Chapter XVI: Part I: The Principles and General Phenomena of Distribution (3)

The divisions in use till quite recently were of two kinds; either those ready made by geographers, more especially the quarters or continents of the globe; or those determined by climate and marked out by certain parallels of latitude or by isothermal lines. Either of these methods was better than none at all; but from the various considerations explained in the preceding chapters, it will be evident, that such divisions must have often been very unnatural, and have disguised many {53}of the most important and interesting phenomena which a study of the distribution of animals presents to us.

The merit of initiating a more natural system, that of determining zoological regions, not by any arbitrary or _a priori_ consideration but by studying the actual ranges of the more important groups of animals, is due to Mr. Sclater, who, in 1857, established six primary zoological regions from a detailed examination of the distribution of the chief genera and families of Birds. Before stating what these regions are, what objections have been made to them, what other divisions have been since proposed, and what are those which we shall adopt in this work, it will be well to consider the general principles which should guide us in the choice between rival systems.

_Principles on which Zoological Regions should be formed._--It will be evident in the first place that nothing like a perfect zoological division of the earth is possible. The causes that have led to the present distribution of animal life are so varied, their action and reaction have been so complex, that anomalies and irregularities are sure to exist which will mar the symmetry of any rigid system. 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 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, with 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 this doubtful territory, or whether it should be formed into a primary region itself. Again, two regions which have now well-marked points of difference, may be shown to have been much more alike at a comparatively recent geological epoch; {54}and this, it may be said, proves their fundamental unity and that they ought to form but one primary region. To obviate some of these difficulties a binary or dichotomous division is sometimes proposed; that portion of the earth which differs most from the rest being cut off as a region equal in rank to all that remains, which is subjected again and again to the same process.

To decide these various points it seems advisable that convenience, intelligibility, and custom, should largely guide us. The first essential is, a broadly marked and easily remembered set of regions; which correspond, as nearly as truth to nature will allow, with the distribution of the most important groups of animals. What these groups are we shall presently explain. In determining the number, extent, and boundaries of these regions, we must be guided by a variety of indications, since the application of fixed rules is impossible. They should evidently be of a moderate number, corresponding as far as practicable with the great natural divisions of the globe marked out by nature, and which have always been recognized by geographers. There should be some approximation to equality of size, since there is reason to believe that a tolerably extensive area has been an essential condition for the development of most animal forms; and it is found that, other things being equal, the numbers, variety and importance of the forms of animal and vegetable life, do bear some approximate relation to extent of area. Although the possession of peculiar families or genera is the main character of a primary zoological region, yet the negative character of the absence of certain families or genera is of equal importance, _when this absence does not manifestly depend on unsuitability to the support of the group_, and especially _when there is now no physical barrier preventing their entrance_. This will become evident when we consider that the importance of the possession of a group by one region depends on its absence from the adjoining regions; and if there is now no barrier to its entrance, we may be sure that there has once been one; and that the possession of the area by a distinct and well balanced set of organisms, which must have been slowly {55}developed and adjusted, is the living barrier that now keeps out intruders.

When it is ascertained that the chief differences which now obtain between two areas did not exist in Miocene or Pliocene times, the fact is one of great interest, and enables us to speculate with some degree of probability as to the causes that have brought about the present state of things; but it is not a reason for uniting these two areas into one region. Our object is to represent as nearly as possible the main features of the distribution of existing animals, not those of any or all past geological epochs. Should we ever obtain sufficient information as to the geography and biology of the earth at past epochs, we might indeed determine approximately what were the Pliocene or Miocene or Eocene zoological regions; but any attempt to exhibit all these in combination with those of our own period, must lead to confusion.

The binary or dichotomous system, although it brings out the fundamental differences of the respective regions, is an inconvenient one in its application, and rather increases than obviates the difficulty as to equality or inequality of regions; for although _a_, _b_, _c_, and _d_, may be areas of unequal zoological rank, _a_ being the most important, and _d_ the least, yet this inequality will probably be still greater if we first divide them into _a_, on one side, and _b_, _c_, and _d_, on the other, and then, by another division, make _b_, an area of the second, and _c_, and _d_, of the third rank only.

