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to species. In the foregoing remarks I have had little to say respecting the range of species, and have tabulated merely genera and families. These tables clearly show that a large proportion of the mammalian genera and families of the northern hemisphere have a circumpolar range, the same genera and families occupying the Arctic and Sub-Arctic lands in both the Old World and the New, and that only a small per cent. of the whole number found here are peculiar to either of the porthern land-areas; that a large part of the genera and families met with in the temperate and warmer latitudes occur on the eastern continent as well as on the western; that again a considerable proportion of the genera and families met with in the warmer parts of the earth occur also both in the Old World and the New, while many others are well known to have been common to the two during the Tertiary period. It has been further shown that there is a greater diversity of life between contiguous climatic belts of the same continent than between corre. sponding belts of the two continents, especially north of the forty-fifth parallel of latitude, and that any marked faunal differentiation of the two continents begins only in the warm-temperate and subtropical latitudes. On each contivent, the arctic, tenperate, and tropical zones are each marked in their general facies respectively by corresponding phases of life. So obvious is this that we have in current use the expressions ** arctic life”, “temperate life”, and “tropical life”, in recognition of certain common features of resemblance by which each of these regions is distinguished as a region from the others. This is in accordance with a law I have termed the law 6 of differentiation from the north southward”,* or in accordance with increase of temperature and the condi. tions resulting therefrom favorable to increased abundance of life.
In this connection it may be well to recall certain general facts pre. viously referred to respecting the geographical relations of the lands of the northern hemisphere and their past history. Of first importance is their present close connection about the northern pole and their former still closer union at a comparatively recent date in their geological history; furthermore, that at this time of former, more intimate relationship, the climatic conditions of the globe were far more uniform than at present, a mild or warm-temperate climate prevailing where now are regions of perpetual ice, and that many groups of animals whose existing representatives are found now only in tropical or semitropical regions lived formerly along our present Arctic coasts. We have, hence, an easy explanation of the present distribution of such groups as Tapirs, Manatees, many genera of Bats, etc., in the tropics of the two hemispheres, on the wholly tenable assumption of a southward migration from a common wide-spread northern habitat, to say nothing of the numerous existing arctopolitan and semi-cosmopolitan genera. The former greater commu. nity of life in the northern hemisphere in preglacial times is further evinced by the wide spread occurrence there of the remains of Camels,
* Bull. Mus. Comp. Zoöl., vol. ii, p. 379.
Elephants, Mastodons, Rhinoceroses, and Horses, which, though extinct in America, have living representatives in the tropies of the so-called "Old World", to say nothing of the evidence afforded by the remains of still earlier types of arctopolitan range. The succeeding epochs of cold caused extensive migrations of some groups and the extinction of others; with the diverse climatic conditions subsequently characterizing high and low latitudes came the more pronounced differentiation of fauna, and the development, doubtless, of many new types adapted to the changed conditions of life-the development of boreal types from a warmtemperate or semi-tropical stock. The accepted theories respecting the modification of type with change in conditions of environment-changes necessarily due mainly to climatic influences-render it certain that if animals are so far under the control of circumstances dependent upon climate, and emphatically upon temperature, as to be either exterminated or greatly modified by them, the same influences must govern their geographical distribution.
Recent discoveries respecting the mammalia inhabiting North Amer. ica during the Tertiary period have shown that many of the leading types of mammals—including not only those above named, but also many others—Dow found only in the eastern hemisphere, originated in North America, and migrated thence to Asia, Europe, and even Africa, either as somewhat generalized types, or after they had nearly reached their present degree of differentiation; in short, so far as mammalian life is concerned, that America is the “Old World” from which the so-called “Old World” has been mainly peopled. The present genetic convergence of life about the northern pole seems to show that not only has there been here a comparatively free intercommunication, but that the mammalian life now existing there has lived there for a long period under similar conditions of environment; and that these conditions are unfavorable, in consequence of a comparatively low temperature, to rapid change of form or structure.
