To every one who has paid the slightest attention to the subject, it is a familiar fact that different parts of the earth have different animals; school-children learn from their geographies that kangaroos are found in Australia, the Hippopotamus in Africa, the Tiger in southern Asia, armadillos and llamas in South America. These examples are all taken from distant lands, yet the zoölogical difference between two given land-areas is by no means proportional to the distance between them. An Englishman landing in Japan finds himself surrounded by animals and plants very like and often identical with those which he left at home, while the narrow Strait of Lombok, east of Java, separates two profoundly different regions. In crossing Mexico from east to west, the traveller meets very different animals in closely adjacent areas; and, at first sight, the arrangement of animals appears to be so capricious as to admit of no formulation in general laws.

In pre-Darwinian times, when it was the almost universal belief that each species had been separately created and was exactly fitted to the region which it inhabits, no explanation of the geographical arrangement of animals was possible, but the acceptance of the theory of evolution demanded that such an explanation should be found. A failure to devise any rational and satisfactory account of the geography of animal life would be a fatal weakness in the evolutionary theory, hence the facts of distribution were subjected to a renewed and searching analysis as one of the best means of critically testing the new doctrine. Not that the subject had received no attention before the publication of Darwin’s book; on the contrary, it had attracted much interest as a study of facts, and this study was one of the principal avenues by which Darwin approached his great generalization. In his autobiographical fragment he tells us: “I had been deeply impressed by discovering in the Pampean formation great fossil animals covered with armour like that on the existing armadillos; secondly, by the manner in which closely allied animals replace one another in proceeding southward over the Continent; and third, by the South American character of most of the productions of the Galapagos archipelago and more especially by the manner in which they differ slightly in each island of the group.”

Obviously, before attempting to explain the facts of the geographical distribution of mammals, we must first ascertain what those facts are. The following brief sketch of the terms used in describing geographical arrangement is summarized from Mr. Wallace’s “Island Life.”

Though with fluctuating boundaries and subject to slow secular changes, a mammalian species is limited to a fairly definite area, which may be of immense or very restricted extent, and throughout which it may be found in greater or less abundance. Many species, however, are not distributed continuously over the areas which they inhabit, but occur only in suitable stations adapted to their habits and mode of life. Thus, some will be found only where there are trees, others in the neighbourhood of water, others only on open plains, etc. A specific area is then the whole extent of country within which the species may be found, while the stations are the limited districts contained in the area which are exactly suited to the habits of the species in question; these stations may be hundreds of miles apart, as in the case of mountain-tops, or they may be close together. A marsh-living species, for example, will occur in all the marshes throughout its area, whether these be many or few, near together or widely scattered; for such a species marshes are its stations.

Generic areas differ in character according as the genus is large, that is, comprising many species, or small and having but few species, or, it may be, a single one. The species, as a rule, occupy each its own area, and the areas may be entirely distinct, or they may be contiguous and more or less extensively overlapping, though it seldom happens that two or more species of the same genus inhabit exactly the same area. Often some physical feature, such as a range of high mountains, a great river, the edge of a forest, plain or desert, exactly defines the limits of species of the same genus. The Amazon, for example, acts as such a boundary to many species. It was to this change of related species from one area to another that Darwin referred in the passage quoted above, saying that he had been deeply impressed “by the manner in which closely allied animals replace one another in proceeding southward over the Continent [i.e. South America].” On the other hand, the overlapping of areas may be very extensive, and one species of great range may cover the whole area of another and much more besides.

A remarkable example of the widely separated areas of species belonging to the same genus is that of the tapirs. Of this genus there are two or three species in Central and South America and one inhabiting the Malay Peninsula and Borneo, almost as wide a separation as the size of the earth permits. Discontinuous distribution of this character can be explained in terms of the evolutionary theory only in one of two ways. Either (1) the American and Asiatic species developed independently of one another from different ancestors, or (2) the regions intervening between these widely separated areas once formed a continuous land, occupied by species of the genus which have become extinct. From all that we know concerning the operation of the evolutionary process, the first alternative may be set aside as altogether improbable, and, even had we no information concerning the history of the tapirs and their former distribution, the second explanation would be chosen as incomparably the more likely. As a matter of fact, we have definite knowledge that tapirs once ranged all over Europe and North America and doubtless over northern Asia, as well, and, further, that North America was joined to Asia by a land occupying the place of the shallow Bering Sea, at a time when the tapirs were able to take advantage of this means of passing from one continent to the other. Such appears to be the invariable explanation of discontinuous distribution, though we may not always be able to give so clear a proof of it.