Coming to the second objection, the often incompatible distribution of different groups of animals, affords ground for opposition to any proposed scheme of zoological regions. There is first the radical difference between land and sea animals; the most complete barriers to the dispersal of the one, sometimes offering the greatest facilities for the emigration of the other, and _vice versa_. A large number of marine animals, however, frequent shallow water only; and these, keeping near the coasts, will agree generally in their distribution with those inhabiting the land. But among land animals themselves there are very great differences of distribution, due to certain specialities {56}in their organization or mode of life. These act mainly in two ways,--1stly, by affecting the facilities with which they can be dispersed, either voluntarily or involuntarily;--2ndly, by the conditions which enable them to multiply and establish themselves in certain areas and not in others. When both these means of diffusion are at a maximum, the dispersal of a group becomes universal, and ceases to have much interest for us. This is the case with certain groups of fungi and lichens, as well as with some of the lower animals; and in a less degree, as has been shown by Mr. Darwin, with many fresh-water plants and animals. At the other extreme we may place certain arboreal vertebrata such as sloths and lemurs, which have no means of passing such barriers as narrow straits or moderately high mountains, and whose survival in any new country they might reach, would be dependent on the presence of suitable forests and the absence of dangerous enemies. Almost equally, or perhaps even more restricted, are the means of permanent diffusion of terrestrial molluscs; since these are without any but very rare and accidental means of being safely transported across the sea; their individual powers of locomotion are highly restricted; they are especially subject to the attacks of enemies; and they often depend not only on a peculiar vegetation, but on the geological character of the country, their abundance being almost in direct proportion to the presence of some form of calcareous rocks. Between these extremes we find animals possessed of an infinite gradation of powers to disperse and to maintain themselves; and it will evidently be impossible that the limits which best define the distribution of one group, should be equally true for all others.

_Which class of Animals is of most importance in determining Zoological Regions._--To decide this question we have to consider which groups of animals are best adapted to exhibit, by their existing distribution, the past changes and present physical condition of the earth's surface; and at the same time, by the abundance of their remains in the various tertiary formations will best enable us to trace out the more recent of the series of changes, both of the earth's surface and {57}of its inhabitants, by which the present state of things has been brought about. For this purpose we require a group which shall be dependent for its means of dispersal on the distribution of land and water, and on the presence or absence of lofty mountains, desert plains or plateaux, and great forests; since these are the chief physical features of the earth's surface whose modifications at successive periods we wish to discover. It is also essential that they should not be subject to dispersal by many accidental causes; as this would inevitably in time tend to obliterate the effect of natural barriers, and produce a scattered distribution, the causes of which we could only guess at. Again, it is necessary that they should be so highly organized as not to be absolutely dependent on other groups of animals, and with so much power of adaptation as to be able to exist in one form or another over the whole globe. And lastly, it is highly important that the whole group should be pretty well known, and that a fairly natural classification, especially of its minor divisions such as families and genera, should have been arrived at; the reason for which last proviso is explained in our next chapter, on classification.

Now in every one of these points the mammalia are preeminent; and they possess the additional advantage of being the most highly developed class of organized beings, and that to which we ourselves belong. We should therefore construct our typical or standard Zoological 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 those long-enduring features of physical geography, on which the distribution of all forms of life fundamentally depend; and all discrepancies in the distribution of other classes of animals must be capable of being explained, 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, the objections of those who study insects or molluscs, for example,--that our regions are not true for their departments of nature--cannot be {58}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 normal distribution of higher animals.

