Darwin and Modern Science Part 27

XVII. GEOGRAPHICAL DISTRIBUTION OF ANIMALS. By Hans Gadow, M.A., Ph.D., F.R.S.

Strickland Curator and Lecturer on Zoology in the University of Cambridge.

The first general ideas about geographical distribution may be found in some of the brilliant speculations contained in Buffon's "Histoire Naturelle". The first special treatise on the subject was however written in 1777 by E.A.W. Zimmermann, Professor of Natural Science at Brunswick, whose large volume, "Specimen Zoologiae Geographicae Quadrupedum"..., deals in a statistical way with the mammals; important features of the large accompanying map of the world are the ranges of mountains and the names of hundreds of genera indicating their geographical range. In a second work he laid special stress on domesticated animals with reference to the spreading of the various races of Mankind.

In the following year appeared the "Philosophia Entomologica" by J.C.

Fabricius, who was the first to divide the world into eight regions. In 1803 G.R. Trevira.n.u.s ("Biologie oder Philosophie der lebenden Natur", Vol. II. Gottingen, 1803.) devoted a long chapter of his great work on "Biologie" to a philosophical and coherent treatment of the distribution of the whole animal kingdom. Remarkable progress was made in 1810 by F.

Tiedemann ("Anatomie und Naturgeschichte der Vogel". Heidelberg, 1810.) of Heidelberg. Few, if any, of the many subsequent Ornithologists seem to have appreciated, or known of, the ingenious way in which Tiedemann marshalled his statistics in order to arrive at general conclusions.

There are, for instance, long lists of birds arranged in accordance with their occurrence in one or more continents: by correlating the distribution of the birds with their food he concludes "that the countries of the East Indian flora have no vegetable feeders in common with America," and "that it is probably due to the great peculiarity of the African flora that Africa has few phytophagous kinds in common with other countries, whilst zoophagous birds have a far more independent, often cosmopolitan, distribution." There are also remarkable chapters on the influence of environment, distribution, and migration, upon the structure of the Birds! In short, this anatomist dealt with some of the fundamental causes of distribution.

Whilst Tiedemann restricted himself to Birds, A. Desmoulins in 1822 wrote a short but most suggestive paper on the Vertebrata, omitting the birds; he combated the view recently proposed by the entomologist Latreille that temperature was the main factor in distribution. Some of his ten main conclusions show a peculiar mixture of evolutionary ideas coupled with the conception of the stability of species: whilst each species must have started from but one creative centre, there may be several "a.n.a.logous centres of creation" so far as genera and families are concerned. Countries with different faunas, but lying within the same climatic zones, are proof of the effective and permanent existence of barriers preventing an exchange between the original creative centres.

The first book dealing with the "geography and cla.s.sification" of the whole animal kingdom was written by W. Swainson ("A Treatise on the Geography and Cla.s.sification of Animals", Lardner's "Cabinet Cyclopaedia" London, 1835.) in 1835. He saw in the five races of Man the clue to the mapping of the world into as many "true zoological divisions," and he reconciled the five continents with his mystical quinary circles.

Lyell's "Principles of Geology" should have marked a new epoch, since in his "Elements" he treats of the past history of the globe and the distribution of animals in time, and in his "Principles" of their distribution in s.p.a.ce in connection with the actual changes undergone by the surface of the world. But as the sub-t.i.tle of his great work "Modern changes of the Earth and its inhabitants" indicates, he restricted himself to comparatively minor changes, and, emphatically believing in the permanency of the great oceans, his numerous and careful interpretations of the effect of the geological changes upon the dispersal of animals did after all advance the problem but little.

Hitherto the marine faunas had been neglected. This was remedied by E.

Forbes, who established nine h.o.m.ozoic zones, based mainly on the study of the mollusca, the determining factors being to a great extent the isotherms of the sea, whilst the 25 provinces were given by the configuration of the land. He was followed by J.D. Dana, who, taking princ.i.p.ally the Crustacea as a basis, and as leading factors the mean temperatures of the coldest and of the warmest months, established five lat.i.tudinal zones. By using these as divisors into an American, Afro-European, Oriental, Arctic and Antarctic realm, most of which were limited by an eastern and western land-boundary, he arrived at about threescore provinces.