The genera of a family are distributed in much the same fashion as the species of a genus, but, as a rule, much more widely. While no genus of terrestrial mammals is cosmopolitan (i.e. universally distributed), at least as genera are defined and limited by most modern systematists, certain families are represented in every continent. If the extremely peculiar and isolated Australian continent be excepted, the number of such cosmopolitan families is considerable and wide separation between the genera is frequent. Of the camel family, for instance, one genus, that of the true Camel (Camelus), is confined to the northern hemisphere and the Old World, the other (Lama), comprising the Llama, Guanaco, etc., is found only in the southern hemisphere and the New World. Less extreme instances of the discontinuous distribution of a family are common enough.

The principles of distribution are the same when applied to families and orders. Most of the mammalian orders are very widely distributed and many are cosmopolitan, except for Australia, though some are confined to one or two continents. The monotremes are limited to Australia and Tasmania, the marsupials to Australia and the Americas, the edentates to the latter, the elephants and hyracoids to Africa and Asia. Carnivores and rodents, on the contrary, are found in every continent, even Australia.

We have next to inquire what is the nature of the obstacles or barriers that prevent the indefinite spread of terrestrial mammals, so that the mammalian fauna of the whole earth, and even of a single continent, is not uniform, but highly variegated. The rate of multiplication of animals is so rapid that, under normal conditions, the animal population is always pressing hard upon the means of subsistence and every species that is increasing in numbers must constantly extend its range in search of food. Every species would increase indefinitely, if there were no countervailing checks. Were all the young to survive and breed in their turn, “even large and slow-breeding mammals, which only have one at a birth, but continue to breed from eight to ten successive years, may increase from a single pair to 10,000,000 in forty years” (Wallace). Evidently, a species must spread from its place of origin until stopped by insuperable obstacles, the most obvious of which are wide seas. A few land mammals are not only excellent swimmers, but will cross straits without hesitation, as the Guanaco has been seen to swim the Straits of Magellan; for the great majority, however, a very few miles of sea form an impassable barrier. As was shown above, a broad or deep river is sufficient to limit many species, as the Santa Cruz River in Patagonia marks the southern boundary of the armadillos.

Important geographical changes, such as the joining of lands that before were separate, or the dividing of continuous lands by transgressions and incursions of the sea, must necessarily have a profound effect upon the distribution of land mammals. Separated land-areas, however similar may have been their faunas at the time of separation, will, through the operation of the divergent evolutionary process, grow more unlike in proportion to the length of time that the separation continues. Regions which have been severed within a short time (in the geological sense of a short time) are zoölogically very similar or even identical, while those that have long been isolated are correspondingly peculiar. Attention has already been called, in another connection, to the contrasted cases of such great continental islands as Great Britain, Java, Sumatra, etc., on the one hand, and Australia, on the other. The continental islands, which have but lately been detached from the neighbouring main lands, are hardly more peculiar zoölogically than equal areas of the adjoining continents, while the long-continued isolation of Australia has made it the most peculiar region of the earth. Climatic changes, which, as we saw in Chapter I, have indubitably taken place many times, have also had a great effect in shifting the distribution of mammals, which in its present form is the outcome of a very long series of geographical and climatic changes, on the one hand, and of adaptive changes in the animals themselves, on the other.

Of almost equal importance as a barrier is climate and especially temperature. Not that similar climates can produce similar forms in separate areas. Regions of almost exactly similar climate in Australia, Africa and South America have totally different faunas, but, within continuous land-areas, the most effective of barriers is temperature. This acts differently in the case of limiting the northward spread of southern forms and the southward spread of northern species. Dr. Merriam’s long study of this problem has led him to the conclusion that southern species are bounded on the north by the temperature of the breeding season, in which the total quantity of heat must reach a certain minimum, while “animals and plants are restricted in southward distribution by the mean temperature of a brief period covering the hottest part of the year.” On the Pacific coast there is a remarkable mingling in the same areas of species which, east of the high mountains, are distributed in sharply separated zones. This is explained by the mild and equable climate of the coastal belt, where the hottest season of the year does not reach the limiting maximum for the northern species, while the total quantity of heat in the breeding season is sufficient to enable southern species to thrive and maintain themselves.

