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Professor Owen has overlooked my views as to the phylogenetic position of this order, and has ascribed to me, by implication, those I do not hold. He then adds others of his own which do not commend themselves to my approval. He observes (l. c. 714),—“To call the Maestricht reptile a Pythonomorph is to raise a delusive beacon, misguiding the Voyager in the discovery of the true course of the organic change." My views as to the course of organic change in this direction are as follows:-"Experience has shown that generalized orders have been the predecessors of the special groups of the existing fauna. The structure of the Pythonomorpha, which has so much in common with orders well distinguished from each other, offers a hint of the character of the primary group from which both have sprung. That this order is not that unknown type is clear, but the indication of affinity to it is equally unmistakable." The structure of the posterior part of the skull demonstrates the correctness of this position, as it is more generalized than that of either Lacertilia or Ophidia, approximating more than either that of the tortoises. In other parts of the skeleton, this order displays the specialization which Professor Owen claims for it.

In closing this discussion of the essay of a master from whom I have learned much, and from whom I expect to learn more, I may say that I have not attempted to exhaust the subject, but have only followed it so far as to set forth my own views so as to prevent any misunderstanding of them.

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ART. XV.-THE GEOGRAPHICAL DISTRIBUTION OF THE MAMMALIA, CONSIDERED IN RELATION TO THE PRINCIPAL ONTOLOGICAL REGIONS OF THE EARTH, AND THE LAWS THAT GOVERN THE DISTRIBUTION OF ANIMAL LIFE.

BY JOEL ASAPH ALLEN.

I-DISTRIBUTION OF MAMMALIAN LIFE IN THE NORTHERN 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 princi ples then stated. In this paper I claimed, in accordance with the views of Humboldt, Wagner, Dana, Agassiz, De Candolle, and 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 confirmed 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 of birds, and since so generally adopted. Their wide acceptation, it seems to me, has 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 work has now rendered a critical presentation of the subject more necessary than be fore, since, instead of seeking in the facts of geographical zoology a 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 Faunas as Elucidating the Past Changes of the Earth's Surface. By Alfred Russel Wallace. Two vols. 8°. With maps and illustrations. London, 1876.

Bull. iv. No. 2

-1

313

ble have been contrasted, and erroneous deductions have been the result. In the division of the northern hemisphere 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, nor 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 liferegions, 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 size, 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 has not worked out in detail the distribu tion of life, species by species, over a diversified area of considerable extent, like, for instance, that of Eastern North America, where an interdigitation of the lesser faunal areas is one of the marked features of the region, as it is elsewhere 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 distribution 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,f 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 not 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 land-areas, and are modified by such physical conditions as mountain 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 limitation 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 bringing 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.

† See Geogr. Distr. Anim., vol. i, pp. 56-58.

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