In 1853 appeared L.K. Schmarda's ("Die geographische Verbreitung der Thiere", Wien, 1853.) two volumes, embracing the whole subject. Various centres of creation being, according to him, still traceable, he formed the hypothesis that these centres were originally islands, which later became enlarged and joined together to form the great continents, so that the original faunas could overlap and mix whilst still remaining pure at their respective centres. After devoting many chapters to the possible physical causes and modes of dispersal, he divided the land into 21 realms which he shortly characterises, e.g. Australia as the only country inhabited by marsupials, monotremes and meliphagous birds.

Ten main marine divisions were diagnosed in a similar way. Although some of these realms were not badly selected from the point of view of being applicable to more than one cla.s.s of animals, they were obviously too numerous for general purposes, and this drawback was overcome, in 1857, by P.L. Sclater. ("On the general Geographical Distribution of the members of the cla.s.s Aves", "Proc. Linn. Soc." (Zoology II. 1858, pages 130-145.)) Starting with the idea, that "each species must have been created within and over the geographical area, which it now occupies,"

he concluded "that the most natural primary ontological divisions of the Earth's surface" were those six regions, which since their adoption by Wallace in his epoch-making work, have become cla.s.sical. Broadly speaking, these six regions are equivalent to the great ma.s.ses of land; they are convenient terms for geographical facts, especially since the Palaearctic region expresses the unity of Europe with the bulk of Asia.

Sclater further brigaded the regions of the Old World as Palaeogaea and the two Americas as Neogaea, a fundamental mistake, justifiable to a certain extent only since he based his regions mainly upon the present distribution of the Pa.s.serine birds.

Unfortunately these six regions are not of equal value. The Indian countries and the Ethiopian region (Africa south of the Sahara) are obviously nothing but the tropical, southern continuations or appendages of one greater complex. Further, the great eastern ma.s.s of land is so intimately connected with North America that this continent has much more in common with Europe and Asia than with South America. Therefore, instead of dividing the world longitudinally as Sclater had done, Huxley, in 1868 ("On the cla.s.sification and distribution of the Alectoromorphae and Heteromorphae", "Proc. Zool. Soc." 1868, page 294.), gave weighty reasons for dividing it transversely. Accordingly he established two primary divisions, Arctogaea or the North world in a wider sense, comprising Sclater's Indian, African, Palaearctic and Neartic regions; and Notogaea, the Southern world, which he divided into (1) Austro-Columbia (an unfortunate subst.i.tute for the neotropical region), (2) Australasia, and (3) New Zealand, the number of big regions thus being reduced to three but for the separation of New Zealand upon rather negative characters. Sclater was the first to accept these four great regions and showed, in 1874 ("The geographical distribution of Mammals", "Manchester Science Lectures", 1874.), that they were well borne out by the present distribution of the Mammals.

Although applicable to various other groups of animals, for instance to the tailless Amphibia and to Birds (Huxley himself had been led to found his two fundamental divisions on the distribution of the Gallinaceous birds), the combination of South America with Australia was gradually found to be too sweeping a measure. The obvious and satisfactory solution was provided by W.T. Blanford (Anniversary address (Geological Society, 1889), "Proc. Geol. Soc." 1889-90, page 67; "Quart. Journ."

XLVI 1890.), who in 1890 recognised three main divisions, namely Australian, South American, and the rest, for which the already existing terms (although used partly in a new sense, as proposed by an anonymous writer in "Natural Science", III. page 289) "Notogaea," "Neogaea" and "Arctogaea" have been gladly accepted by a number of English writers.

After this historical survey of the search for larger and largest or fundamental centres of animal creation, which resulted in the mapping of the world into zoological regions and realms of after all doubtful value, we have to return to the year 1858. The eleventh and twelfth chapters of "The Origin of Species" (1859), dealing with "Geographical Distribution," are based upon a great amount of observation, experiment and reading. As Darwin's main problem was the origin of species, nature's way of making species by gradual changes from others previously existing, he had to dispose of the view, held universally, of the independent creation of each species and at the same time to insist upon a single centre of creation for each species; and in order to emphasise his main point, the theory of descent, he had to disallow convergent, or as they were then called, a.n.a.logous forms. To appreciate the difficulty of his position we have to take the standpoint of fifty years ago, when the immutability of the species was an axiom and each was supposed to have been created within or over the geographical area which it now occupies. If he once admitted that a species could arise from many individuals instead of from one pair, there was no way of shutting the door against the possibility that these individuals may have been so numerous that they occupied a very large district, even so large that it had become as discontinuous as the distribution of many a species actually is. Such a concession would at once be taken as an admission of multiple, independent, origin instead of descent in Darwin's sense.