Dr. Merriam thus sums up the effects of climatic factors upon distribution: “Humidity and other secondary causes determine the presence or absence of particular species in particular localities within their appropriate zones, but temperature pre-determines the possibilities of distribution; it fixes the limits beyond which species cannot pass.” “Concurrently with these changes in vegetation from the south northward occur equally marked differences in the mammals, birds, reptiles, and insects. Among mammals the tapirs, monkeys, armadillos, nasuas, peccaries, and opossums of Central America and Mexico are replaced to the northward by wood-rats, marmots, chipmunks, foxes, rabbits, short-tailed field-mice of several genera, shrews, wild-cats, lynxes, short-tailed porcupines, elk, moose, reindeer, sables, fishers, wolverines, lemmings, musk-oxen, and polar bears.”

Dr. J. A. Allen has reached closely similar conclusions. “Of strictly climatic influences, temperature is by far the most important, although moisture plays an influential part. Where a low temperature prevails life, both animal and vegetable, is represented by comparatively few forms; under a high temperature it is characterized by great diversity and luxuriance. Within the Arctic Circle the species of both animals and plants are not only few, but they are widely distributed, being for the most part everywhere the same. Under the tropics they are a hundred fold more numerous and of comparatively restricted distribution.” “The influence of temperature is perhaps most strikingly displayed in the distribution of life upon the slopes of a high mountain, especially if situated near the tropics. While its base may be clothed with palms and luxuriant tropical vegetation, its summit may be snow-capped and barren.... The animal life becomes likewise correspondingly changed, tropical forms of mammals, birds, and insects of the lower slopes gradually giving place to such as are characteristic of arctic latitudes.” “The effect of humidity upon plant life is thus obvious, but it is equally potent, though less evident, upon animal life. Many animals ... are so fitted for a forest life, as regards both food and shelter, that their very existence depends upon such surroundings.... Thus moisture alone may determine the character of life over extensive regions.”

While climate is thus the most important of the barriers which determine distribution in continuous land-areas, the absence of any particular species from a given region is no proof that the climate is unsuitable to that species. This is sufficiently shown by the manner in which animals introduced into a new country often run wild and multiply to an incredible extent, as the rabbits have done in Australia, the Mongoose in Jamaica, horses on our western plains, horses and cattle on the Pampas of Argentina, etc.

Topographical features, such as great mountain-ranges and plateaus, also limit many species, not only by the difficulty of crossing them, but also by the effect which they have upon temperature and moisture. For this reason long ranges of mountains and table-lands may carry a northern fauna very far to the south of its ordinary range, as do the mountain-systems of North America in a very conspicuous manner. The great Mexican plateau is zoölogically a part of North America, while the low coastal lands as far as southeastern Texas have Central American affinities.

A different kind of obstacle to the spread of a species into a new area may be the pre-occupation of that area by another species. The pre-occupier may be one that plays so similar a part in the economy of nature as to leave no opportunity for the newcomer to establish itself. On the other hand, the obstructing form may be an active enemy and of a totally different character from the intruder, as in the case of the Tse-tse Fly in parts of Africa. The bite of the fly is fatal to horses and oxen, so that these mammals are unable to enter the fly-infested regions. Many times in the course of the Tertiary period various mammals reached North America from the south or from the Old World, which were unable to gain a permanent foothold and speedily died out. At this distance of time it is seldom, if ever, possible to explain why a species which succeeded in reaching this continent could not maintain itself, though the most probable assumption is that the forms already in possession of the land were an insuperable obstacle to the intruders.

The rate of dispersal of a species into new areas may be fast or slow, according as the conditions are more or less favourable. Newly introduced insect-pests, like the Gypsy and the Brown-tailed Moths in New England, often spread with portentous rapidity; and introduced mammals have frequently taken possession of vast areas in a surprisingly short time. One of the most remarkable of these cases is cited by Darwin. “In the time of Sarmiento (1580) these Indians had bows and arrows, now long since disused; they then also possessed some horses. This is a very curious fact, showing the extraordinarily rapid multiplication of horses in South America. The horse was first landed at Buenos Ayres in 1537 and the colony being then for a time deserted, the horse ran wild; in 1580, only forty-three years afterwards, we hear of them at the Strait of Magellan!” (“Voyage of a Naturalist,” pp. 232-233.) In this example, something must be allowed for human agency, but even so, it is very surprising.