We shall thus be led to an intelligent comprehension of the phenomena of distribution in all groups, which would not be the case if every specialist formed regions for his own particular study. In many cases we should find that no satisfactory division of the earth could be made to correspond with the distribution even of an entire class; but we should have the coleopterist and the lepidopterist each with his own Geography. And even this would probably not suffice, for it is very doubtful if the detailed distribution of the Longicornes, so closely dependent on woody vegetation, could be made to agree with that of the Staphylinidæ or the Carabidæ which abound in many of the most barren regions, or with that of the Scarabeidæ, largely dependent on the presence of herbivorous mammalia. And when each of these enquirers had settled a division of the earth into "regions" which exhibited with tolerable accuracy the phenomena of distribution of his own group, we should have gained nothing whatever but a very complex mode of exhibiting the bare facts of distribution. We should then have to begin to work out the causes of the divergence of one group from another in this respect; but as each worker would refer to his own set of regions as the type, the whole subject would become involved in inextricable confusion. These considerations seem to make it imperative that one set of "regions" should be established as typical for Zoology; and it is hoped the reasons here advanced will satisfy most naturalists that these regions can be best determined, in the first place, by a study of the distribution of the mammalia, supplemented in doubtful cases by that of the other vertebrates. We will now proceed to a discussion of what these regions are.

_Various Zoological Regions proposed since 1857._--It has already been pointed out that a very large number of birds are limited by the same kind of barriers as mammalia; it will therefore not be surprising that a system of regions formed to suit the {59}one, should very nearly represent the distribution of the other. Mr. Sclater's regions are as follows:--

1. The Palæarctic Region; including Europe, Temperate Asia, and N. Africa to the Atlas mountains.

2. The Ethiopian Region; Africa south of the Atlas, Madagascar, and the Mascarene Islands, with Southern Arabia.

3. The Indian Region; including India south of the Himalayas, to South China, and to Borneo and Java.

4. The Australian Region; including Celebes and Lombock, eastward to Australia and the Pacific Islands.

5. The Nearctic Region; including Greenland, and N. America, to Northern Mexico.

6. The Neotropical Region; including South America, the Antilles, and Southern Mexico.

This division of the earth received great support from Dr. Günther, who, in the _Proceedings of the Zoological Society_ for 1858, showed that the geographical distribution of Reptiles agreed with it very closely, the principal difference being that the reptiles of Japan have a more Indian character than the birds, this being especially the case with the snakes. In the volume for 1868 of the same work, Professor Huxley discusses at considerable length the primary and secondary zoological divisions of the earth. He gives reasons for thinking that the most radical primary division, both as regards birds and mammals, is into a Northern and Southern hemisphere (Arctogæa and Notogæa), the former, however, embracing all Africa, while the latter includes only Australasia and the Neotropical or Austro-Columbian region. Mr. Sclater had grouped his regions primarily into Palæogæa and Neogæa, the Old and New Worlds of geographers; a division which strikingly accords with the distribution of the passerine birds, but not so well with that of mammalia or reptiles. Professor Huxley points out that the Nearctic, Palæarctic, Indian, and Ethiopian regions of Mr. Sclater have a much greater resemblance to each other than any one of them has to Australia or to South America; and he further suggests that New Zealand alone has peculiarities which might entitle it to rank as a primary region {60}along with Australasia and South America; and that a Circumpolar Province might be conveniently recognised as of equal rank with the Palæarctic and Nearctic provinces.

In 1866, Mr. Andrew Murray published a large and copiously illustrated volume on the _Geographical Distribution of Mammals_, in which he maintains that the great and primary mammalian regions are only four: 1st. The Palæarctic region of Mr. Sclater, extended to include the Sahara and Nubia; 2nd. the Indo-African region, including the Indian and Ethiopian regions of Mr. Sclater; 3rd. the Australian region (unaltered); 4th. the American region, including both North and South America. These are the regions as _described_ by Mr. Murray, but his coloured map of "Great Mammalian Regions" shows all Arctic America to a little south of the Isothermal of 32° Fahr. as forming with Europe and North Asia one great region.

