Darwin, and After Darwin Volume I Part 9

[Ill.u.s.tration: FIG. 90.--Transformation of _Strombus_. (After Heilprin.) 1, 1_a_, _Strombus Leidy_ (1, typical), Pliocene; 2, 2_a_, _Strombus accipitrinus_ (2_a_ typical) Recent.]

Lastly, attention may here again be directed to the very instructive series of sh.e.l.ls which has already been shown in a previous chapter, and which serves to ill.u.s.trate the successive geological forms of _Paludina_ from the Tertiary beds of Slavonia, as depicted by Prof. Neumayr of Vienna. (Fig. 1, p. 19.)

CHAPTER VI.

GEOGRAPHICAL DISTRIBUTION.

The argument from geology is the argument from the distribution of species in time. I will next take the argument from the distribution of species in s.p.a.ce--that is, the present geographical distribution of plants and animals.

Seeing that the theory of descent with adaptive modification implies slow and gradual change of one species into another, and progressively still more slow and gradual changes of one genus, family, or order into another genus, family, or order, we should expect on this theory that the organic types living on any given geographical area would be found to resemble or to differ from organic types living elsewhere, according as the area is connected with or disconnected from other geographical areas. For instance, the large continental islands of Australia and New Zealand are widely disconnected from all other lands of the world, and deep sea soundings show that they have probably been thus disconnected, either since the time of their origin, or, at the least, through immense geological epochs. The theory of evolution, therefore, would expect to find two general facts with regard to the inhabitants of these islands.

First, that the inhabitants should form, as it were, little worlds of their own, more or less unlike the inhabitants of any other parts of the globe. And next, that some of these inhabitants should present us with independent information touching archaic forms of life. For it is manifestly most improbable that the course of evolutionary history should have run exactly parallel in the case of these isolated oceanic continents and in continents elsewhere. Australia and New Zealand, therefore, ought to present a very large number, not only of peculiar species and genera, but even of families, and possibly of orders. Now this is just what Australia and New Zealand do present. The case of the dog being doubtful, there is an absence of all mammalian life, except that of one of the oldest and least highly developed orders, the Marsupials. There even occurs a unique order, still lower in the scale of organization--so low, in fact, that it deserves to be regarded as but nascent mammalian: I mean, of course, the Monotremata. As regards Birds, we have the peculiar wingless forms alluded to in a previous chapter (viz. that on Morphology); and, without waiting to go into details, it is notorious that the faunas of Australia and New Zealand are not only highly peculiar, but also suggestively archaic. Therefore, in both the respects above mentioned, the antic.i.p.ations of our theory are fully borne out. But as it would take too long to consider, even cursorily, the faunas and floras of these immense islands, I here allude to them only for the sake of ill.u.s.tration. In order to present the argument from geographical distribution within reasonable limits, I think it is best to restrict our examination to smaller areas; for these will better admit of brief and yet adequate consideration. But of course it will be understood that the less isolated the region, and the shorter the time that it has been isolated, the smaller amount of peculiarity should we expect to meet with on the part of its present inhabitants. Or, conversely stated, the longer and the greater the isolation, the more peculiarity of species would our theory expect to find. The object of the present chapter will be to show that these, and other cognate expectations, are fully realized by facts; but, before proceeding to do this, I must say a few words on the antecedent standing of the argument.

Where the question is, as at present, between the rival theories of special creation and gradual trans.m.u.tation, it may at first sight well appear that no test can be at once so crucial and so easily applied as this of comparing the species of one geographical area with those of another, in order to see whether there is any constant correlation between differences of type and degrees of separation. But a little further thought is enough to show that the test is not quite so simple or so absolute--that it is a test to be applied in a large and general way over the surface of the whole earth, rather than one to be relied upon as exclusively rigid in every special case.