This is shown not only by the great diversity of life met with in the intertropical regions, as compared with the uniformity met with in the semi-frigid regions (equal areas being, of course, compared), but by the coincident occurrence of a simple, homogeneous arctic marine fauna, with the low temperature over the sea-floor far to the southward of where sucb forms occur in the warmer surface and shore-waters. The iutimate relation between temperature and the distribution of life is most forci. bly shown by the existence under the same parallel of latitude of diverse faunæ not only at different elevations above the sea on mountain.slopes, but at different depths beneath the surface of the ocean, where the several faunæ are characterized not only by the presence of different species, but by the prevalence of different genera, and even families. In fact, it is to me a matter of surprise that, with our present knowledge of the subject, any naturalist of note should assume that temperature has nothing to do with the circumscription of faunæ, or that any law based on it can have "little bearing on the great features of zoological geography--the limitation of groups of genera and families to certain areas".
II.-MAMMALIAN REGIONS OF THE GLOBE.
The influence of temperature as a limiting agent in the distribution of life, as well the “lar of the distribution of life in circumpolar zones”, was fully recognized by Humboldt nearly three-fourths of a century ago, and later, practically if not explicitly, by Ritter, De Candolle, Agassiz, Wagner, Forbes, Dava, Günther, Meyen, Middendorff, and inany other leading zoölogists and botanists. While this law must incontrovertibly underlie every philosophic scheme of lief-regions, the number of zones to be recognized, as well as their boundaries, must in a measure be open to diversity of opinion. Professor Dana, in 1852, recognized five primary zones for marine animals, namely, a torrid, a north and a south temperate, and a north and a south frigid. The torrid and temperate were subdivided, the first into three, the others each into five sub-zones, the two frigid being left undivided. Mr. A. Agassiz, in treating of the distribution of the Echini,* recognizes also five zones, a torrid, two temperate, and two frigid. These five primary zones prove to be applicable also to the mammalia, and even their subdivisions may be readily traced, but are rather too detailed for practical use. Owing to the irregular surface of the land areas, occasioned by elevated plateaus and mountain-chains, these zones of distribution have of course a less regular breadth and trend than they preserve over the oceans. Their boundaries, however, approximate to the courses of the isotherms, by certain of which they may be considered as in a general way limited.
In recognition of these zones, and also of the law of differentiation of life with the relative isolation of the principal land-areas, I proposed in a former paper (l. C., p. 380) a division of the land-areas into eight “Realıns", wamely: I, Arctic; II, North Temperate; III, American Tropical; IV, Indo-African; V, South American Temperate; VI, Afri. can Temperate; VII, Antarctic; VIII, Australian. A subdivision of most of these primary regions was provisionally suggested, but only the North American was treated with any degree of detail, and this mainly with reference to the birds, and more especially those of its castern portion. Subsequent study of the distribution of mammalian life over the globe bas led me to modify some of the views then ex. pressed, especially in relation to the divisions of the Australian Realm, and to unite the South African Temperate with the Indo-African, as a division of the latter, and also to recognize Madagascar and the Mascarene Islands as forming together an independent primary region, in accordance with the views of Sclater, Wallace, and others. Whether or not the Arctic and Antarctic Regions should stand as primary divi. sions seems also open to question. While perhaps tenable on general
* Iliustr. Cat. Mus. Comp. Zoöl., No. vii, 1872, pls. A-F. Bull. iv. No. 2- 3
grounds, they are hardly required for the elucidation of the distribution of the mammalia, since they must be mainly characterized negatively.
Beginning with the Arctic Region, we meet, as already shown, and as is almost universally admitted, a continuous homogeneous fauna, of considerable geographical area, but mainly characterized by what it lacks. Its southern boundary may be considered as the northern limit of forest vegetation. Continuing southward, few other than arctopoli. tan genera of mammals are met with north of the mean annual of 360 F. This considerable belt bence includes what may be termed the coldtemperate zone. The American and Europæo-Asiatic portions of this zone are only to a slight degree differentiated, while each is essentially homogeneous.