At the meeting of the British Association at Exeter in 1869, Mr. W. T. Blanford read a paper on the Fauna of British India, in which he maintained that a large portion of the peninsula of India had derived its Fauna mainly from Africa; and that the term "Indian region" of Mr. Sclater was misleading, because India proper, if it belongs to it at all, is the least typical portion of it. He therefore proposes to call it the "Malayan region," because in the Malay countries it is most highly developed. Ceylon and the mountain ranges of Southern India have marked Malay affinities.

In 1871 Mr. E. Blyth published in _Nature_ "A suggested new Division of the Earth into Zoological Regions," in which he indicates seven primary divisions or regions, subdivided into twenty-six sub-regions. The seven regions are defined as follows: 1. The Boreal region; including the whole of the Palæarctic and Nearctic regions of Mr. Sclater along with the West Indies, Central America, the whole chain of the Andes, with Chili and Patagonia. 2. The Columbian region; consisting of the remaining part of South America. 3. The Ethiopian region; comprising besides that region of Mr. Sclater, the valley of the Jordan, Arabia, and the desert country towards India, with all the plains and table lands of India and the northern {61}half of Ceylon. 4. The Lemurian region; consisting of Madagascar and its adjacent islands. 5. The Austral-Asian region; which is the Indian region of Mr. Sclater without the portion taken to be added to the Ethiopian region. 6. The Melanesian region; which is the Australian region of Mr. Sclater without New Zealand and the Pacific Islands, which form 7. the Polynesian region. Mr. Blyth thinks this is "a true classification of zoological regions as regards mammalia and birds."

In an elaborate paper on the birds of Eastern North America, their distribution and migrations (_Bulletin of Museum of Comparative Zoology, Cambridge, Massachusetts_, Vol. 2), Mr. J. A. Allen proposes a division of the earth in accordance with what he terms, "the law of circumpolar distribution of life in zones," as follows: 1. Arctic realm. 2. North temperate realm. 3. American tropical realm. 4. Indo-African tropical realm. 5. South American tropical realm. 6. African temperate realm. 7. Antarctic realm. 8. Australian realm. Some of these are subdivided into regions; (2) consisting of the American and the Europæo-Asiatic regions; (4) into the African and Indian regions; (8) into the tropical Australian region, and one comprising the southern part of Australia and New Zealand. The other realms each form a single region.

_Discussion of proposed Regions._--Before proceeding to define the regions adopted in this work, it may be as well to make a few remarks on some of the preceding classifications, and to give the reasons which seem to render it advisable to adopt very few of the suggested improvements on Mr. Sclater's original proposal. Mr. Blyth's scheme is one of the least natural, and also the most inconvenient. There can be little use in the knowledge that a group of animals is found in the Boreal Region, if their habitat might still be either Patagonia, the West Indies, or Japan; and it is difficult to see on what principle the Madagascar group of islands is made of equal rank with this enormous region, seeing that its forms of life have marked African affinities. Neither does it seem advisable to adopt the Polynesian Region, or that comprising New Zealand alone (as hinted at by Professor Huxley and since adopted by {62}Mr. Sclater in his Lectures on Geographical Distribution at the Zoological Gardens in May 1874), because it is absolutely without indigenous mammalia and very poor in all forms of life, and therefore by no means prominent or important enough to form a primary region of the earth.