In the first place, there is the obvious consideration that lands or seas which are discontinuous now may not always have been so, or not for long enough to admit of the effects of separation having been exerted to any considerable extent upon their inhabitants. Next, there is the scarcely less important consideration, that although land areas may long have been separated from one another by extensive tracts of ocean, birds and insects may more or less easily have been able to fly from one to the other; while even non-flying animals and plants may often have been transported by floating ice or timber, wind or water currents, and sundry other means of dispersal. Again, there is the important influence of climate to be taken into account. We know from geological evidence that in the course of geological time the self-same continents have been submitted to enormous changes of temperature--varying in fact from polar cold to almost tropical heat; and as it is manifestly impossible that forms of life suited to one of these climates could have survived during the other, we can here perceive a further and most potent cause interfering with the test of geographical distribution as indiscriminately applied in all cases. When the elephant and hippopotamus were flouris.h.i.+ng in England amid the luxuriant vegetation which these large animals require, it is evident that scarcely any one species of either the fauna or the flora of this country can have been the same as it was when its African climate gave place to that of Greenland. Therefore, as Mr. Wallace observes, "If glacial epochs in temperate lands and mild climates near the poles have, as now believed by men of eminence, occurred several times over in the past history of the earth, the effects of such great and repeated changes both on migration, modification, and extinction of species, must have been of overwhelming importance--of more importance perhaps than even the geological changes of sea and land."

But although for these, and certain other less important reasons which I need not wait to detail, we must conclude that the evidence from geographical distribution is not to be regarded as a crucial test between the rival theories of creation and evolution in all cases indiscriminately, I must next remark that it is undoubtedly one of the strongest lines of evidence which we possess. When we once remember that, according to the general theory of evolution itself, the present geographical distribution of plants and animals is "the visible outcome or residual product of the whole past history of the earth," and, therefore, that of the conditions determining the characters of life inhabiting this and that particular area continuity or discontinuity with other areas is but one,--when we remember this, we find that no further reservation has to be made: all the facts of geographical distribution speak with one consent in favour of the naturalistic theory.

The first of these facts which I shall adduce is, that although the geographical range of any given species is, as a rule, continuous, such is far from being always the case. Very many species have more or less discontinuous ranges--the mountain-hare, for instance, extending from the Arctic regions over the greater portion of Europe to the Ural Mountains and the Caucasus, and yet over all this enormous tract appearing only in isolated or discontinuous patches, where there happen to be either mountain ranges or climates cold enough to suit its nature.

Now, in all such cases of discontinuity in the range of a species the theory of evolution has a simple explanation to offer--namely, either that some representatives of the species have at some former period been able to migrate from one region to the other, or else that at one time the species occupied the whole of the range in question, but afterwards became broken up as geographical, climatic, or other changes rendered parts of the area unfit for the species to inhabit. Thus, for instance, it is easy to understand that during the last cold epoch the mountain-hare would have had a continuous range; but that as the Arctic climate gradually receded to polar regions, the species would be able to survive in southern lat.i.tudes only on mountain ranges, and thus would become broken up into many discontinuous patches, corresponding with these ranges. In the same way we can explain the occurrence of Arctic vegetation on the Alps and Pyrenees--namely, as left behind by the retreat of the Arctic climate at the close of the glacial period.

But now, on the other hand, the theory of special creation cannot so well afford to render this obvious explanation of discontinuity. In the case of the Arctic flora of the Alps, for instance, although it is true that much of this vegetation is of an Arctic type, it is not true that the species are all identical with those which occur in the Arctic regions. Therefore the theory of special creation would here have to a.s.sume that, although the now common species were left behind on the Alps by the retreat of glaciation northwards, the peculiar Alpine species were afterwards created separately upon the Alps, and yet created with such close affinities to the pre-existing species as to be included with them under the same genera. Looking to the absurdity of this supposition, as well as of others which I need not wait to mention, certain advocates of special creation have sought to take refuge in another hypothesis--namely, that species which present a markedly discontinuous range may have had a corresponding number of different centres of creation, the same specific type having been turned down, so to speak, on widely separated areas. But to me it seems that this explanation presents even greater difficulty than the other. If it is difficult to say why the Divinity should have chosen to create new species of plants on the Alps on so precisely the same pattern as the old, much more would it be difficult to say why, in addition to these new species, he should also have created again the old species which he had already placed in the Arctic regions.