For the so-called multiple, independently repeated creation of species as an explanation of their very wide and often quite discontinuous distribution, he subst.i.tuted colonisation from the nearest and readiest source together with subsequent modification and better adaptation to their new home.

He was the first seriously to call attention to the many accidental means, "which more properly should be called occasional means of distribution," especially to oceanic islands. His specific, even individual, centres of creation made migrations all the more necessary, but their extent was sadly baulked by the prevailing dogma of the permanency of the oceans. Any number of small changes ("many islands having existed as halting places, of which not a wreck now remains"

("The Origin of Species" (1st edition), page 396.).) were conceded freely, but few, if any, great enough to permit migration of truly terrestrial creatures. The only means of getting across the gaps was by the principle of the "flotsam and jetsam," a theory which Darwin took over from Lyell and further elaborated so as to make it applicable to many kinds of plants and animals, but sadly deficient, often grotesque, in the case of most terrestrial creatures.

Another very fertile source was Darwin's strong insistence upon the great influence which the last glacial epoch must have had upon the distribution of animals and plants. Why was the migration of northern creatures southwards of far-reaching and most significant importance?

More northerners have established themselves in southern lands than vice versa, because there is such a great ma.s.s of land in the north and greater continents imply greater intensity of selection. "The productions of real islands have everywhere largely yielded to continental forms." (Ibid. page 380.)... "The Alpine forms have almost everywhere largely yielded to the more dominant forms generated in the larger areas and more efficient workshops of the North."

Let us now pa.s.s in rapid survey the influence of the publication of "The Origin of Species" upon the study of Geographical Distribution in its wider sense.

Hitherto the following thought ran through the minds of most writers: Wherever we examine two or more widely separated countries their respective faunas are very different, but where two faunas can come into contact with each other, they intermingle. Consequently these faunas represent centres of creation, whence the component creatures have spread peripherally so far as existing boundaries allowed them to do so.

This is of course the fundamental idea of "regions." There is not one of the numerous writers who considered the possibility that these intermediate belts might represent not a mixture of species but transitional forms, the result of changes undergone by the most peripheral migrants in adaptation to their new surroundings. The usual standpoint was also that of Pucheran ("Note sur l'equateur zoologique", "Rev. et Mag. de Zoologie", 1855; also several other papers, ibid. 1865, 1866, and 1867.) in 1855. But what a change within the next ten years!

Pucheran explains the agreement in coloration between the desert and its fauna as "une harmonie post-etablie"; the Sahara, formerly a marine basin, was peopled by immigrants from the neighbouring countries, and these new animals adapted themselves to the new environment. He also discusses, among other similar questions, the Isthmus of Panama with regard to its having once been a strait. From the same author may be quoted the following pa.s.sage as a strong proof of the new influence: "By the radiation of the contemporaneous faunas, each from one centre, whence as the various parts of the world successively were formed and became habitable, they spread and became modified according to the local physical conditions."

The "multiple" origin of each species as advocated by Sclater and Murray, although giving the species a broader basis, suffered from the same difficulties. There was only one alternative to the old orthodox view of independent creation, namely the bold acceptance of land-connections to an extent for which geological and palaeontological science was not yet ripe. Those who shrank from either view, gave up the problem as mysterious and beyond the human intellect. This was the expressed opinion of men like Swainson, Lyell and Humboldt. Only Darwin had the courage to say that the problem was not insoluble. If we admit "that in the long course of time the individuals of the same species, and likewise of allied species, have proceeded from some one source; then I think all the grand leading facts of geographical distribution are explicable on the theory of migration... together with subsequent modification and the multiplication of new forms." We can thus understand how it is that in some countries the inhabitants "are linked to the extinct beings which formerly inhabited the same continent."