Below this, non-arctopolitan genera, or those restricted to more or less limited areas, become more frequent, and, indeed, form a considerable proportion of the genera represented. This belt occupies the remainder of the north-temperate zone, extending to about the mean isotherm of 700 F., and may be termed the warm-temperate zone. Unlike the cold-temperate zone, it is divisible on each continent into several well-marked minor regions, which are, however, more strongly differentiated, inter se, in the Old World than in the New.
The tropical zone embraces, of course, in its fullest extension, a much greater latitudinal breadth than the temperate, but its southern landborder is very irregular, its only considerable development south of the equator being in South America and Africa. It is also so much diver: sified in many parts by mountain-chains that subdivision into secondary zones seems less feasible than in the case with the north-temperate zone. A central torrid and a north and a south sub-torrid zones might, however, be readily made, but such a division has not been attempted in the present connection. A northern sub-torrid division may indeed be very conveniently recognized, extending from about the annual isotherm of 670 to that of about 74° F., and including a transitional region consisting of the extreme southern border of what has been above defined as the warm-temperate zone and the northern border of the tropical.
In like manner, the distribution of life seems to warrant the recogni. tion, in Africa and South America, of a corresponding transitional belt between the two torrid and the southern warm-temperate zones. Aside from these divisions, the Torrid Zone admits of others of a more practical or useful character. These become at once obvious, since they result from the position and configuration of its component land elements. The first is a primary separation into two “realms”, an American and an Indo-African. Each of these is again divisible into several minor portions or “provinces"; but the Indo-African admits also of division into two " regions”, an African and an Indian, which are divisions of secondary rank, each having several “ provinces”.
The South Temperate Zone has a very limited land-surface, consisting
of the southern third of South America, a small portion of Southern Africa, and the greater portion of Australia. Extra-tropical South Africa is all comprised within the Warm Temperate Zone, and is so small in area and so intimately related, both geographically and faunally, with Tropical Africa, that its formal séparation, while, perhaps, warranted in the abstract, is hardly practically necessary. Temperate South America is exceedingly irregular in its northern outline, owing to peculiarities of configuration, resulting from the presence of the great Andean Plateau, by means of which it extends along the western border of South America far northward of the southern tropic. Temperate Australia is clearly separable from the tropical portion of the Australian Realm. The South Temperate Zone hence consists of three comparatively small land-areas, widely separated from each other, and consequently, as would be supposed, have little in common.
The Antarctic Region has a very limited amount of land-surface, and the few species that compose its fauna are almost wholly either marine or pelagic. As previously stated, as a mammalian region it has little significance.
This hasty sketch shows that the differentiation of the land-surface of the earth into realms, regions, and minor divisions has relation pot only to climate, but to the divergence and isolation of the different principal land areas; that at the northward, where the lands converge, there is no partitioning in conformity with continental areas, the temperate and colder portions of the northern hemisphere all falling into a single primary division, and that only the southern half is susceptible of divisions of the second rank. Within the tropics, on the other hand, the lands of the eastern and western hemispheres fall at once into different primary regions, and one of these is again divisible into regions of second rank. Beyond the tropics, the land-surfaces are of small extent, widely separated, and faunally have almost nothing in common.
With these preliminary remarks, we may now pass to a detailed consideration of the several primary regions and their subdivisions.
I.-ARCTIC REALM. Whether or not an Arctic Region should be recoguized as a division of the first rank is a question not easy to satisfactorily answer. Naturalists who have made the distribution of animal life in the boreal regions a subject of special study very generally agree in the recogni. tion of a hyperboreal or circumpolar fauna, extendivg in some cases far southward over the Temperate Zone. The Arctic portion of this hyper. borean region has been frequently set off as a secondary division, or subregion,* and generally recognized as possessing many features not
* It forms Mr. Blyth's “Arctic Subregion” (Nature, vol. iii, p. 427, March 30, 1871), Mr. Brown's “Circumpolar" division (Proc. Zool. Soc., Lond., 1868, p. 337), and Dr. von Middendorfi's “ Zirkumpolar-Fauna” (Sibirische Reise, Bd. iv, p. 910, 1867). It also accords very nearly with Agassiz's " Arctic Realm” (Nott and Gliddor's Types of Mankind. 1854, p. 1x and map).