So much, then, for discontinuity of distribution. The next general fact to be adduced is, that there is no constant correlation between habitats and animals or plants suited to live upon them. Of course all the animals and plants living upon any given area are well suited to live upon that area; for otherwise they could not be there. But the point now is, that besides the area on which they do live, there are usually many other areas in different parts of the globe where they might have lived equally well--as is proved by the fact that when transported by man they thrive as well, or even better, than in their native country. Therefore, upon the supposition that all species were separately created in the countries where they are respectively found, we must conclude that they were created in only some of the places where they might equally well have lived. Probably there is at most but a small percentage either of plants or animals which would not thrive in some place, or places, on the earth's surface other than that in which they occur; and hence we must say that one of the objects of special creation--if this be the true theory--was that of depositing species in only some among the several parts of the earth's surface equally well suited to support them.

Now, I do not contend that this fact in itself raises any difficulty against the theory of special creation. But I do think that a very serious difficulty is raised when to this fact we add another--namely, that on every biological region we encounter species related to other species in genera, and usually also genera related to other genera in families. For if each of all the const.i.tuent species of a genus, and even of a family, were separately created, we must hence conclude that in depositing them there was an unaccountable design manifested to make areas of distribution correspond to the natural affinities of their inhabitants. For example, the humming-birds are geographically restricted to America, and number 120 genera, comprising over 400 species. Hence, if this betokens 400 separate acts of creation, it cannot possibly have been due to chance that they were all performed on the same continent: it must have been design which led to every species of this large family of birds having been deposited in one geographical area. Or, to take a case where only the species of a single genus are concerned. The rats and mice proper const.i.tute a genus which comprises altogether more than 100 species, and they are all exclusively restricted to the Old World. In the New World they are represented by another genus comprising about 70 species, which resemble their Old World cousins in form and habits; but differ from them in dent.i.tion and other such minor points. Now, the question is,--Why should all the 100 species have been separately created on one side of the Atlantic with one pattern of dent.i.tion, and all the 70 species on the other side with another pattern? What has the Atlantic Ocean got to do with any "archetypal plan" of rats' teeth?

Or again, to recur to Australia, why should all the mammalian forms of life be restricted to the one group of Marsupials, when we know that not only the Rodents, such as the rabbit, but all other orders of mammals, would thrive there equally well. And similarly, of course, in countless other instances. Everywhere we meet with this same correlation between areas of distribution and affinities of cla.s.sification.

Now, it is at once manifest how completely this general fact harmonizes with the theory of evolution. If the 400 species of humming-birds, for instance, are all modified descendants of common ancestors, and if none of their const.i.tuent individuals have ever been large enough to make their way across the oceans which practically isolate their territory from all other tropical and sub-tropical regions of the globe, then we can understand why it is that all the 400 species occupy the same continent. But on the special-creation theory we can see no reason why the 400 species should all have been deposited in America. And, as already observed, we must remember that this correlation between a geographically restricted habitat and the zoological or botanical affinities of its inhabitants, is repeated over and over and over again in the faunas and floras of the world, so that merely to enumerate the instances would require a separate chapter.

Furthermore, the general argument thus presented in favour of descent with continuous modification admits of being enormously strengthened by three different cla.s.ses of additional facts.

The first is, that the correlation in question--namely, that between a geographically restricted habitat and the zoological or botanical affinities of its inhabitants--is not limited to the now existing species, but extends also to the extinct. That is to say, the dead species are allied to the living species, as we should expect that they must be, if the latter are modified descendants of the former. On the alternative theory, however, we have to suppose that the policy of maintaining a correlation between geographical restriction and natural affinity extends very much further back than even the existing species of plants and animals; indeed we must suppose that a practically infinite number of additional acts of separate creation were governed by the same policy, in the case of long lines of species long since extinct.

Thus far, then, the only answer which an advocate of special creation can adduce is, that for some reason unknown to us such a policy may have been more wise than it appears: it may have served some inscrutable purpose that allied products of distinct acts of creation should all be kept together on the same areas. Well, in answer to this unjustifiable appeal to the argument from ignorance, I will adduce the second of the three considerations. This is, that in cases where the geographical areas are not restricted the policy in question fails. In other words, where the inhabitants of an area are free to migrate to other areas, the policy of correlating affinity with distribution is most significantly forgotten. In this case species wander away from their native homes, and the course of their wanderings is marked by the origination of new species springing up en route. Now, is it reasonable to suppose that the mere circ.u.mstance of some members of a species being able to leave their native home should furnish any occasion for creating new and allied species upon the tracts over which they travel, or the territories to which they go? When the 400 existing species of humming-birds have all been created on the same continent for some reason supposed to be unknown, why should this reason give way before the accident of any means of migration being furnished to humming-birds, so that they should be able to visit, say the continents of Africa and Asia, there gain a footing beside the sun-birds, and henceforth determine a new centre for the separate creation of additional species of humming-birds peculiar to the Old World--as has happened in the case of the majority of species which, unlike the humming-birds, have been at any time free to migrate from their original homes?

