DISTRIBUTION OF MAMMALS 161 vances of such nature are likely to be shared by all the animals of a region.
The ways in which mammal species and mammal faunas move from their favorable original environment into some newly available but just as favorable one, seem almost analogous to the creeping expansion of pools of water here or there on a dry, nearly level floor. This analogy is useful, provided that one remembers that the water (a fauna) is a solution of many liquids (species) which are slowly changing into other liquids, sometimes meeting and dissolving or obliterating still others, and that the floor itself and the local climate above it are also gradually changing.
Geographical movements of species depend fundamentally upon random movements of individuals through successive generations. Several deer, for example, born at locality "A," move fifty miles to the northeast and produce offspring at "B." If this takes place within the established range of the species it has little significance, but if it represents penetration into new territory for that species, perhaps repeated a thousand times along the perimeter of the range, it may well have significance. Such movements, generation by generation, represent the way by which land animals spread into favorable environments. This procedure continues until stopped by contact with unfavorable conditions. But random movement into and accidental transportation into the unfavorable environment persists. Rarely accident permits passage through a barrier into favorable conditions.
THE PASSAGE OF NATURAL BARRIERS Barriers to the spread of animals on the continents are sometimes incomplete because not sufficiently extensive, or are completely efficacious against one species while scarcely hindering another. The functioning of a barrier may vary with the season of the year, or perhaps with sun-spot cycles. In the course of
162 MAMMALS OF THE PACIFIC WORLD geological time, as has been indicated, some barriers develop, and others disappear. Barriers have been spoken of as chiefly climatic or physiographical but others based upon disease, competition with other species, and physiology also occur. All are likely to be by-passed or penetrated sooner or later through temporarily favorable circumstances or because of permanent modification of the affected organisms.
Even when barriers seem to be complete, as at the edges of wide oceans, accident or the laws of chance eventually permit occasional species to cross to other land masses or to islands. Wherever land and sea meet, animals have repeatedly — not once or twice, but thousands of times during the passage of time — fallen into or been forced into the waters. Trees with climbing mammals clinging to their branches or living in hollows in their trunks fall into rivers and are carried out to sea. Ground-living creatures are sometimes accidentally taken for rides when banks cave in. The debris from such undermined banks and tangled masses of tree-trunks and water weeds sometimes forms extensive rafts. Bats, especially those that migrate, may be blown out to sea by storms. If the distance across a water barrier is short, some of the larger mammals may even swim across ; deer and pigs possibly do this. Natural swimmers, such as otters, can be carried by flooded rivers into the ocean but are likely to swim back to the nearby shore.
The natural rafts mentioned above have been one of the commonest means of transportation across seas. Trees and masses of vegetation become undermined by rivers during the rainy season and are carried out to sea. In the East Indies, where the currents between the innumerable islands are changeable, such rafts have good chances to be stranded quickly, but in the open ocean the arrival of one at a remote island must indeed be a rare occurrence. The mammals so transported are likely to be small, tree-living species, and the raft must be large enough and carry sufficient food to preserve life until the accidental landfall is made.
DISTKIBUTION OF MAMMALS 163 ESTABLISHMENT OF SPECIES ON ISLANDS Few of those surviving species are able to establish themselves permanently. The reason for this is that the new environment must not only be favorable and provide approximately the kind of climate and the kind of food the animal needs, but the newcomer must either arrive at the same time as one of its opposite sex or be a pregnant female in order that the process of colonization of the island may begin. It is not surprising then that the species of animals on all islands except those near mainlands are few and assorted. A few mammals have traveled as accidental "hitch-hikers" on native canoes and on the ships of explorers, traders, and whalers. Several species of rats have by this means spread to almost all parts of the world, including even the most isolated islands. Domestic animals that escape and go wild also add island species. The game animals of various lands have been introduced, especially into New Zealand where they flourish.