In the case of lands newly raised above the sea and connecting formerly separated areas, it is necessary that they should first be taken possession of by vegetation, before they can become passable by animals, for the migration of mammals from continent to continent is an entirely distinct phenomenon from the annual migration of birds. The latter, though a fact familiar to every one, is an unexplained mystery, and it is somewhat unfortunate that the same term should be used for the completely different process of the spread of mammals into newly opened land. This spread is purely unconscious and is due to the pressure of increasing numbers upon the means of subsistence, each new generation ranging farther and farther from the original home of the species and continuing so to extend until some insuperable obstacle is encountered. When a sea-barrier is removed by upheaval and the newly formed land rendered habitable for mammals through the invasion of plants, the interrupted process is resumed and an interchange of species between the areas thus connected is brought about. The interchange is, however, always an incomplete one, certain forms not being able so to extend their range, because of climatic differences, pre-occupation or some such barrier.

It is customary to give a graphic expression to the facts of animal distribution by dividing the land surface of the earth into districts which are characterized by their faunas. It is not possible to construct a geographical scheme which will be equally satisfactory for all classes of animals, because the geological date of most rapid development and diffusion was so different in the various classes. The geographical and climatic conditions which favoured a particular geographical arrangement of one class had been so completely altered that the class coming in later could not attain a similar distribution. For this reason, land mammals are chosen as affording the best criteria; their adaptability is such that they are found all over the earth, their dispersal is primarily dependent upon the arrangement and connections of the continental land-masses, modified by the topographical and climatic conditions, and they, with the birds, are the latest of the vertebrate classes to assume a dominating importance. Their history is the most fully known and falls within the best understood portion of the earth’s history, making it possible to follow their migrations with a precision which is seldom feasible for the other classes of animals, and thus to correlate the successive physical and organic changes. A particularly great advantage which mammals possess for this purpose is that the mutual relationships of the various kinds are better understood than in the case of most other groups of animals. It is true that we shall find a great many unsolved problems, upon which the most divergent opinions are held, but the main outlines of the scheme are quite generally agreed upon.

Many plans for the zoölogical division of the continental areas have been proposed by various writers on the subject, some differing very radically from others. It would be useless and tedious to review even the more important of the many proposals and suggestions which have been made in the last half-century; and we may, with advantage, adopt an eclectic scheme which has been slowly reached by successive approximations to a satisfactory arrangement.

Just as in political geography it is found necessary to recognize divisions of different rank and scope, like nation, state, county, township, the facts of zoölogical geography require divisions of different orders of importance. Thus, in descending order, the terms realm, region, subregion, province, etc. are commonly employed, but unfortunately they are often used loosely and even interchangeably; yet it is desirable to attach a more or less precise significance to each and more terms are needed for an accurate expression of the many complex facts.

The extreme zoölogical peculiarity of Australia is recognized by making that continent and its adjoining islands one of the great primary divisions, of which the other includes all the rest of the world; the former is characterized by its almost exclusively marsupial fauna, while the other continents are inhabited by the Monodelphia or placental mammals. Aside from Australia, by far the most isolated and peculiar region of the earth is South America, and this fact is expressed by constituting it into a realm, or division of the second order, and to this realm is given the name Neogæa. The remaining continents, North America, Europe, Asia and Africa, make up the other realm, Arctogæa, in which there is an unmistakable general likeness among the mammals. The three continents of the Old World form a vast, connected land-mass, and the final separation of North America from this great complex is an event of geologically recent date. For reasons that will be made clear in the course of the history, the junction of the two Americas has had comparatively little effect upon the zoölogy of the northern continent, except in its tropical portion. It is obvious from a glance at the map, that the great zoölogical divisions are of very unequal size, but the arrangement is made on the basis of degrees of difference in the mammalian faunas. These degrees of difference are, in turn, an expression of length of separation or of the difficulty of communication between connected lands.

The following table gives the major divisions of the earth apart from Australia:

North America, as is expressed by this scheme, is zoölogically composite; the northern half, including nearly all of Canada, belongs to the vast Holarctic Region, which also comprises Europe, Africa north of the Sahara and Asia north of the Himalaya Mountains. The remainder of the continent, exclusive of the Mexican coastal lowlands, is set off as the Sonoran Region. Inasmuch as we have here to do with broadly continuous land-areas, not demarcated by great physical features, and as the genera and species of mammals differ greatly in regard to their ability to withstand a wide range of climatic variations, it is not to be expected that the boundaries between the regions which make up North America should be very sharply drawn. It is not surprising, therefore, to find a transition zone, extending all across the continent, in which the Holarctic and Sonoran faunas mingle, or that Central America should, in considerable measure, be transitional to South America, though zoölogically a part of the latter.