Lastly, my third consideration is, that the supposed policy in question does not extend to affinities which are wider than those between species and genera--more rarely to families, scarcely ever to orders, and never to cla.s.ses. In other words, nature shows a double correlation in her geographical distribution of organic types:--first, that which we have already considered between geographical restriction and natural affinity among inhabitants of the same areas; second, another of a more detailed character between _degrees_ of geographical restriction and _degrees_ of natural affinity. The more distant the affinity, the more general is the extension. This, of course, is what we should expect on the theory of descent with modification, because the more distant the affinity, and therefore, _ex hypothesi_, the larger and the older the original group of organisms, the greater must be the chance of dispersal. The 400 species of humming-birds may well be unable to migrate from their native continent; but it would indeed have been an unaccountable fact if no other species of all the cla.s.s of birds had ever been able to have crossed the Atlantic Ocean. Thus, on the theory of evolution, we can well understand the second correlation now before us--namely, between remoteness of affinity and generality of dispersal,--so that there is no considerable portion of the habitable globe without representatives of all the cla.s.ses of animals, few portions without representatives of all the orders, but many portions without many of the families, innumerable portions without innumerable genera, and, of course, all portions without the great majority of species. Now, while this general correlation thus obviously supports the theory of natural descent with progressive modification, it makes directly against the opposite theory of special creation. For we have recently seen that when we restrict our view to the case of species and genera, the theory of special creation is obliged to suppose that for some inscrutable reason the Deity had regard to systematic affinity while determining on what large areas to create his species[20]. But now we see that he must be held to have neglected this inscrutable reason (whatever it was) when he pa.s.sed beyond the range of genera--and this always in proportion to the remoteness of systematic affinity on the part of the species concerned.

[20] I say "_large_ areas" for the sake of argument; but the same correlation between distribution and affinity extends likewise to _small_ areas where only _small_ differences of affinity are concerned. Thus, for instance, speaking of smaller areas, Moritz Wagner says:--"The broader and more rapid the river, the higher and more regular the mountain-chain, the calmer and more extensive the sea, the more considerable, as a general rule, will be the taxonomic separation between the populations"; and he shows that, in correlation with such differences in the _degrees_ of separation, are the _degrees_ of diversification--i. e., the _numbers_ of species, and even of varieties, which these topographical barriers determine.

I cannot well conceive a _reductio ad absurdum_ more complete than this.

But, having now presented these most general facts of geographical distribution in their relation to the issue before us, we may next proceed to consider a few ill.u.s.trations of them in detail, for in this way I think that their overwhelming weight may become yet more abundantly apparent.

It will a.s.sist us in dealing with these detailed ill.u.s.trations if we begin by considering the means of dispersal of organisms from one place to another. Of course the most ordinary means is that of continuous wandering, or emigration; but where geographical barriers of any kind have to be surmounted, organisms may only be able to pa.s.s them by more exceptional and accidental means. The princ.i.p.al barriers of a geographical kind are oceans, rivers, mountain-chains, and desert-tracts, in the case of terrestrial organisms; and, in the case of aquatic organisms, the presence of land. But it is to be observed that, as regards marine organisms, any considerable difference in the temperature of the water may const.i.tute a barrier as effectual as the presence of land; and also that, in the case of all shallow-water faunas, a tract of deep ocean const.i.tutes almost as complete a barrier as it does to terrestrial faunas.

