lydekker, "that Australia has been isolated from the Asiatic Continent from some very remote geological epoch, at which period it is probable that monotremes and marsupials were the dominant if not the sole representatives of the mammalia then existing. Consequently Australia has never been able to receive an influx of the Eutherian orders, which have probably swept away all the marsupials except the small American opossums from the rest of the globe. Again, the large island of Madagascar, which has a fauna of an African type, but still very markedly different from that of the mainland, may be considered to have been connected with the latter at a time when the Eutheria had become the dominant forms, but has been separated for a sufficiently long period to have enabled a large number of its species and genera to have become distinct from those of the adjacent continent. Similarly there is evidence to show that South America was probably cut off for a considerable period from the northern half of the American Continent, in consequence of which its lowly organized fauna of Edentates were enabled to attain such a remarkable development in the later geological periods. Consult Wallace, Island Life (New York, 1880). See Isolation.

“The above instances [and their converse, exemplified by the identity of insular and continental faunas where a land union is known to have existed lately] are sufficient to show . . . how largely the present distribution of mammalian life is bound up with the past history of our globe. We must, however, not omit to mention another very important agency of past times which has likewise had great influence on the present distribution of the various faunas of the Northern Hemisphere. This is the so-called Glacial Epoch, which took place immediately before the establishment of the present state of things, and appears to have been the cause of the extinction of many of the larger mammalian types which formerly inhabited Europe.”

But while many species have had a far wider distribution in past ages than now (and in a general way a widely distributed form may be regarded as an old form), many species seem never to have expanded much, some being limited to a short stretch of coast, or a single river-valley, or to a certain mountain or island.

Deserts, Mountains, etc., as Barriers. Other barriers than the sea, therefore, have been effective in impeding the spread of crescent varieties, species, and groups. Most prominent among these is a desert region. The Sahara cuts off almost completely the fauna of Europe from that of Africa; and the continuation of the arid treeless area northeastward across Asia divides the northern Asiatic animals from the southern in a most emphatic way. Similarly, the fauna of North America stops and that of the southern continent begins where the plains and tablelands of Arizona and northern Mexico interpose a waterless, scantily planted space of semi-desert and sterile hills, uninviting to either set of animals as a whole, yet invaded by both.

Long and lofty mountain ranges stand next in importance, probably, as physical barriers, but this is more marked in the warmer than in the cooler parts of the world, since animals accustomed to the torrid conditions of tropical plains cannot or will not endure the cold and lack of customary food encountered even on the passes, and so do not cross over the highlands. The mountains of North America seem to make less difference with the distribution of our animals, as between the Atlantic and Pacific slopes, than does the central-plains area; and that of Europe has been little affected by the presence of mountains. The Atlas range is more marked as a boundary, but that is because of the neighboring desert; and similarly the great Central-Asian ranges are part of the lofty, cold, and arid region which as a whole forms probably the most effective inland barrier in the world.

Deep and broad rivers or straits of the sea may interrupt the spread of many species. The Hudson seems never to have been crossed by the opossum, which could neither get over nor around it. The Mississippi put an end to the eastward progress of the jaguar along the Gulf region, and much smaller rivers might stop many other creatures, such as monkeys (which are unable to swim); yet none would prove a barrier to flying or amphibious animals, and many serve to extend hundreds of miles the range of aquatic and semi-aquatic ones.

Frequently, however, neither these nor various other ‘barriers’ that might be mentioned seem to have prevented the growth of species which nevertheless are scant in numbers and extremely restricted in area of range. The blaubok (q.v.) of South Africa was unknown beyond a single valley, and has almost become extinct. This is a sharp example of many puzzling cases where probably the restraining influence has been competition. Each locality supplies food, water, and shelter for only a certain quota of animals. The ablest and best-fitted in each class for that region will get the most out of it and will increase and spread. The ‘struggle for existence’ is everywhere a real daily fight for space and subsistence, and the weakest will gradually succumb or become prisoners within areas so peculiarly favorable to them that there they are able to hold their own. The powerful and gregarious cattle long ago forced the sheep to keep to the mountain pastures. Active enemies must also be considered, certain animals not being able to exist in the same territory. A river infested with crocodiles might forever prove uncrossable for small quadrupeds that otherwise might soon pass over it. The faunal distinctions between forest and prairie, mountain and plain, diurnal and nocturnal methods, are effects of enmity as well as of competition.