We can see why two areas, having nearly the same physical conditions, should often be inhabited by very different forms of life,... and "we can see why in two areas, however distant from each other, there should be a correlation, in the presence of identical species... and of distinct but representative species." ("The Origin of Species" (1st edition), pages 408, 409.)

Darwin's reluctance to a.s.sume great geological changes, such as a land-connection of Europe with North America, is easily explained by the fact that he restricted himself to the distribution of the present and comparatively recent species. "I do not believe that it will ever be proved that within the recent period continents which are now quite separate, have been continuously, or almost continuously, united with each other, and with the many existing oceanic islands." (Ibid. page 357.) Again, "believing... that our continents have long remained in nearly the same relative position, though subjected to large, but partial oscillations of level," that means to say within the period of existing species, or "within the recent period." (Ibid. page. 370.) The difficulty was to a great extent one of his own making. Whilst almost everybody else believed in the immutability of the species, which implies an enormous age, logically since the dawn of creation, to him the actually existing species as the latest results of evolution, were necessarily something very new, so young that only the very latest of the geological epochs could have affected them. It has since come to our knowledge that a great number of terrestrial "recent" species, even those of the higher cla.s.ses of Vertebrates, date much farther back than had been thought possible. Many of them reach well into the Miocene, a time since which the world seems to have a.s.sumed the main outlines of the present continents.

In the year 1866 appeared A. Murray's work on the "Geographical Distribution of Mammals", a book which has perhaps received less recognition than it deserves. His treatment of the general introductory questions marks a considerable advance of our problem, although, and partly because, he did not entirely agree with Darwin's views as laid down in the first edition of "The Origin of Species", which after all was the great impulse given to Murray's work. Like Forbes he did not shrink from a.s.suming enormous changes in the configuration of the continents and oceans because the theory of descent, with its necessary postulate of great migrations, required them. He stated, for instance, "that a Miocene Atlantis sufficiently explains the common distribution of animals and plants in Europe and America up to the glacial epoch."

And next he considers how, and by what changes, the rehabilitation and distribution of these lands themselves were effected subsequent to that period. Further, he deserves credit for having cleared up a misunderstanding of the idea of specific centres of creation. Whilst for instance Schmarda a.s.sumed without hesitation that the same species, if occurring at places separated by great distances, or apparently insurmountable barriers, had been there created independently (multiple centres), Lyell and Darwin held that each species had only one single centre, and with this view most of us agree, but their starting point was to them represented by one individual, or rather one single pair.

According to Murray, on the other hand, this centre of a species is formed by all the individuals of a species, all of which equally undergo those changes which new conditions may impose upon them. In this respect a new species has a multiple origin, but this in a sense very different from that which was upheld by L. Aga.s.siz. As Murray himself puts it: "To my multiple origin, communication and direct derivation is essential.

The species is compounded of many influences brought together through many individuals, and distilled by Nature into one species; and, being once established it may roam and spread wherever it finds the conditions of life not materially different from those of its original centre."

(Murray, "The Geographical Distribution of Mammals", page 14. London, 1866.) This declaration fairly agrees with more modern views, and it must be borne in mind that the application of the single-centre principle to the genera, families and larger groups in the search for descent inevitably leads to one creative centre for the whole animal kingdom, a condition as unwarrantable as the myth of Adam and Eve being the first representatives of Mankind.

It looks as if it had required almost ten years for "The Origin of Species" to show its full effect, since the year 1868 marks the publication of Haeckel's "Naturliche Schoepfungsgeschichte" in addition to other great works. The terms "Oecology" (the relation of organisms to their environment) and "Chorology" (their distribution in s.p.a.ce) had been given us in his "Generelle Morphologie" in 1866. The fourteenth chapter of the "History of Creation" is devoted to the distribution of organisms, their chorology, with the emphatic a.s.sertion that "not until Darwin can chorology be spoken of as a separate science, since he supplied the acting causes for the elucidation of the hitherto acc.u.mulated ma.s.s of facts." A map (a "hypothetical sketch") shows the monophyletic origin and the routes of distribution of Man.