Now, the means whereby barriers admit of being accidentally or occasionally surmounted are, of course, various; and they differ in the case of different organisms. Birds, bats, and insects, on account of their powers of flight, are particularly apt to be blown out great distances to sea, and hence of all animals are most likely to become the involuntary colonists of distant sh.o.r.es. Floating timber serves to convey seeds and eggs of small animals over great distances; and Darwin has shown that many kinds of seeds are able of themselves to float for more than a month in sea-water without losing their powers of germination. For instance, out of 87 kinds, 64 germinated after an immersion of 28 days, and a few survived an immersion of 137 days. As a result of all his experiments he concludes, that the seeds of at least ten per cent. of the species of plants of any country might be floated by sea-currents during 28 days, without losing their powers of germination; and this, at the average rate of flow of several Atlantic currents, would serve to transport the seeds to a distance of at least 900 miles. Again, he proved that even seeds which are quickly destroyed by contact with sea-water admit of being successfully transported during 30 days, if they be contained within the crop of a dead bird. He also proved that living birds are most active agents in the work of dissemination, and this not only by taking seeds into their crops (where, so long as they remain, the seeds are uninjured), but likewise by carrying seeds (and even young mollusks) attached to their feet and feathers. In the course of these experiments he found that a small cup-full of mud, which he gathered from the edges of three ponds in February, was so charged with seeds that when sown in the ground these few ounces of mud yielded no less than 537 plants, belonging to many different species. It is therefore evident what opportunities are thus afforded for the transportation of seeds on the feet and bills of wading-birds. Lastly, floating ice is well known to act as a carrier of any kind of life which may prove able to survive this mode of transit.

Such being the nature of geographical barriers, and the means that organisms of various kinds may occasionally have of overcoming them, I will now give a few detailed ill.u.s.trations of the argument from geographical distribution, as previously presented in its general form.

To begin with aquatic animals. As Darwin remarks, "the marine inhabitants of the Eastern and Western sh.o.r.es of South America are very distinct; with extremely few sh.e.l.ls, crustacea, or echinodermata in common." Again, westward of the sh.o.r.es of America, a wide s.p.a.ce of open ocean extends, which, as we have seen, furnishes as effectual a barrier as does the land to any emigration of shallow-water animals. Now, as soon as this reach of deep water is pa.s.sed, we meet in the eastern islands of the Pacific with another and totally distinct fauna. "So that three marine faunas range northward and southward in parallel lines not far from each other, under corresponding climates": they are, however, "separated from each other by impa.s.sable barriers, either of land or open sea": and it is in exact coincidence with the course of these barriers that we find so remarkable a differentiation of the faunas[21].

Obviously, therefore, it is impossible to suggest that this correlation is accidental. Altogether many thousands of species are involved, and within this comparatively limited area they are sharply marked off into three groups as to their natural affinities, and into three groups as to their several basins. Hence, if all these species were separately created, there is no escape from the conclusion that for some reason or another the act of creation was governed by the presence of these barriers, so that species deposited on the Eastern sh.o.r.es of South America were formed with one set of natural affinities, while species deposited on the Western sh.o.r.e were formed with another set; and similarly with regard to the third set of species in the third basin, which, extending over a whole hemisphere to the coast of Africa without any further barrier, nowhere presents, over this vast area, any other case of a distinct marine fauna. But what conceivable reason can there have been thus to consult these geographical barriers in the original creation of specific types? Even if such a case stood alone, it would be strongly suggestive of error on the part of the special creation theory. But let us take another case, this time from fresh-water faunas.

[21] The only exception is in the case of the fish on each side of the isthmus of panama, where about 30 per cent, of the species are identical. But it is possible enough that at some previous time this narrow isthmus may have been even narrower than at present, if not actually open. At all events, the fact that this partial exception occurs just where the land-barrier is so narrow, is more suggestive of migration than of independent creation.

Although the geographical distribution of fresh-water fish and fresh-water sh.e.l.ls is often surprisingly extensive and apparently capricious, this may be explained by the means of dispersal being here so varied--not only aquatic birds, floods, and whirlwinds, but also geographical changes of water-shed having all a.s.sisted in the process.