Climate and Altitude as Factors. Of climate as a determinant in geographical distribution, perhaps too much has been made in the past. Climate, within its extremes, seems influential upon animal life mainly as cold or prevailing drought affects the higher forms of vegetation (see Distribution of Plants), and consequently the food of herbivorous and insectivorous animals. Such species as can take a varied fare, and can by migration, storage of food, or hibernation, escape or provide against storm and scarcity, defy climatic bounds. The big cats and bears range from torrid jungles to snowy mountain-tops and subarctic latitudes. Transplanted species frequently flourish in climates the opposite of that to which they are native. In a general way, nevertheless, similar animals are found in similar climates, and within the larger regions lesser faunal divisions often conform closely to isothermal belts, a fact more noticeable in North America than anywhere else. Here, too, comes in the so-called ‘vertical’ distribution of life observable in high mountains, where various altitudes exhibit an animal and plant life identical with or similar to that of northern latitudes, where a corresponding average of temperature prevails. Thus mountain ranges lying north and south carry far toward the equator along their cool ridges species which exist near sea-level only in high latitudes. Our bighorn, once numerous along the Rockies south to New Mexico, is an example of this feature; and the guanaco of Patagonia, following the Andes north to the equator in Ecuador, is another. Terrestrial life is everywhere most plentiful near sea-level and in warm regions, and becomes more scanty as the poles are approached or mountains are ascended. Mountain-tops, however, frequently form refuges for animals elsewhere known only in subarctic regions, which were left on their lofty and now isolated homes from the time when semi-glacial conditions prevailed over the whole district.

Distribution Within the Sea. Related to climatic influences on land are those conditions in the sea which set invisible bounds to the spread of most marine animals, even when apparently at full liberty to go anywhere. A few species of big whales, sharks, and predatory fishes are known in all parts of the oceanic world, and even individuals are often very far-ranging. Some others are spread throughout all the northern seas, or all the southern, as the 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 exhibits local restriction as fully as does that of land animals. Marine animals may be divided into three classes as relates to the present theme, namely the Littoral Fauna, Pelagic Fauna, and Abyssal Fauna. To the first belongs the crowded life of the shore region, where the rocks and forests of kelp, coral reefs, and natural bottom from high-water mark down to 100 fathoms or so of depth, are the home of a vast number and variety of fixed and moving 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 mud are nearly barren, but rocky shores are populous. Currents of cold water affect a coast unfavorably, while warm currents bring and sustain many species. Thus physical and climatic conditions influence the amount and distribution 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 be able to swim actively and so move at will here or there, and such form the class ‘nekton,’ or, like the jellyfishes and salps, may only float and be drifted about by winds and currents (the class ‘plankton’). Even here, however, certain 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 distance north of Florida.

Bathymetric Distribition. 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 distribution, or that according to height above sea-level. Layers of animal life, as it were, may be observed from the shore line to the greatest explored depths. Most of the creatures to be found between tide-marks are absent or rapidly decrease below a few fathoms, while many rarely approach the shore, but are numerous on bottoms covered by 100 to 300 fathoms of water. Another zone belongs mainly or exclusively below that; and the globigerina ooze (q.v.), covering 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 2500 fathoms the sea-bottom is formed of red clay, in which shells are absent, having apparently been dissolved during their descent to the greater depth. Even here, however, is found an abyssal fauna chiefly of fishes, “often of a very grotesque appearance.”

This bathymetric distribution, from the 100-fathom line down, depends upon the factors of temperature and density. The former would be influenced by the ocean currents, and the animal life in the path of an influx of water from the polar regions would be different from that in the path of a warm current. The abyssal fauna is one habituated to such a degree of cold as would instantly kill much of the littoral or surface fauna. In addition to this powerful localizing influence, that of density, increasing with depth of water, is supreme as limiting the upward and downward range of animals habituated 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 members are world-wide or restricted to local areas; but, as the conditions in the deep ocean-basins are nearly uniform and undisturbed, it is probable that all the life is widely distributed. See Deep-Sea Exploration.

Fresh-Water Life. The fresh-water fauna presents certain characteristic features, and is divisible into ‘fluviatile’ forms, inhabiting streams and rivers, and ‘lacustrine’ forms, inhabiting large lakes, where, as in the sea, the life is divisible into ‘littoral,’ ‘pelagic,’ and ‘deep-water.’