It may be as well here to notice what appears to be a serious objection to making New Zealand, or any similar isolated district, one of the great zoological regions, comparable to South America, Australia, or Ethiopia; which is, that its claim to that distinction rests on grounds which are liable to fail. It is because New Zealand, in addition to its negative merits, possesses three families of birds (Apterygidæ living, Dinornithidæ and Palapterygidæ extinct), and a peculiar lizard-like reptile, _Hatteria_, which has to be classed in a distinct order, Rhynchocephalina, that the rank of a Region is claimed for it. But supposing, what is not at all improbable, that other Rhynchocephalina should be discovered in the interior of Australia or in New Guinea, and that Apterygidæ or Palapterygidæ should be found to have inhabited Australia in Post-Pliocene times, (as Dinornithidæ have already been proved to have done) the claims of New Zealand would entirely fail, and it would be universally acknowledged to be a part of the great Australian region. No such reversal can take place in the case of the other regions; because they rest, not upon one or two, but upon a large number of peculiarities, of such a nature that there is no room upon the globe for discoveries that can seriously modify them. Even if one or two peculiar types, like Apterygidæ or _Hatteria_, should permanently remain characteristic of New Zealand alone, we can account for these by the extreme isolation of the country, and the absence of enemies, which have enabled these defenceless birds and reptiles to continue their existence; just as the isolation and protection of the caverns of Carniola have enabled the _Proteus_ to survive in Europe. But supposing that the _Proteus_ was the sole representative of an order of Batrachia, and that two or three other equally curious and isolated forms occurred with it, no one would propose that these caverns or the district containing them, should form one of the {63}primary divisions of the earth. Neither can much stress be laid on the negative characteristics of New Zealand, since they are found to an almost equal extent in every oceanic island.

Again, it is both inconvenient and misleading to pick out certain tracts from the midst of one region or sub-region and to place them in another, on account of certain isolated affinities which may often be accounted for by local peculiarities. Even if the resemblance of the fauna of Chili and Patagonia to that of the Palæarctic and Nearctic regions was much greater than it is, this mode of dealing with it would be objectionable; but it is still more so, when we find that these countries have a strongly marked South American character, and that the northern affinities are altogether exceptional. The Rodentia, which comprise a large portion of the mammalia of these countries, are wholly South American in type, and the birds are almost all allied to forms characteristic of tropical America.

For analogous reasons the Ethiopian must not be made to include any part of India or Ceylon; for although the Fauna of Central India has some African affinities, these do not preponderate; and it will not be difficult to show that to follow Mr. Andrew Murray in uniting bodily the Ethiopian and Indian regions of Mr. Sclater, is both unnatural and inconvenient. The resemblances between them are of the same character as those which would unite them both with the Palæarctic and Nearctic regions; and although it may be admitted, that, as Professor Huxley maintains, this group forms one of the great primary divisions of the globe, it is far too extensive and too heterogeneous to subserve the practical uses for which we require a division of the world into zoological regions.

_Reasons for adopting the six Regions first proposed by Mr. Sclater._--So that we do not violate any clear affinities or produce any glaring irregularities, 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. 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 {64}the antipodes of each other, would be most inconvenient, even if there were not such difference of opinion about them. There can be little doubt, for example, that the most radical zoological division of the earth is made by separating the Australian region from the rest; but although it is something useful and definite to know that a group of animals is peculiar to Australia, it is exceedingly vague and unsatisfactory to say of any other group merely that it is extra-Australian. Neither can it be said that, from any point of view, these two divisions are of equal importance. The next great natural division that can be made is the separation of the Neotropical Region of Mr. Sclater from the rest of the world. We thus have three primary divisions, which Professor Huxley seems inclined to consider as of tolerably equal zoological importance. But a consideration of all the facts, zoological and palæontological, indicates, that the great northern division (Arctogæa) is fully as much more important than either Australia or South America, as its four component parts are less important; and if so, convenience requires us to adopt the smaller rather than the larger divisions.