Moreover, in some cases it is possible that the habits of more widely distributed fresh-water fish may have originally been wholly or partly marine--which, of course, would explain the existing discontinuity of their existing fresh-water distribution. But, be this as it may (and it is not a question that affects the issue between special creation and gradual evolution, since it is only a question as to how a given species has been dispersed from its original home, whether or not in that home it was specially created), the point I desire to bring forward is, that where we find a barrier to the emigration of fresh-water forms which is more formidable than a thousand miles of ocean--a barrier over which neither water-fowl nor whirlwinds are likely to pa.s.s, and which is above the reach of any geological changes of water-shed,--where we find such a barrier, we always find a marked difference in the fresh-water faunas on either side of it. The kind of barrier to which I allude is a high mountain-chain. It may be only a few miles wide; yet it exercises a greater influence on the diversification of specific types, where fresh-water faunas are concerned, than almost any other. But why should this be the case on any intelligible theory of special creation? Why, in the depositing of species of newly created fresh-water fish, should the presence of an impa.s.sable mountain-chain have determined so uniformly a difference of specific affinity on either side of it? The question, so far as I can see, does not admit of an answer from any reasonable opponent.

Turning now from aquatic organisms to terrestrial, the body of facts from which to draw is so large, that I think the s.p.a.ce at my disposal may be best utilized by confining attention to a single division of them--that, namely, which is furnished by the zoological study of oceanic islands.

In the comparatively limited--but in itself extensive--cla.s.s of facts thus presented, we have a particularly fair and cogent test as between the alternative theories of evolution and creation. For where we meet with a volcanic island, hundreds of miles from any other land, and rising abruptly from an ocean of enormous depth, we may be quite sure that such an island can never have formed part of a now submerged continent. In other words, we may be quite sure that it always has been what it now is--an oceanic peak, separated from all other land by hundreds of miles of sea, and therefore an area supplied by nature for the purpose, as it were, of testing the rival theories of creation and evolution. For, let us ask, upon these tiny insular specks of land what kind of life should we expect to find? To this question the theories of special creation and of gradual evolution would agree in giving the same answer up to a certain point. For both theories would agree in supposing that these islands would, at all events in large part, derive their inhabitants from accidental or occasional arrivals of wind-blown or water-floated organisms from other countries--especially, of course, from the countries least remote. But, after agreeing upon this point, the two theories must part company in their antic.i.p.ations. The special-creation theory can have no reason to suppose that a small volcanic island in the midst of a great ocean should be chosen as the theatre of any extraordinary creative activity, or for any particularly rich manufacture of peculiar species to be found nowhere else in the world. On the other hand, the evolution theory would expect to find that such habitats are stocked with more or less peculiar species. For it would expect that when any organisms chanced to reach a wholly isolated refuge of this kind, their descendants should forthwith have started upon an independent course of evolutionary history. Protected from intercrossing with any members of their parent species elsewhere, and exposed to considerable changes in their conditions of life, it would indeed be fatal to the general theory of evolution if these descendants, during the course of many generations, were not to undergo appreciable change. It has happened on two or three occasions that European rats have been accidentally imported by s.h.i.+ps upon some of these islands, and even already it is observed that their descendants have undergone a slight change of appearance, so as to const.i.tute them what naturalists call local varieties. The change, of course, is but slight, because the time allowed for it has been so short. But the longer the time that a colony of a species is thus completely isolated under changed conditions of life the greater, according to the evolution theory, should we expect the change to become. Therefore, in all cases where we happen to know, from independent evidence of a geological kind, that an oceanic island is of very ancient formation, the evolution theory would expect to encounter a great wealth of peculiar species. On the other hand, as I have just observed, the special-creation theory can have no reason to suppose that there should be any correlation between the age of an oceanic island and the number of peculiar species which it may be found to contain.

Therefore, having considered the principles of geographical distribution from the widest or most general point of view, we shall pa.s.s to the opposite extreme, and consider exhaustively, or in the utmost possible detail, the facts of such distribution where the conditions are best suited to this purpose--that is, as I have already said, upon oceanic islands, which may be metaphorically regarded as having been formed by nature for the particular purpose of supplying naturalists with a crucial test between the theories of creation and evolution. The material upon which my a.n.a.lysis is to be based will be derived from the most recent works upon geographical distribution--especially from the magnificent contributions to this department of science which we owe to the labours of Mr. Wallace. Indeed, all that follows may be regarded as a condensed filtrate of the facts which he has collected. Even as thus restricted, however, our subject-matter would be too extensive to be dealt with on the present occasion, were we to attempt an exhaustive a.n.a.lysis of the floras and faunas of all oceanic islands upon the face of the globe. Therefore, what I propose to do is to select for such exhaustive a.n.a.lysis a few of what may be termed the most oceanic of oceanic islands--that is to say, those oceanic islands which are most widely separated from mainlands, and which, therefore, furnish the most unquestionable of test cases as between the theories of special creation and genetic descent.