Dispersion. The slow or rapid spread of a species from its point of origination will depend upon its powers of locomotion and its adaptability to new circumstances. In the mammalia this is a matter of walking and swimming. 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 fliers. Reptiles and amphibians are poorly provided with means of locomotion, and are often very sluggish, while fresh-water fishes, except anadromous ones, usually dwell in confined waters. As for the mollusks, lower orders of insects, worms, and small sedentary animals of the shores, their principal, and often sole, resource is in the spread of their eggs or free-swimming larvæ. Mechanical aids, however, render important assistance in the dispersal of certain species, as is the case with so many plants. Land animals, large or small, frequently 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 adhering to the feet of birds. Infusoria, the eggs of rotifers, and other microscopic forms may be transported in the dried condition by the wind. Darwin brought forward much curious information on this subject in the second volume of his Origin of Species (London, 1882).

It will be seen that the varying abilities among the different groups of animals would make their distribution equally different one from the other. Salt water is a poison to larval amphibians, while birds may fly across more than 1000 miles of sea space. Mammals may run about rapidly, while the reptiles and small invertebrate creatures must creep slowly or not at all. Finally, marine animals occupy an area and medium entirely different from that held by terrestrial animals. These variations must be borne in mind in attempting to reduce to scheme and system the vastly diverse phenomena of zoögeography.

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Faunal Regions. Heretofore we have been considering only the dispersion and restriction of a species. But as a rule several kindred co-exist in any given area, and these usually differ in their range, while occasionally a species of the same genus is to be found somewhere else, entirely disconnected from its fellows. This shows that the geographical area covered by a genus is greater than that of a species; and, carrying the same inquiry further, it appears true of families, orders, and classes. The larger the group in the scheme of classification the wider its geographical area. To ascertain and record the spread, past and present, of the groups of animals is the business of zoögeography. The earlier students drew up a map in which they confidently set apart a series of realms, provinces, and subprovinces. A vast amount of such information had been tabulated by Sclater, Darwin, Schmarda, Murray, and others previously to the publication, in 1876. of Alfred Russell Wallace's monumental work on the subject. Wallace, following Sclater (1857), divided the globe into six grand ‘regions,’ each characterized by groups of animals absent from, or very scantily represented in, any other. These were:

Palearctic Region—Europe, Africa north of the Sahara, and Asia north of the Himalayas.

Oriental Region.—India, Malaya, southern China, Sumatra, Java, Borneo, the Philippines, and included islands.

Australian Region.—Australia and the islands north of it from Celebes eastward, New Zealand and the South Sea archipelagoes. This was divided from the Oriental region by Wallace's line — a name gracefully given by Huxley in 1868 to the zoögeographical demarcation discovered by A. R. Wallace (Ibis, London, 1859; Proceedings Zoölogical Society, London, 1863; Malay Archipelago, London and New York, 1869), which passes along the narrow straits between the Philippine and Sulu islands, and southward between Celebes and Borneo, Lombok and Java. He found the birds and mammals strikingly different on opposite sides of this line of deep channels. “The great contrast between the two divisions of the archipelago,” he informs us, “is nowhere so abruptly exhibited as in passing from the island of Bali to that of Lombok, where the two regions are in closest proximity. . . . The strait is here 15 miles wide, so that we may pass in two hours from one great division of the earth to another, differing as essentially in their animal life as Europe does from America. If we travel from Java or Borneo to Celebes or the Moluccas, the difference is still more striking. In the first the forests abound in monkeys of many kinds, wild-cats, deer, civets, and otters, and numerous varieties of squirrels are constantly met with. In the latter none of these occur, but the prehensile-tailed cuscus is almost the only terrestrial mammal seen. . . . The birds, which are most abundant in the western islands, are woodpeckers, barbets, trogons, fruit-thrushes, and leaf-thrushes; they are seen daily and form the great ornithological feature of the country. In the eastern islands these are absolutely unknown, honeysuckers and small lories being the most common birds; so that the naturalist feels himself in a new world.”

Paleotropical (or Ethiopian) Region. — Africa south of the Sahara, and Madagascar.

Nearctic Region.— North America and the elevated central region of Mexico.

Neotropical Region.—South America, Central America, and the West Indies.

Each of these regions was divided into four subregions or "provinces." In Africa, Cape Colony and the southeast coast formed a province, the Congo and Niger basins together another, Madagascar and the Mascarene Islands a third, and all the rest of the continent south of the Sahara a fourth. In North America, all Canada north of Lake Huron and the Saskatchewan was one province, the eastern United States as far as the central dry plains formed a second, the Rocky Mountain country and ‘great basin’ a third, and the Pacific Coast the fourth. And so on. For the faunal characteristics of each of these regions, see the articles under their names; also Holarctic Region; Neogæa; Notogæa.

