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* Report of a Lecture before the Royal Geographical Society, in Geogr. Mag., vol. iv, August, 1877, p. 221.
Geogr. Dist. Anim., vol. i, p. 37.
As is well known, and almost universally admitted, the animal and plant life of the Arctic lands is nearly everywhere the same, many of the species having a circumpolar range, while the genera are mainly, and the families almost entirely, the same throughout. Especially is this the case with mammals. To show how gradual is the change from almost absolute uniformity in the Arctic regions to the ultimate diversity met with in the intertropical latitudes it is only necessary to divide latitud. inally the so-called “Nearctic” and “Palæarctic” regions into several minor areas, and to tabulate and compare the genera found in each. Adopting as our first division the region approximately bounded southward by the isotherm of 360 F., and bence embracing the Arctic, SubArctic, and Cold Temperate lands of the northern hemisphere, we find that of the fifty-four commonly recognized genera of non-pelagic mammals occurring north of this boundary, five are subcosmopolitan; twenty seven, or more than one balf, are strictly circumpolar, being represented throughout the greater part of the region north of this boundary; that five more are found on both shores of the Atlantic, and that five others are common to both sbores of the Pacific. This leaves only twelve-less than one-fourth-that are peculiar to either the northern portion of North America or to the corresponding portion of the Old World, of which eight are restricted to America and four to the Europæo-Asiatic continent. These genera and their distribution are approximately shown in the subjoined table. Genera of mammals of the Arctic and Cold Temperate portions of the northern hemisphere (the
region north of the mean annual of 36° F.).
t Scotophilus of American authors, not of Dobson.
The above-given statistics show most clearly that the mammals of the northern third of the northern hemisphere present few generic or subgeneric forms that are peculiar to either North America or to the Europæo-Asiatic continent. In many cases, these are closely representative forms; in other cases, the peculiar genera extend but a short dis. tance into the region, being temperate forms rather than hyperboreal.
The close relativnship of the mammalian life of the northern lands, as compared with the diversity met with between that of the northern and southern portions of the two northern continents, is further shown by a tabulation of the genera met with in the region intervening between the cold-temperate and sub-tropical zones of life, the northern and southern boundaries of which may be considered respectively as the isotherms of 360 and 680 to 700 F. Rather more than one-balf of the above.enumerated genera extend also over a large portion of this more southern belt, and impart thereby a general similarity to the facies of the mammalian faune of the two regious. In addition to these, however, we find in North America thirty-one genera and seven subgenera that are not found much, if any, to the northward of the isotherm of 36° F., and about the same proportion of new generic and subgeneric types make their appearance in the corresponding region of the Old World. Turuing first to North America, we find that of these added forms one has so wide a distribution that it may be properly considered as subcos. mopolitan, being found in the corresponding region of the EuropæoAsiatic continent as well as far to the southward of the region under notice. One other occurs also in Eastern Asia and six more belong rather to Tropical America than to Temperate North America. Excluding these, leaves about thirty as strictly American and twenty-two that are almost wholly restricted to Temperate North America; there is, hence, twice as great a difference between the mammalian faunæ of the middle temperate region of North America and the colder portion of the same continent as there is between those of the colder parts of the two wortherm continents, or the northern portions of the so-called “ Nearctic" and “ Palæarctic Regions”. But we get in Temperate North America not only twenty-two generic and subgeneric forms peculiar to this region, but a differentiation of this region into three well-marked faunal areas, differing more from each other than do the boreal parts of the New World (“Nearctic Region") from the boreal parts of the Old World " Palæarctic Region”). While thirteen of the genera, or about one
third, have a general distribution throughout Temperate North America, there are four genera and ono subgenus peculiar to the so-called Eastern Province, five genera and one subgenus mainly restricted to the Middle Province, and fire genera and two subgenera almost wholly limited to the geographically much smaller Western Province. In addition to this, there are five other genera and one subgenus common to the greater part of the Middle and Western Provinces that are not found in the Eastern.* The genera that may be regarded as characterizing the middle temperate region of North America and their relative distribution is shown in the subjoined table.