Natural Selection may be all-mighty, all-sufficient, but it requires time, so much that the countless aeons required for the evolution of the present fauna were soon felt to be one of the most serious drawbacks of the theory. Therefore every help to ease and shorten this process should have been welcomed. In 1868 M. Wagner (The first to formulate clearly the fundamental idea of a theory of migration and its importance in the origin of new species was L. von Buch, who in his "Physikalische Beschreibung der Canarischen Inseln", written in 1825, wrote as follows: "Upon the continents the individuals of the genera by spreading far, form, through differences of the locality, food and soil, varieties which finally become constant as new species, since owing to the distances they could never be crossed with other varieties and thus be brought back to the main type. Next they may again, perhaps upon different roads, return to the old home where they find the old type likewise changed, both having become so different that they can interbreed no longer. Not so upon islands, where the individuals shut up in narrow valleys or within narrow districts, can always meet one another and thereby destroy every new attempt towards the fixing of a new variety." Clearly von Buch explains here why island types remain fixed, and why these types themselves have become so different from their continental congeners.--Actually von Buch is aware of a most important point, the difference in the process of development which exists between a new species b, which is the result of an ancestral species a having itself changed into b and thereby vanished itself, and a new species c which arose through separation out of the same ancestral a, which itself persists as such unaltered. Von Buch's prophetic view seems to have escaped Lyell's and even Wagner's notice.) came to the rescue with his "Darwin'sche Theorie und das Migrations-Gesetz der Organismen". (Leipzig, 1868.) He shows that migration, i.e. change of locality, implies new environmental conditions (never mind whether these be new stimuli to variation, or only acting as their selectors or censors), and moreover secures separation from the original stock and thus eliminates or lessens the reactionary dangers of panmixia. Darwin accepted Wagner's theory as "advantageous." Through the heated polemics of the more ardent selectionists Wagner's theory came to grow into an alternative instead of a help to the theory of selectional evolution.

Separation is now rightly considered a most important factor by modern students of geographical distribution.

For the same year, 1868, we have to mention Huxley, whose Arctogaea and Notogaea are nothing less than the reconstructed main ma.s.ses of land of the Mesozoic period. Beyond doubt the configuration of land at that remote period has left recognisable traces in the present continents, but whether they can account for the distribution of such a much later group as the Gallinaceous birds is more than questionable. In any case he took for his text a large natural group of birds, cosmopolitan as a whole, but with a striking distribution. The Peristeropodes, or pigeon-footed division, are restricted to the Australian and Neotropical regions, in distinction to the Alectoropodes (with the hallux inserted at a level above the front toes) which inhabit the whole of the Arctogaea, only a few members having spread into the South World.

Further, as Asia alone has its Pheasants and allies, so is Africa characterised by its Guinea-fowls and relations, America has the Turkey as an endemic genus, and the Grouse tribe in a wider sense has its centre in the holarctic region: a splendid object lesson of descent, world-wide spreading and subsequent differentiation. Huxley, by the way, was the first--at least in private talk--to state that it will be for the morphologist, the well-trained anatomist, to give the casting vote in questions of geographical distribution, since he alone can determine whether we have to deal with h.o.m.ologous, or a.n.a.logous, convergent, representative forms.

It seems late to introduce Wallace's name in 1876, the year of the publication of his standard work. ("The Geographical Distribution of Animals", 2 vols. London, 1876.) We cannot do better than quote the author's own words, expressing the hope that his "book should bear a similar relation to the eleventh and twelfth chapters of the "Origin of Species" as Darwin's "Animals and Plants under Domestication" does to the first chapter of that work," and to add that he has amply succeeded.

Pleading for a few primary centres he accepts Sclater's six regions and does not follow Huxley's courageous changes which Sclater himself had accepted in 1874. Holding the view of the permanence of the oceans he accounts for the colonisation of outlying islands by further elaborating the views of Lyell and Darwin, especially in his fascinating "Island Life", with remarkable chapters on the Ice Age, Climate and Time and other fundamental factors. His method of arriving at the degree of relationship of the faunas of the various regions is eminently statistical. Long lists of genera determine by their numbers the affinity and hence the source of colonisation. In order to make sure of his material he performed the laborious task of evolving a new cla.s.sification of the host of Pa.s.serine birds. This statistical method has been followed by many authors, who, relying more upon quant.i.ty than quality, have obscured the fact that the key to the present distribution lies in the past changes of the earth's surface. However, with Wallace begins the modern study of the geographical distribution of animals and the sudden interest taken in this subject by an ever widening circle of enthusiasts far beyond the professional brotherhood.