More species are found on large islands than small ones. This is not merely because of the larger size of those islands but especially because, offering a far wider choice of habitat, they are likely to meet the requirements of specialized kinds of mammals. Almost continuous invasion by random drift may be imagined on islands which are favorably situated. However, very small populations anywhere are liable to extermination, and island populations are usually small. Large mammals are especially liable to this risk. On a small island their numbers are so limited that disease, an unfavorable year, or hunting by human inhabitants may kill them all. The fauna of an island may thus have formerly included species that were unable to persist.
ARCHAIC MAMMALS PRESERVED ON ISLANDS Some of the ancient types of animals found upon deep-sea islands are the altered descendants of colonizers which must
164 MAMMALS OF THE PACIFIC WORLD have succeeded in establishing themselves when the world of mammals was comparatively young. The classical example of these is the pouch-bearing mammals of Australia. This land, even though called a "continent" because of its large size, is in reality a big, deep-sea island. Marsupials were present in America, Europe, and probably Asia at the time when they are believed to have reached Australia. Upon arrival they found a fertile and varied territory probably devoid of mammalian competitors. In their evolutionary experiments they produced pouched replicas of flying squirrels and tree squirrels, mice, lemurs, woodchucks, weasels, wolves, and moles. They failed to develop either a marsupial bat or a marsupial swimmer. During the late prehuman and possibly early human period of Australia, many giant marsupials, which apparently died out rather recently, still existed. Huge hippopotamus-like animals, a marsupial "lion" (which probably ate vegetation), and enormous kangaroos have left their bones in caves and in the mud of dried-up lakes in Australia. New Guinea, representing the tropical phase of the Australian fauna, has much the same type of animals.
Archaic animals of other islands are by contrast less arresting. It is unlikely that any other islands contain mammals whose time of arrival dates as far back as the marsupial movement. Random colonization of islands has been a continuous process, and many animals that are not "recent" in terms of existing continental forms can still be found on islands. Such are the peculiar small cattle of the Philippines and Celebes, the pig-like babirusa and the black ape of Celebes. Similarly, many kinds of rat-like animals on those islands and also on New Guinea and Australia no longer possess close relatives in Asia. BIOLOGICAL REGIONS The land surface of the globe is divided into five major zoogeographical regions, which include a number of subregions.
DISTRIBUTION OF MAMMALS 165 The northern region, comprising Europe, northern Asia, and North America, is called the Holarctic Region. The several land masses which project from it are respectively named the Neotropical or South American Region, the Ethiopian or African Region, and the Oriental or South Asiatic Region. The Fig. 69 — Fanned Regions of the World, showing probable greatest extent of land during the Ice Age (unshaded areas beyond coast lines) isolated Australian continent together with New Guinea forms the fifth, the Australian Region. This relationship of the faunal regions is indicated by a map (Fig. 69).
Some of these regions are divided into two or more subregions; for example, the Holarctic consists of the Nearctic or North American, and the Palearctic or Eurasian Subregions. The boundaries of these regions and subregions, unlike those of political areas, are not definite, for animals go wherever they
166 MAMMALS OF THE PACIFIC WORLD are not stopped by impassable barriers or prevented by other species.
The zoological regions or subregions which are related to the Pacific and East Indian area comprise parts of the cold Palearctic or Eurasian (northeastern Asia) and Nearctic (Alaska), a part of the warm Oriental (southeastern Asia), the Australian (Australia, New Guinea, and the Solomon Islands), and the Neotropical (limited to a few strays from Central and South America).
An important if anomalous area containing faunas of mixed archaic and "modern" mammals lies between the tropical Oriental and Australian Regions. This area, because its fauna is chiefly Asiatic in origin and contains few animals derived from Australia, can well be regarded as a subregion of the Oriental region. It includes four principal island groups, each of which is to some extent f aunistically distinct : the Philippines, Celebes, the Moluccas, and the Lesser Sunda Islands. It has been variously named the Indo-Australian Subregion, the Australo-Oriental Subregion, and Wallacea, named for one of its early explorers, Alfred Russell Wallace. New Guinea, with the Solomon Islands, is held to be a subregion of the Australian Region.
