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MALIA, CONSIDERED IN RELATION TO THE PRINCIPAL
BY JOEL ASAPH ALLEN.
I.--DISTRIBUTION OF MAMMALIAN LIFE IN THE NORTH. ERN HEMISPHERE, CONSIDERED IN RELATION TO LAWS OF GEOGRAPHICAL DISTRIBUTION.
When, in 1871, I published* a few preliminary remarks concerning the general subject of geographical zoology, it was my intention soon to present more fully the facts whereon were based the few general priuci. ples then stated. In this paper I claimed, in accordance with the views of Humboldt, Wagner, Dana, Agassiz, De Candolle, aud others, that life is distributed in circumpolar zones, which conform with the climatic zones, though not always with the parallels of the geographer. Subsequent study of the subject has coufirmed the convictions then expressed. These are directly antagonistic to the scheme of division of the earth's surface into the life-regions proposed by Dr. Sclater in 1857, based on the distribution oł birds, and since so generally adopted. Their wide acceptation, it seems to me, bas resulted simply from the fact that so few have taken the trouble to sift the facts bearing upon the subject, or to carefully examine the basis on which Dr. Sclater's divisions are founded. The recent appearance of Mr. Wallace's laborious and in many respects excellent and praiseworthy workt has now rendered a critical presentation of the subject more necessary than be. fore, since, instead of seeking in the facts of geograpbical zoölogy & basis for a natural scheme of division, he has unhesitatingly accepted Dr. Sclater's ontological regions and marshalled his facts and arranged his work wholly in conformity with this, as I shall presently attempt to show, grossly misleading scheme. The source of error, as I hope to make evident, lies in method of treatment. Assuming apparently that the larger or continental land areas are necessarily coincident with natural ontological regions, divisions of the earth's surface wholly incompara
* On the Geographical Distribution of the Birds of Eastern North America, with special reference to the Number and Circumscription of the Ornithological Fauna. < Bull. Mus. Comp. Zoöl., vol. ii, No. 3, pp. 375-450. April, 1871.
The Geographical Distribution of Animals. With a Study of Living and Extinct Fauvas as Elucidating the Past Changes of the Earth's Surface. By Alfred Russel Wallace. Two vols. 80. With maps and illustrations. London, 1876. Bull. iv. No. 2- 1
ble have been contrasted, and erroneous deductions have been the resalt. In the division of the northern bemisphere into two primary regions, the so-called “ Nearctic" and "Palæarctic", no account has been taken of the almost homogeneous character of life throughout the Arctic and Sub-Arctic regions, and the equally important principle of temperature as a powerful limiting agent, por of the facts of the rapid increase of organic forms and the consequent differentiation of life from the Arctic regions toward the Equatorial in an ever increasing ratio in proportion to the extent and divergence of the principal land areas. at the northward, this method of division separates, into primary life. regions, areas of the closest ontological resemblances, while at the southward these divisions each embrace faunæ so unlike those of their northern portions respectively that the two extremes of either region have little in common, scarcely more than have the southern portions of these two regions as compared with each other. It is the neglect of the above-stated fundamental facts and principles that forms the fatal weakness of the scheme of life-regions proposed by Dr. Sclater, and so widely and thoughtlessly accepted. That the facts and principles above alluded to are fundamental,-in other words, that life is distributed in circumpolar zones under the controlling influence of climate and mainly of temperature,- I propose to show by a tabular presentation of the facts of distribution of mammalian life in the northern hemisphere.
One of the reasons given by Mr. Wallace for adopting Dr. Sclater's regions is that “it is a positive, and by no means an unimportant advantage to have our named regions approximately equal in sizo, and with easily defined, and therefore easily remembered, boundaries”, providing that " we do not violate any clear affinities or produce any glar. ing irregularities”. It is further claimed that “all elaborate definitions of interpenetrating frontiers, as well as regions extending over three fourths of the land surface of the globe, and including places which are the antipodes of each other, would be most inconvenient, even if there were not such difference of opinion about them”.*
These arguments can be scarcely characterized as otherwise than trivial, since they imply that truth, at least to a certain degree, should be regarded as secondary to convenience. They further show that the author of these propositions bas not worked out in detail the distribu. tion of life, species by species, over a diversitied area of considerable extent, like, for instance, that of Eastern North America, where an in. terdigitation of the lesser faunal areas is one of the marked features of the region, as it is elsewbere wherever there is a varied topography and consequent inequality of climate under the same parallels of latitude. Again, Mr. Wallace says,—“On two main points every system yet proposed, or that probably can be proposed, is open to objection; they are,-1stly, that the several regions are not of equal rank;—2ndly, that they are not equally applicable to all classes of animals. As to the first objection, it will be found impossible to form any three
* Geogr. Dist. Anim., vol. i, pp. 63, 64.