Dr. Merriam’s arrangement, which deals only with North America without reference to the Old World, divides the land into a series of transcontinental zones, which he calls the Arctic, Boreal, Upper and Lower Sonoran and Tropical. These zones have very irregular and sinuous boundaries, which follow lines of equal temperature (isothermal lines) during the breeding season, May, June and July, the tortuous boundaries being conditioned by topographical features, which deflect the isothermal lines.

The Arctic zone is part of a circumpolar area, which is very much the same in North America, Asia and Europe; and in any of these continents the fauna differs much more from that of the contiguous zone to the south than from the Arctic fauna of another continent. There are some local differences, but the characteristic mammals of this Arctic zone are the Polar Bear, Arctic Fox, Musk Ox, Barren-ground Caribou, Lemming, Arctic Hare, and a marmot. Most, if not all, of these forms are of Old World origin.

The American portion of the great Holarctic region is called by Mr. Lydekker, who uses Wallace’s term, the “Canadian subregion,” and by Dr. Merriam the “Boreal region.” Not that there is any difference of principle involved in this varying nomenclature, for Dr. Merriam says: “It so happens that the Boreal element in America resembles that of Eurasia so closely that in the judgment of many eminent authorities the two constitute a single primary region—a view in which I heartily concur.” The Canadian or Boreal subregion of the Holarctic is the great belt of coniferous forest, which extends obliquely across North America from Alaska to New England; its frontier with the Arctic zone is the northern limit of trees and it is divided from the Transition zone approximately by the line of latitude 45° N., though with a sinuous course, and it is carried far to the south by the wooded heights of the Appalachian, Rocky and Sierra Nevada Mountains, and along the Pacific coast, the mixed character of which has already been explained; it extends almost to San Francisco. The subregion is further divisible into northern and southern belts, called the Hudsonian and Canadian faunas, the limit between them approximately following the isothermal line of 57° F. The mammals of this subregion are largely of Old World origin, many of them coming in with the great immigrations of the Pliocene and Pleistocene epochs; but there are also native American elements and even one genus of South American origin, the Short-tailed or Canada Porcupine (Erethizon).

In considering the mammals of this subregion, it should be remembered that they are not uniformly distributed throughout even one subdivision, but in a scattering way and in accordance with their habits and stations, and also in accordance with a gradual change to the south, following the changing temperature. The Muskrat will not be found far from water or the Porcupine from woods. Especially characteristic of the Canadian subregion are the Old World types of deer, none of which range farther south than the Transition zone. The Wapiti, erroneously called the Elk (Cervus canadensis), is very closely allied to the European Stag (C. elaphus) and still more closely to the Stag of the Thian Shan in Central Asia (C. eustephanus). So great is the resemblance, that some naturalists would refer all three forms to a single species. The Moose (Alce americanus), which should be called the Elk, is so near to the Scandinavian Elk (A. machlis) that it is hardly distinguishable as a separate species, and the Woodland Caribou (Rangifer caribou) is the American representative of the Lapland Reindeer (R. tarandus). The so-called Rocky Mountain Goat (Oreamnos montanus), a peculiar and aberrant form of the Chamois subfamily of the Antelopes, is confined to the subregion. The Mountain Sheep (Ovis montana, O. dalli) are represented by three or four species, one of which extends into the Sonoran region, as does also the Bison, wrongly called Buffalo (Bison bison), which is nearly allied to the European B. bonasus. In Cæsar’s time the European Bison (German, Wisent) ranged through Germany and is described in his account of the Hercynian Forest; but the advance of civilization has almost exterminated it, only a few small herds being maintained by the most rigid protection in Russia and in the Carpathian Mountains. Of the Carnivora, the weasels, martens, Fisher, Mink and Ermine are Boreal, as are the Wolverene (Gulo) and the Grey Wolf (Canis), the three last-named extending also into the Arctic zone. Essentially Boreal, though reaching and entering the Sonoran, are the bears (Ursus), the red foxes (Vulpes), the otters (Lutra) and the Old World shrews (Sorex), while the Star-nose Mole (Condylura) and the mole-shrews (Urotrichus) do not extend south of the Transition zone. Probable intruders from the south into the Boreal subregion are the pumas, or “mountain lions,” which just enter the subregion, the Canada Lynx (Lynx rufus) and one species of skunks (Mephitis), the Raccoon (Procyon lotor), Badger (Taxidea americana) and the American deer (Odocoileus). A large number of rodents are characteristically Boreal: marmots, or woodchucks (Marmota), the Sewellel (Aplodontia rufa), lemmings (Myodes), Jumping Mouse (Zapus), the Canada Porcupine (Erethizon dorsatus) and the pikas, “tailless or whistling hares” (Ochotona). Boreal rodents that enter the Sonoran are the chipmunks (Tamias), beavers (Castor), meadow-mice (Microtus), the Muskrat (Fiber zibethicus). The white-footed mice (Sitomys) and the wood-rats (Neotoma) are southern rodents that reach or enter the Boreal.