Terrestrial genera and subgenera of Middle North America (between the mean annuals of 360
and 680 F.), not found in the Arctic and Cold Temperate latitudes.
(NOTE:-Subgepera are enclosed in parentheses.)
Of general distribution.
Limited to the Limited to the Limited to the Common to the Mid-
* Mr. Wallace, in his late work (Geogr. Dist. Anim., vol. i, p. 6), refers to the Rocky Mountains as forming a barrier to species, “almost all the mammals, birds, and insects" belonging to different species on the two sides of the Rocky Mountains. Nothing, so far as mammals and birds are concerned (and I am informed by good authorities that the same is true of insects), could well be further from the truth. Only in rare instances do the Rocky Mountains form such a barrier, the division between the Eastern and Middle Provinces being more than six hundred miles to the eastward of this range, while the boundary between the Middle and Western Provinces is formed by the Sierra Nevada chain. The same species, as a rule, range over the greater part of the great elevated interior plateau, of which the Rocky Mountains constitute the axis. So far as the distribution of both birds and mammals is concerned, the presence or absence of forests, and the accompanying diverse climatic conditions, have far more to do with the limitation of habitat than the commonly so-called “Rocky Mountain barrier". This is obviously due to the longitudinal direction of this supposed barrier, which, if trending in a latitudinal direction, would certainly form an impassable obstacle to very many species.
Between the warm-temperate belt we have been considering and the zone next to the southward-the subtropical--the faunal differences are far greater than between the warm-temperate and colder zones. Aside from the few subcosmopolitan genera still present, and the few essentially tropical genera that range northward into the warmer temperate zone, there is little in common to the mammalian fauna of these two regions. At or near this boundary (the isotherm of about 680 F.-say 680 to 700 F.) several strictly tropical families first make their appear. ance, and tropical genera begin largely to replace those of the colder region to the north ward.
In respect to the Europæo-Asiatic continent, we have already seen how small a proportion of the genera of mammals met with porth of the thirty-sixth isotherm are really peculiar to this region, the number being less than twelve per cent., the remainder being circumpolar. Pass. ing, however, to the warm-temperate division of this Europæo-Asiatic continent, or that portion between the isotherms of 360 and 680 to 700 F., and we meet with many genera not found to the northward. While many circumpolar genera still prevail, at least three-fourths of the whole number are bere first met with. A considerable proportion (about one-fifth) are properly southern or subtropical, and extend far to the southward of the warm-temperate zone. About one-half, howerer, are peculiar to this zone, and belong to groups (families of subfamilies) especially characteristic of the North Temperate Realm. In adopting the isotherm of 70°F. as its southern boundary, we include not only the Mediterranean Province (and hence Northern Africa), but all of Asia north of the great Himalayan chain, together with Northern China and the Persian Peninsula. Hence quite a number of such southern forms occur as Macacus, Herpestes, Genetta, Hyæna, llystrix, etc., that are more properly members of the intertropical fauna. Owing to the great extent of this region, we meet with many genera peculiar to special districts, giving a higher proportion of peculiar forms than is met with in the corresponding portion (but far more limited in area) of North America. Of about fifty genera met with here that do not occur to the northward, about one-fourth may be thrown out as more properly tropical, since they in most cases barely enter the southern border.
Of the remainder, fully one-half are restricted in their range wholly or almost wholly to this region, the rest extending far into or throughout the Old World tropics. There is thus more than thrice as great a difference between the mammalian fauna of the boreal parts of the Europæo-Asiatic continent and that of the warmer parts of the same con. tinent as between the fauna of the boreal parts of the Europæo-Asiatic continent and the corresponding region of North America. The differentiation is here again, as in North America, from the north southward, not through the rapid increase of land area and diversity of physical structure, but purely from climatic conditions,-through the multipli. cation of life in consequence of increase of temperature and means of