or more regions, each of which differs from the rest in an equal degree or in the same manner. One will surpass all others in the possession of peculiar families; another will have many characteristic genera; while a third will be mainly distinguished by negative characters. There will also be found many intermediate districts, which possess some of the characteristics of two well-marked regions, and a few special features of their own, or perhaps with none; and it will be a difficult question to decide in all cases which region should possess the doubtful territory, or whether it should be formed into a primary region by itself."*
In geographical zoology, as in the genetic relation of animals, we find, as a rule, no strongly marked boundary-lines, and in the liferegions, especially those of lesser rank, the boundaries can be given only approximately, owing to the intergradation of contiguous faunæ and floræ, contingent upon the gradual modification of climatic conditions; yet it is not hard to find boundary-lines that shall be, if not sharply definable, at least easy of recognition. This at least proves to be the case wherever the distribution of specific forms is thoroughly known. The first objection, “ that the several regions are not of equal rank,” forms to my mind no objection at all, since it matters little whether they are equal or unequal if they correctly indicate the distri. bution of life.
The second objection Mr. Wallace has himself satisfactorily answered, in discussing the question “Which class of animals is of most importance in determining Zoological Regions.” As Mr. Wallace here points out, and as must become apparent to every careful investigator of this question, the mammalia are pre-eminently of the greatest importance in determining zoological regions. To summarize Mr. Wallace's argument on this point,t their dispersal is less dependent on fortuitous circumstances than that of the representatives of other classes; from their high organization they are less dependent upon “other groups of animals”, and have so much power of adaptation that they are able to exist in one form or another over the whole globe", as is certainly pot the case with two of the lower classes of vertebrates, the reptilia and amphibia. Their distribution and dispersal are dependent on the distribution of the lapd-areas, and are modified by such physical conditions as mount. ain barriers, areas of forest, and grassy or desert plateaus. Furthermore, their geological history, as well as their geographical range, is better known than that of most other classes, and there is also a greater unanimity of opinion respecting their natural affinities and the limita. tion of families and genera in this class than in most others. “We should therefore”, says Mr. Wallace (and I heartily agree with the remark), “construct our typical or standard Zoölogical Regions in the first place, from a consideration of the distribution of mammalia, only bring. ing to our aid the distribution of other groups to determine doubtful points. Regions so established will be most closely in accordance with
* Geogr. Dist. Anim., vol. i, p. 53. t See Geogr. Distr. Anim., vol. I, pp. 56-58.
those long-enduring features of physical geography, on which the distribution of all forms of life fundamentally depends;* and all discrepancies in the distribution of other classes of animals must be capable of being ex. plained, either by their exceptional means of dispersion or by special conditions affecting their perpetuation and increase in each locality." “ If these considerations are well founded,” he continues, “ the objections of those who study insects or molluscs, for example,--that our regions are not true for their departments of nature cannot be maintained. For they will find, that a careful consideration of the exceptional means of dispersal and conditions of existence of each group, will explain most of the divergences from the vormal distribution of higher animals." +
In the present paper I shall consequently, in my discussion of the zoological regions of the northern hemisphere, confine myself primarily to mammals. Throwing aside, for the moment, all theoretical considerations, I shall endeavor first to present the facts of the case, and then consider what generalizations may be legitimately drawn from them.
A. word, however, first in respect to the conformation and distribution of the land-areas. In reference to this part of the subject I can hardly do better than to again quote the words of Mr. Wallace, who has thus forcibly presented the subject :- "One great peculiarity of the distribution of land lies in its freedom from complete isolation ... The continents, indeed, resembling as they do a huge creeping plant, with roots at the North Pole, and the matted stems and branches of which cover a large part of the northern hemisphere and send three great offshoots toward the South Pole, offer great facilities for the transmission of varied forms of animal life. There is evidence to prove that during the greater part of the Tertiary period the relative positions of our conti* The italicizing is my own.
+ The question, which class of animals is best fitted to form the basis of a division of the earth's surface into life-regions ? has a wider bearing than might be at first supposed, since the same power of adaptation to diverse climatic conditions that results in a wide distribution in some cases and a limited rango in others would also impart different degrees of ability to resist the influence of geological changes, and is hence related to the question, Which class forms the best index for marking geological time? The relative importance of different groups as geological indices is necessarily connected with their power to resist unfavorable influences, and hence groups that succumb most readily would give the best clue to such changes in the past. Among vertebrates the mammalia are undoubtedly, as a class, the best able to survive a wide range of climatic conditions. Birds are to so great a degree migratory that they are in great measure able to avoid seasonal extremes of climate by a change of habitat. Extremes that mammals readily survive prove quickly fatal to reptiles and amphibians.
Climate, though in itself a powerful geological agent, is, of course, subject to profound modification due to geological causes. Any great amount of upheaval or subsidence of the earth's crust, or the gradual uplifting of mountain chains, must necessarily induce changes in the climate of the regions where such disturbances occur, the eflect of which must extend over an area far greater than that of the disturbed district. A comparatively slight change of climate, either in respect to temperature or humidity, has a most marked influence upon vegetation, and especially upon the distribution of forests. The presence or absence of particular species of plants is well known to determine the presence or absence of many species of insects, while the distribution of whole families of the latter is determined wholly by the character of the vegetation;