Between the Boreal subregion and the Sonoran region is the Transition zone, which follows all the complex windings of the boundary lines. It covers most of New England, New York, Pennsylvania and southern Ontario; passing through southern Michigan and Wisconsin, it bends northward over Minnesota and covers most of North Dakota, Manitoba and the plains of the Saskatchewan, then turns abruptly southward and includes eastern Montana and parts of South Dakota and Nebraska. Crossing Wyoming, it follows around the northern edge of the Great Basin to the plains of the Columbia. The three great mountain-systems carry the zone far to the south and arms of it extend along the Appalachians to northern Georgia, along the Rockies to New Mexico, and it follows the Sierras to southern California. “The Transition zone, as its name indicates, is a zone of overlapping Boreal and Sonoran types. Many Boreal genera and species here reach the extreme southern limits of their distribution and many Sonoran genera and species their northern limits. But a single mammalian genus (Synaptomys) [one of the field mice] is restricted to the Transition zone.... A number of species, however, seem to be nearly or quite confined to this zone” (Merriam).

The most characteristic portion of North America, zoölogically speaking, is the Sonoran region of Dr. Merriam, the Warm Temperate of Dr. Allen. It crosses the continent from ocean to ocean, its northern boundary following for most of the way the 43d parallel of latitude, but over the Great Plains and Great Basin, on each side of the Rocky Mountains and the high plateaus, it extends to lat. 48°. On the south, it takes in the greater part of Mexico, covering all of the table-land of that country, the lowlands of which belong to the South American or Neotropical region. The Sonoran is invaded from the north by the long branches from the Boreal and Transition zones, which follow the three great mountain-systems in the manner already explained, and the Mexican plateau permits the similar invasion of Neotropical territory by the Sonoran fauna. Characteristic Sonoran genera, none of which extend into the Boreal, are the opossums (Didelphis), in the southern part a peccary (Tagassu) or “Wild Texas Pig,” representative of a family of swine quite different from the true pigs of the Old World, and an armadillo (Tatu). A very isolated form is the Prong-horned Antelope (Antilocapra americana); there are several species of the typically American deer (Odocoileus) which differ in important respects from those of the eastern hemisphere, and the Bison was very abundant until exterminated by Man. Bison, antelope and deer also reach or extend into the Boreal zone, but the former, or Wood Bison, is probably a different species from the plains animal.

The grey foxes (Urocyon), Coyote (Canis latrans), large Timber Wolf (Canis occidentalis), the Caxomistle (Bassariscus), the Coati (Nasua), Raccoon (Procyon), Badger (Taxidea), three genera of skunks, pumas, several species of lynx and some bears (Ursus) represent the Carnivora, though one species each of raccoon, skunk, badger, puma and lynx range into the Boreal. The American types of shrews (Blarina) and moles (Scalops and Scapanus) are characteristic of the Sonoran, though partially shared with the Boreal. A great many peculiar rodents inhabit the Sonoran; cotton-rats (Sigmodon), pocket-gophers (Geomys, etc.), several genera of the beautiful little kangaroo-rats (Dipodomys, etc.); while the prairie-dogs (Cynomys), the white-footed mice (Sitomys), wood-rats (Neotoma) and one genus of pocket-gophers (Thomomys) are chiefly Sonoran, but have Boreal representatives. The flying squirrels (Sciuropterus), true squirrels (Sciurus), ground-squirrels (Spermophilus), rabbits (Lepus), wolves (Canis) and otters (Lutra) have a very wide range through both the Boreal and Sonoran, but have many more species in the latter region.

The Sonoran region may be divided into the upper and lower Sonoran zones, which are demarcated by temperature and are of transcontinental extent. Each of these zones may, in turn, be subdivided into arid and humid provinces, but our purpose does not necessitate entering into such refinements.