This question, of comparative importance or equivalence of value, is very difficult to determine. It may be considered from the point of view of speciality or isolation, or from that of richness and variety of animal forms. In isolation and speciality, determined by what they want as well as what they possess, the Australian and Neotropical regions are undoubtedly each comparable with the rest of the earth (Arctogæa). But in richness and variety of forms, they are both very much inferior, and are much more nearly comparable with the separate regions which compose it. Taking the families of mammalia as established by the best authors, and leaving out the Cetacea and the Bats, which are almost universally distributed, and about whose classification there is much uncertainty, the number of families represented in each of Mr. Sclater's regions is as follows:

I. Palæarctic region has 31 families of terrestrial mammalia.
II. Ethiopian " " 40 " " "
III. Indian " " 31 " " "
IV. Australian " " 14 " " "
V. Neotropical " " 26 " " "
VI. Nearctic " " 23 " " "

{65}We see, then, that even the exceedingly rich and isolated Neotropical region is less rich and diversified in its forms of mammalian life than the very much smaller area of the Indian region, or the temperate Palæarctic, and very much less so than the Ethiopian region; while even the comparatively poor Nearctic region, is nearly equal to it in the number of its family types. If these were united they would possess fifty-five families, a number very disproportionate to those of the remaining two. Another consideration is, that although the absence of certain forms of life makes a region more isolated, it does not make it zoologically more important; for we have only to suppose some five or six families, now common to both, to become extinct either in the Ethiopian or the Indian regions, and they would become as strongly differentiated from all other regions as South America, while still remaining as rich in family types. In birds exactly the same phenomenon recurs, the family types being less numerous in South America than in either of the other tropical regions of the earth, but a larger proportion of them are restricted to it. It will be shown further on, that the Ethiopian and Indian, (or, as I propose to call it in this work, Oriental) regions, are sufficiently differentiated by very important groups of animals peculiar to each; and that, on strict zoological principles they are entitled to rank as regions of equal value with the Neotropical and Australian. It is perhaps less clear whether the Palæarctic should be separated from the Oriental region, with which it has undoubtedly much in common; but there are many and powerful reasons for keeping it distinct. There is an unmistakably different facies in the animal forms of the two regions; and although no families of mammalia or birds, and not many genera, are wholly confined to the Palæarctic region, a very considerable number of both have their metropolis in it, and are very richly represented. The distinction between the characteristic forms of life in tropical and cold countries is, on the whole, very strongly marked in the northern hemisphere; and to refuse to recognise this in a subdivision of the earth which is established for the very purpose of expressing such contrasts more clearly and concisely than by ordinary geographical terminology, would be both illogical and {66}inconvenient. The one question then remains, whether the Nearctic region should be kept separate, or whether it should form part of the Palæarctic or of the Neotropical regions. Professor Huxley and Mr. Blyth advocate the former course; Mr. Andrew Murray (for mammalia) and Professor Newton (for birds) think the latter would be more natural. No doubt much is to be said for both views, but both cannot be right; and it will be shown in the latter part of this chapter that the Nearctic region is, on the whole, fully as well defined as the Palæarctic, by positive characters which differentiate it from both the adjacent regions. More evidence in the same direction will be found in the Second Part of this work, in which the extinct faunas of the several regions are discussed.

A confirmation of the general views here set forth, as to the distinctness and approximate equivalence of the six regions, is to be found in the fact, that if any two or more of them are combined they themselves become divisions of the next lower rank, or "sub-regions;"--and these will be very much more important, both zoologically and geographically, than the subdivisions of the remaining regions. It is admitted then that these six regions are by no means of precisely equal rank, and that some of them are far more isolated and better characterized than others; but it is maintained that, looked at from every point of view, they are more equal in rank than any others that can be formed; while in geographical equality, compactness of area, and facility of definition, they are beyond all comparison better than any others that have yet been proposed for the purpose of facilitating the study of geographical distribution. They may be arranged and grouped as follows, so as to exhibit their various relations and affinities.

Regions.
{ NEOTROPICAL Austral zone Notogæa.
Neogæa {
{ NEARCTIC } }
} Boreal zone }
{ PALÆARCTIC } }
{ } Arctogæa.
{ ETHIOPIAN } }
Palæogæa { } Palæotropical zone }
{ ORIENTAL } }
{
{ AUSTRALIAN Austral zone Notogæa.

The above table shows the regions placed in the order followed in the Fourth Part of this work, and the reasons for which are {67}explained in