Arctogæa and Notogæa. The apportionment above sketched has proved too artificial. The propriety of separating North America from Europe and Asia was soon disputed, and they were united by various authorities in a single circumpolar region called Holarctic (or Periarctic). The equatorial countries, mostly separated by oceans, cannot be so easily combined, and for some time the only serious change in the classic arrangement was the erection of the New Zealand group from secondary to primary rank. Much is to be said in favor of this movement, but it seems not to be generally acceptable.

As knowledge of both the living and fossil animals of the southern continents and islands has increased, and criteria have become largely available outside of the groups of birds and mammals upon which earlier conclusions were mainly based, it becomes more and more apparent that even these regional distinctions are vague. The best opinion at the opening of the twentieth century, following Huxley (1868), held that only two prime regions might be recognized in zoögeography — Arctogæa, a northern world, and Notogæa, a southern world. These names, however, are not precisely descriptive. Arctogæa includes not only the whole Northern hemisphere, but also Paleotropica (Africa, India, and the East Indies as far as Wallace's line). Notogæa is formed of South and Central America and Australasia. Within these primary realms the old subdivisions seem to hold pretty well, except that North America does not seem separable from Eurasia, both now forming the single Holarctic Region (q.v.) of recent students.

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Lesser subdivisions must be determined independently, if at all, for each group under study. Nor have even the largest zoögeographical divisions hard and fast boundaries. They overlap and blend, forming transitional zones or debatable grounds. The stretch of mountainous deserts from the western Sahara to Manchuria is such a zone, where representatives of northern and southern faunas mingle, and where dwell many species not known elsewhere in either. The so-called ‘Sonoran’ subregion (southwest United States to the highlands of Central America) is another borderland of debatable validity. The distinction between the Malayan and Australian subregions insisted upon by Wallace disappears entirely in respect to frogs and toads. In truth, the lesser subdivisions that have been so numerously and exactly marked off by specialists have no real existence. “They depend,” to use Gadow's words (1902), “upon the class, or even order, of animals which we may happen to study. The faunistic distribution of the Urodela is not that of the Anura, and both follow separate lines of dispersal, different from those of the various orders of reptiles, birds, and mammals. This must be so. There is no doubt that the distribution of land and water was totally different in the Coal Age from what it is now. The face of the globe at the Jurassic Age can scarcely be compared with the aspect which the world had assumed in the Miocene Period. This leads to another consideration often neglected. We know that the various classes, orders, families, etc., of animals have appeared successively on the stage. A group which arose in the Coal Age followed lines of dispersal different from one which was not evolved until Jurassic times, the post-Cretaceous creatures could not avail themselves of what assisted their ancestors, and vice versa. Speaking generally, the older a group the more likely it is to be widely distributed. If it appears scattered, this may be due to extinction in intermediate countries, or to submergence of former land connexions. . . . It is the morphologist who is ultimately responsible for the establishment of faunistic regions, not the systematist, least of all he who accepts an elaborate classification, and then mechanically, mathematically, by lists of genera and species, maps out the world.”

Bibliography. L. K. Schmarda, Die geographische Verbreitung der Thiere (Vienna, 1853); P. L. Sclater, Proceedings Linnæan Society (Zoölogy), vol. ii. (London, 1857); id., The Ibis (London, 1891); A. Murray, Geographical Distribution of Animals (London, 1866); Thos. Huxley, Proceedings Zoölogical Society of London, for 1868; A. R. Wallace, The Geographical Distribution of Animals (New York, 1876); A. Heilprin, The Geographical and Geological Distribution of Animals (New York, 1887); H. Gadow, chapter on “Geographical Distribution,” in Bronn's Thierreich (Berlin, 1893); A. Newton, Dictionary of Birds (London, 1893-96); F. E. Beddard, A Text-book of Zoögeography (Cambridge, 1895); R. Lydekker, Geographical History of Mammals (Cambridge, 1896); W. L. and P. L. Sclater, The Geography of Mammals (London, 1899); H. Gadow, Amphibia and Reptiles (London, 1902). For North America, consult especially Heilprin and Newton, and the following: S. F. Baird, “Distribution and Migration of North American Birds,” in American Journal Sciences and Arts, series 2, vol. xli. (New Haven, 1866); J. A. Allen, “Bird Faunæ of Eastern North America,” in Bulletin Museum of Comparative Zoölogy, vol. ii. (Cambridge, Mass., 1871); Allen, “Geographical Distribution of the Mammalia,” in Bulletin United States Geological Survey (vol. iv., No. 2, Washington, 1878); Merriam, Proceedings Biological Society of Washington, vol. vii. (Washington, 1892); and “Biological Survey of the San Francisco Mountain Region,” in North American Fauna, No. 3 (Department of Agriculture, Washington, 1890).