A considerable literature has since grown up, almost bewildering in its range, diversity of aims and style of procedure. It is a chaos, with many paths leading into the maze, but as yet very few take us to a position commanding a view of the whole intricate terrain with its impenetrable tangle and pitfalls.

One line of research, not initiated but greatly influenced by Wallace's works, became so prominent as to almost const.i.tute a period which may be characterised as that of the search by specialists for either the justification or the amending of his regions. As cla.s.s after cla.s.s of animals was brought up to reveal the secret of the true regions, some authors saw in their different results nothing but the faultiness of previously established regions; others looked upon eventual agreements as their final corroboration, especially when for instance such diverse groups as mammals and scorpions could, with some ingenuity, be made to harmonise. But the obvious result of all these efforts was the growing knowledge that almost every cla.s.s seemed to follow principles of its own. The regions tallied neither in extent nor in numbers, although most of them gravitated more and more towards three centres, namely Australia, South America and the rest of the world. Still zoologists persisted in the search, and the various modes and capabilities of dispersal of the respective groups were thought sufficient explanation of the divergent results in trying to bring the mapping of the world under one scheme.

Contemporary literature is full of devices for the mechanical dispersal of animals. Marine currents, warm and cold, were favoured all the more since they showed the probable original homes of the creatures in question. If these could not stand sea-water, they floated upon logs or icebergs, or they were blown across by storms; fishes were lifted over barriers by waterspouts, and there is on record even an hypothetical land tortoise, full of eggs, which colonised an oceanic island after a perilous sea voyage upon a tree trunk. Accidents will happen, and beyond doubt many freaks of discontinuous distribution have to be accounted for by some such means. But whilst sufficient for the scanty settlers of true oceanic islands, they cannot be held seriously to account for the rich fauna of a large continent, over which palaeontology shows us that the immigrants have pa.s.sed like waves. It should also be borne in mind that there is a great difference between flotsam and jetsam. A current is an extension of the same medium and the animals in it may suffer no change during even a long voyage, since they may be brought from one litoral to another where they will still be in the same or but slightly altered environment. But the jetsam is in the position of a pa.s.senger who has been carried off by the wrong train. Almost every year some American land birds arrive at our western coasts and none of them have gained a permanent footing although such visits must have taken place since prehistoric times. It was therefore argued that only those groups of animals should be used for locating and defining regions which were absolutely bound to the soil. This method likewise gave results not reconcilable with each other, even when the distribution of fossils was taken into account, but it pointed to the absolute necessity of searching for former land-connections regardless of their extent and the present depths to which they may have sunk.

That the key to the present distribution lies in the past had been felt long ago, but at last it was appreciated that the various cla.s.ses of animals and plants have appeared in successive geological epochs and also at many places remote from each other. The key to the distribution of any group lies in the configuration of land and water of that epoch in which it made its first appearance. Although this sounds like a plat.i.tude, it has frequently been ignored. If, for argument's sake, Amphibia were evolved somewhere upon the great southern land-ma.s.s of Carboniferous times (supposed by some to have stretched from South America across Africa to Australia), the distribution of this developing cla.s.s must have proceeded upon lines altogether different from that of the mammals which dated perhaps from lower Tria.s.sic times, when the old south continental belt was already broken up. The broad lines of this distribution could never coincide with that of the other, older cla.s.s, no matter whether the original mammalian centre was in the Afro-Indian, Australian, or Brazilian portion. If all the various groups of animals had come into existence at the same time and at the same place, then it would be possible, with sufficient geological data, to construct a map showing the generalised results applicable to the whole animal kingdom.

But the premises are wrong. Whatever regions we may seek to establish applicable to all cla.s.ses, we are necessarily mixing up several principles, namely geological, historical, i.e. evolutionary, with present day statistical facts. We might as well attempt one compound picture representing a chick's growth into an adult bird and a child's growth into manhood.

In short there are no general regions, not even for each cla.s.s separately, unless this cla.s.s be one which is confined to a comparatively short geological period. Most of the great cla.s.ses have far too long a history and have evolved many successive main groups.

