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subsistence. This is still more strikingly shown by a comparison of the fauna of the middle portion of the so-called “Palæarctic Region" with that of its southern border, at which point the truly tropical forms begin to appear. The genera of a zone, say two degrees in width, at these two points would be not only in large part different, but those of the southern belt would he far more numerous. Genera of mammals of the warm-temperate portions of the eastern hemi
sphere (between the isotherms of 360 and 680 to 700 F.), not occurring to
the northward of the 36th isotherm. * Macacus. † Moschus.
Rhinolophus. Vectogale. Felis. + Hydropotes. * Plecotus.
Spalax. * Genetta. . + Poëphagus. * Synotus.
Rhizomys. * Herpestes. * Addar.
Scotopbilus. + Siphneus.
* Nyctinomus. Cricetulus.
1 Scaptochirus. , | Alactaga. t Ælurus. + Saiga. + Scapton yr.
* Gerbillus. * Equus. + Pantholops. tanusorer.
* Dipus. Camelus. + Budorcas. + Mygale.
Muscardinus. t Dama, + Rupicapra. Urotricbus.
Eliomys. + Elaphodus. Nemorhædus. + Uropsilus.
* Hystrix. + Lophotragus. Capra.
A comparison of the families represented in different portions of the porthern bemisphere north of the isotherm of 700 F. brings into prominence some of the points already stated, without the confusion of detail incident to a comparison on the basis of genera, and gives also a more convenient standard for the next stage of comparison, namely, a comparison of the faudæ of the temperate zones with those of the tropical, as well as with the fauna of the two great land-areas of the northern hemisphere. Of thirty-three families of non-pelagicmammals found north of about the isotherm of 700 F. (680 to 700), thirteen have a nearly cosmopolitan distribution, and six others are common to both tbe Old World and the New, leaving fourteen, or about one-third, peculiar to either North America or to Europe and Asia. Three of these are essentially subtropicopolitan or tropicopolitan, having merely straggling representatives north of the 68th isotherm, and five others are represented each by only a single species. Seven of these fourteen families (four only according to many systematistst)are North American and seven European
* Occurring in southern portions only; chiefly tropical. + Peculiar to the region and mostly of restricted range.
I here admit to family rank Antilocapridæ, Zapodidæ, and Geomyidæ, the two former of which are treated by Mr. Wallace as subfamilies of subcosmopolitan families, while the other is not commonly recognized as distinct from Saccomyidæ. On the other hand, I refer the Cercolabide to the Hystricidæ.
and Asiatic. One or two others barely touch, or possibly overlap slightly, the above-given boundary. North of the isotherm of 360 F. not more than two or three families are met with that are not cosmopolitan, and two of these have each but a single species north of this line.
The following is a list of the families referred to above, with approxi. mate indications of their distribution.
Families of non-pelagic mammals occurring north of the mean annual of 700 F.
* Formerly occurring on the shores of the North Pacific only, but now extinct. + Tropical; one species only found north of 70th isotherm. Represonted by a single species.
In regard to the southern extension of these thirty-three families, thirteen range farinto, and most of them over, the greater part of Intertropical America, and eighteen far into, and most of them over, the greater part of the intertropical portion of the Old World.
In Intertropical America, only thirty families are represented. Of these, thirteen occur over much of Temperate North America, while eleven are subcosmopolitan, and the same number are peculiar to the region, while one-half of the whole do not range much beyond the northern tropic. Seven are semitropicopolitan, or occur also in the warmer parts of the Old World ; but of these, three are Chiroptera and another is marine. The approximate range of the families represented in Intertropical America is indicated in the annexed table.
* Five only are exclusively North American.
Families of non-pelagic mammals occurring in Intertropical America
(between the northern and sovthern isotherms of 70° F.).
Fifty families are represented in the intertropical portions of Asia and Africa. Of these nearly thirty do not range much beyond the Northern Tropic, of which about twenty-three are limited to this region. Of the thirty-two families occurring in the north-temperate zone (of which only six or seven are exclusively Europæo-Asiatic), nearly one-half range over most of the Indo-African tropics. The following is a list of the families represented in the Old World tropics, exclusive of those limited to Madagascar and the Australian Realm.