The "subregions" of the open Pacific, the Polynesian, Micronesian, and Melanesian Subregions, are ethnological rather than biological conceptions. Their limited faunas are derived from one or other of the biotic regions of the adjoining continents. FAUNAS OF THE PACIFIC ISLANDS Numerous different kinds of animals, as shown beyond in the geographical index, inhabit the various groups of islands of the Pacific. It has been pointed out that the ancestors of animals living on islands came originally from the continents during the course of an almost unimaginably long period of time. Those continental sources, which have contributed very unequally to
DISTRIBUTION OF MAMMALS 167 the population of the islands, are three: Asia, America, and Australia. The last, though technically an island, is so large and has been occupied by its specialized marsupial fauna for so long a period that it is conveniently treated as a third continent. Asia, particularly the East Indies, has greatly out-distanced Australia and the Americas in providing fauna! elements for the population of the islands. The Americas, on the contrary, have provided but a handful of land-dwelling species, most of these reaching islands rather near the American coast and only one of them going as far as Hawaii. A few representatives of the great evolutionary outburst of marsupials in Australia and New Guinea have reached Talaut, Celebes, the easternmost Lesser Sundas, and the Solomons.
The Aleutian Islands, included in the Holarctic Region, have a mammal fauna that decreases from the Alaskan end of the chain westward. The first island, Unimak, separated from the Alaska Peninsula by a shallow, narrow strait, has almost all of the mammals that occur on the treeless part of the peninsula. Some species have not yet been recorded, but this is probably because of inadequate collecting. The islands west of Umnak, which is itself one hundred and forty miles west of Unimak, may have no native land mammals. Caribou, bear, Arctic fox, wolf, wolverine, mink, weasel, the Arctic hare, marmot, ground squirrel, lemming mouse and lemmings, and a water shrew (Sorex) may occur on the landward islands ; these are all North American forms, but they have close relatives in northern Asia. The Kurile Islands similarly have a limited mammal fauna, chiefly species common to both Old and New Worlds. The arctic fox, red fox, brown bear, pygmy weasel, red-toothed shrew, varying hare, and a mouse-eared bat are such forms, but the wood mouse and the long-eared bat (Plecotus) occur only in the Old World Palearctic Subregion.
Japan, too, has a fauna consisting chiefly of local forms of northern continental groups — various bats, bears, the red fox, wolf, varying hare, pika, chipmunk, meadow mouse or vole, and
168 MAMMALS OF THE PACIFIC WORLD red-backed mouse. Some are related to the Eurasian types : certain insectivorous bats, badger, otter, pigs, moles, dormouse, wood mouse, and tree squirrel. Still other mammals have their affinities with the South Asiatic animals ; some of the bats, including a species of flying fox, the Asiatic black bear, the raccoon dog, sika deer, the serow (a goat-antelope), the water shrew, the Japanese monkey, and the flying squirrels are among these.
The Oriental Region is usually understood to include India, Burma, southern China, Indo-China, Siam, and the Malay Peninsula. It extends to the Luchus and Formosa, and from Sumatra east to the Moluccas. The islands between Lombok and Ceram form a debatable area — ^Wallacea or the Australo- Oriental Subregion — where palm civet, deer, and wild pigs of southern Asiatic types overlap with mammals of Australian relationships, like the phalangers. Only a single genus of the phalangers, however, extends west beyond the Moluccas ; no marsupials are known from any of the Lesser Sunda Islands other than Timor and nearby Wetar. The origins of the poor faunas of this debatable area are chiefly the Oriental Region, at least as far as mammals are concerned.
The Luchus are inhabited by few kinds of mammals. The indigenous Oriental mammals comprise flying fox bats, a species of leaf-nosed bat, and one of the horseshoe bats (other bats are Eurasian species). The dugong, a sea cow, reaches this far north. Pigs and the house shrew probably came in with man. The Luchu hare, found only in these islands, seems to be a relict form related to extinct rabbits.
Formosa is better populated with mammals than the Luchus. Almost all are South Asiatic forms. A few with northern affinities, such as the wood mouse, which occur on the island, are also found in the Chinese fauna. Not all of the species of mammals living on the nearby mainland are found in Formosa. In some cases these mainland species may have reached their present home only after the island was cut off ; in other cases the island