Let us take the mammals. Marsupials live now in Australia and in both Americas, because they already existed in Mesozoic times; Ungulata existed at one time or other all over the world except in Australia, because they are post-Cretaceous; Insectivores, although as old as any Placentalia, are cosmopolitan excepting South America and Australia; Stags and Bears, as examples of comparatively recent Arctogaeans, are found everywhere with the exception of Ethiopia and Australia. Each of these groups teaches a valuable historical lesson, but when these are combined into the establishment of a few mammalian "realms," they mean nothing but statistical majorities. If there is one at all, Australia is such a realm backed against the rest of the world, but as certainly it is not a mammalian creative centre!

Well then, if the idea of generally applicable regions is a mare's nest, as was the search for the Holy Grail, what is the object of the study of geographical distribution? It is nothing less than the history of the evolution of life in s.p.a.ce and time in the widest sense. The attempt to account for the present distribution of any group of organisms involves the aid of every branch of science. It bids fair to become a history of the world. It started in a mild, statistical way, restricting itself to the present fauna and flora and to the present configuration of land and water. Next came Oceanography concerned with the depths of the seas, their currents and temperatures; then enquiries into climatic changes, culminating in irreconcilable astronomical hypotheses as to glacial epochs; theories about changes of the level of the seas, mainly from the point of view of the physicist and astronomer. Then came more and more to the front the importance of the geological record, hand in hand with the palaeontological data and the search for the natural affinities, the genetic system of the organisms. Now and then it almost seems as if the biologists had done their share by supplying the problems and that the physicists and geologists would settle them, but in reality it is not so. The biologists not only set the problems, they alone can check the offered solutions. The mere fact of palms having flourished in Miocene Spitzbergen led to an hypothetical shifting of the axis of the world rather than to the a.s.sumption, by way of explanation, that the palms themselves might have changed their nature. One of the most valuable aids in geological research, often the only means for reconstructing the face of the earth in by-gone periods, is afforded by fossils, but only the morphologist can p.r.o.nounce as to their trustworthiness as witnesses, because of the danger of mistaking a.n.a.logous for h.o.m.ologous forms. This difficulty applies equally to living groups, and it is so important that a few instances may not be amiss.

There is undeniable similarity between the faunas of Madagascar and South America. This was supported by the Centetidae and Dendrobatidae, two entire "families," as also by other facts. The value of the Insectivores, Solenodon in Cuba, Centetes in Madagascar, has been much lessened by their recognition as an extremely ancient group and as a case of convergence, but if they are no longer put into the same family, this amendment is really to a great extent due to their widely discontinuous distribution. The only systematic difference of the Dendrobatidae from the Ranidae is the absence of teeth, morphologically a very unimportant character, and it is now agreed, on the strength of their distribution, that these little arboreal, conspicuously coloured frogs, Dendrobates in South America, Mantella in Madagascar, do not form a natural group, although a third genus, Cardioglossa in West Africa, seems also to belong to them. If these creatures lived all on the same continent, we should unhesitatingly look upon them as forming a well-defined, natural little group. On the other hand the Aglossa, with their three very divergent genera, namely Pipa in South America, Xenopus and Hymenochirus in Africa, are so well characterised as one ancient group that we use their distribution unhesitatingly as a hint of a former connection between the two continents. We are indeed arguing in vicious circles. The Rat.i.tae as such are absolutely worthless since they are a most heterogeneous a.s.sembly, and there are untold groups, of the artificiality of which many a zoo-geographer had not the slightest suspicion when he took his statistical material, the genera and families, from some systematic catalogues or similar lists. A lamentable instance is that of certain flightless Rails, recently extinct or sub-fossil, on the isalnds of Mauritius, Rodriguez and Chatham. Being flightless they have been used in support of a former huge Antarctic continent, instead of ruling them out of court as Rails which, each in its island, have lost the power of flight, a process which must have taken place so recently that it is difficult, upon morphological grounds, to justify their separation into Aphanapteryx in Mauritius, Erythromachus in Rodriguez and Diaphorapteryx on Chatham Island.