It thus appears that only about three-fifths as many families of mammals occur in the intertropical parts of the New World as in the corresponding parts of the Old World. The disproportion in the same direction in respect to genera and species is still greater. This is obviously due to the difference in size and configuration of the two areas. The Old World intertropical land surface is not only several times greater than the American (embracing thrice as great a breadth longitudinally), but is differentiated into one continental (Africa), two large peninsular (India and China) areas, and a group of large, highly differentiated islands (Malay Archipelago), while the intertropical region of America forms a single unindented region, with a single narrow isthmic prolongation. In the one case (America) we have a striking uniformity of mammalian life throughout, corresponding with the gen. eral uniformity of the climatic conditions characteristic of this area, contrasting with well-marked subdivisions in the other, and a much greater diversity of environing circumstances, originating geologically far back in the history of these several land-masses. As Mr. Wallace has remarked, "To those who accept the theory of development as worked out by Mr. Darwin, and the views as to the general permanence and immense antiquity of the great continents and oceans so ably de. veloped by Sir Charles Lyell, it ceases to be a matter of surprise that the tropics of Africa, Asia, and America should differ in their productions, but rather that they should have anything in common. Their similarity, not their diversity, is the fact that most frequently puzzles us."*
In the foregoing remarks, no reference has been made to Madagascar or to Australia, for the reason that they belong to distinct primary liferegions having little in common with the great Europæo-Asiatic landarea (of which Africa, on the other hand, is an inseparable appendage), which, with America, form the regions to wbich the discussion has thus far been intentionally limited. As will be more fully considered later, the intertropical Old World area is divisible into secondary regions, which for the present need not enter into the questions immediately at issue. These are, first, Does that portion of the northern hemisphere north of the northern subtropical zone admit of division into two primary life-regions, conforming in their boundaries to the configuration of the two great northern land-areas? And, secondly, Iu accordance with what principle does the life of the northern hemisphere become differentiated from the homogeneity characteristic of the northern regions
* Geogr. Dist. Anim., vol. i, p. 51.
to the great diversity met with under tropical latitudes? The fundamental question which underlies the whole subject is, Is, or is not, the life of the globe distributed in circumpolar zones? The second is, How and under what influences does it become differentiated ?
To the first of these questions, I ventured some six years since,* to give an affirmative answer, in accordance not only with the views of numerous high authorities on the subject of the geographical distribution of life, but with what seemed to me to be incontrovertibly the facts in the case. While this view has since received the support of other high authorities, it has been altogether ignored by the advocates of Dr. Sclater's division of the earth's surface. Mr. Wallace, who faithfully reflects the views of the Sclaterian school, in referring to this subject says :" Mr. Allen's system of “realms' founded on climatic zones ... calls for a few remarks. The author continually refers to the law of the distribution of life in circumpolar zones', as if it were one generally accepted and that admits of no dispute. But this supposed 'law' only applies to the smallest details of distribution-to the range and increas. ing or decreasing numbers of species as we pass from north to south, or the reverse ; while it has little bearing on the great features of zoologi. cal geography—the limitation of groups of genera and families to certain areas. It is analogous to the law of adaptation' in the organization of animals, by wbich members of various groups are suited for an aerial, an aquatic, a desert, or an arboreal life; are herbivorous, carniv. orous, or insectivorous; are fitted to live underground, or in fresh waters, or on polar ice. It was once thought that these adaptive peculiarities were suitable foundations for a classification,—that whales were fishes, and bats birds; and even to this day there are naturalists who cannot recognize the essential diversity of structure in such groups as swifts and swallows, sun-birds and humming-birds, under the superficial disguise caused by adaptation to a similar mode of life. The application of Mr. Allen's principle leads to equally erroneous results, as may be well seen by considering his separation of the southern third of Australia' to unite it with New Zealand as one of his secondary zoological divisions.”+
Leaving Mr. Wallace's last-quoted objection for notice in another connection (see a foot-note beyond, under the sub-heading “Australian Realm”), I unblushingly claim, in answer to the main point, that the geographical distribution of life is by necessity in accordance with a “law of adaptation”, namely, of climatic adaptation ; that such a law is legitimate in this connection, and that the reference to the superficial disguise” adapting essentially widely different organisms to similar modes of life is wholly irrelevant to the point at issue,-a comparison of things that are in any true sense incomparable; furthermore, that the law of distribution of life in circumpolar zones" does apply as well in a.gen. eral sense as to details—6 to groups of genera and families” as well as
* Bull. Mus. Comp. Zoöl., vol. ii, p. 376, 1871. Geogr. Dist. Anim., vol. i, p. 67.