Morphologically they may well form but one genus, since they have sprung from the same stock and have developed upon the same lines; they are therefore monogenetic: but since we know that they have become what they are independently of each other (now unlike any other Rails), they are polygenetic and therefore could not form one genus in the old Darwinian sense. Further, they are not a case of convergence, since their ancestry is not divergent but leads into the same stratum.

THE RECONSTRUCTION OF THE GEOGRAPHY OF SUCCESSIVE EPOCHS.

A promising method is the study by the specialist of a large, widely distributed group of animals from an evolutionary point of view.

Good examples of this method are afforded by A.E. Ortmann's ("The geographical distribution of Freshwater Decapods and its bearing upon ancient geography", "Proc. Amer. Phil. Soc." Vol. 41, 1902.) exhaustive paper and by A.W. Grabau's "Phylogeny of Fusus and its Allies"

("Smithsonian Misc. Coll." 44, 1904.) After many important groups of animals have been treated in this way--as yet sparingly attempted--the results as to hypothetical land-connections etc. are sure to be corrective and supplementary, and their problems will be solved, since they are not imaginary.

The same problems are attacked, in the reverse way, by starting with the whole fauna of a country and thence, so to speak, letting the research radiate. Some groups will be considered as autochthonous, others as immigrants, and the directions followed by them will be inquired into; the search may lead far and in various directions, and by comparison of results, by making compound maps, certain routes will a.s.sume definite shape, and if they lead across straits and seas they are warrants to search for land-connections in the past. (A fair sample of this method is C.H. Eigenmann's "The Freshwater Fishes of South and Middle America", "Popular Science Monthly", Vol. 68, 1906.) There are now not a few maps purporting to show the outlines of land and water at various epochs.

Many of these attempts do not tally with each other, owing to the lamentable deficiencies of geological and fossil data, but the bolder the hypothetical outlines are drawn, the better, and this is preferable to the insertion of bays and similar detail which give such maps a fallacious look of certainty where none exists. Moreover it must be borne in mind that, when we draw a broad continental belt across an ocean, this belt need never have existed in its entirety at any one time. The features of dispersal, intended to be explained by it, would be accomplished just as well by an unknown number of islands which have joined into larger complexes while elsewhere they subsided again: like pontoon-bridges which may be opened anywhere, or like a series of superimposed dissolving views of land and sea-scapes. Hence the reconstructed maps of Europe, the only continent tolerably known, show a considerable number of islands in puzzling changes, while elsewhere, e.g. in Asia, we have to be satisfied with sweeping generalisations.

At present about half-a-dozen big connections are engaging our attention, leaving as comparatively settled the extent and the duration of such minor "bridges" as that between Africa and Madagascar, Tasmania and Australia, the Antilles and Central America, Europe and North Africa. (Not a few of those who are fascinated by, and satisfied with, the statistical aspect of distribution still have a strong dislike to the use of "bridges" if these lead over deep seas, and they get over present discontinuous occurrences by a former "universal or sub-universal distribution" of their groups.) This is indeed an easy method of cutting the knot, but in reality they shunt the question only a stage or two back, never troubling to explain how their groups managed to attain to that sub-universal range; or do they still suppose that the whole world was originally one paradise where everything lived side by side, until sin and strife and glacial epochs left nothing but scattered survivors?

The permanence of the great ocean-basins had become a dogma since it was found that a universal elevation of the land to the extent of 100 fathoms would produce but little changes, and when it was shown that even the 1000 fathom-line followed the great ma.s.ses of land rather closely, and still leaving the great basins (although transgression of the sea to the same extent would change the map of the world beyond recognition), by general consent one mile was allowed as the utmost speculative limit of subsidence. Naturally two or three miles, the average depth of the oceans, seems enormous, and yet such a difference in level is as nothing in comparison with the size of the Earth. On a clay model globe ten feet in diameter an ocean bed three miles deep would scarcely be detected, and the highest mountains would be smaller than the unavoidable grains in the glazed surface of our model. There are but few countries which have not be submerged at some time or other.

CONNECTION OF SOUTH EASTERN ASIA WITH AUSTRALIA. Neumayr's Sino-Australian continent during mid-Mesozoic times was probably a much changing Archipelago, with final separations subsequent to the Cretaceous period. Henceforth Australasia was left to its own fate, but for a possible connection with the antarctic continent.