_Azores._--A group of volcanic islands, nine in number, about 900 miles from the coast of Portugal, and surrounded by ocean depths of 1,800 to 2,500 fathoms. There is geological evidence that the origin of the group dates back at least as far as Miocene times. There is a total absence of all terrestrial Vertebrata, other than those which are known to have been introduced by man. Flying animals, on the other hand, are abundant; namely, 53 species of birds, one species of bat, a few species of b.u.t.terflies, moths, and hymenoptera, with 74 species of indigenous beetles. All these animals are unmodified European species, with the exception of one bird and many of the beetles. Of the 74 indigenous species of the latter, 36 are not found in Europe; but 19 are natives of Madeira or the Canaries, and 3 are American, doubtless transplanted by drift-wood. The remaining 14 species occur nowhere else in the world, though for the most part they are allied to other European species.

There are 69 known species of land-sh.e.l.ls, of which 37 are European, and 32 peculiar, though all allied to European forms. Lastly, there are 480 known species of plants, of which 40 are peculiar, though allied to European species.

_Bermudas._--A small volcanic group of islands, 700 miles from North Carolina. Although there are about 100 islands in the group, their total area does not exceed 50 square miles. The group is surrounded by water varying in depth from 2,500 to 3,800 fathoms. The only terrestrial Vertebrate (unless the rats and mice are indigenous) is a lizard allied to an American form, but specifically distinct from it, and therefore a solitary species which does not occur anywhere else in the world. None of the birds or bats are peculiar, any more than in the case of the Azores; but, as in that case, a large percentage of the land-sh.e.l.ls are so--namely, at least one quarter of the whole. Neither the botany nor the entomology of this group has been worked out; but I have said enough to show how remarkably parallel are the cases of these two volcanic groups of islands situated in different hemispheres, but at about the same distance from large continents. In both there is an extraordinary paucity of terrestrial vertebrata, and of any peculiar species of bird or beast. On the other hand, there is in both a marvellous wealth of peculiar species of insects and land-sh.e.l.ls. Now these correlations are all abundantly intelligible. It is a difficult matter for any terrestrial animal to cross 900, or even 700, miles of ocean: therefore only one lizard has succeeded in doing so in one of the two parallel cases; and, living cut off from intercrossing with its parent form, the descendants of that lizard have become modified so as to const.i.tute a peculiar species. But it is more easy for large flying animals to cross those distances of ocean: consequently, there is only one instance of a peculiar species of bird or bat--namely, a bull-finch in the Azores, which, being a small land-bird, is not likely ever to have had any other visitors from its original parent species coming over from Europe to keep up the original breed. Lastly, it is very much more easy for insects and land-mollusca to be conveyed to such islands by wind and floating timber than it is for terrestrial mammals, or even than it is for small birds and bats; but yet such means of transit are not sufficiently sure to admit of much recruiting from the mainland for the purpose of keeping up the specific types. Consequently, the insects and the land-sh.e.l.ls present a much greater proportion of peculiar species--namely, one half and one fourth of the land-sh.e.l.ls in the one case, and one eighth of the beetles in the other. All these correlations, I say, are abundantly intelligible on the theory of evolution; but who shall explain, on the opposite theory, why orders of beetles and land-mollusca should have been chosen from among all other animals for such superabundant creation on oceanic islands, so that in the Azores alone we find no less than 32 of the one and 14 of the other?

And, in this connexion, I may again allude to the peculiar species of beetles in the island of Madeira. Here there are an enormous number of peculiar species, though they are nearly all related to, or included under the same genera as, beetles on the neighbouring continent. Now, as we have previously seen, no less than 200 of these species have lost the use of their wings. Evolutionists explain this remarkable fact by their general laws of degeneration under disuse, and the operation of natural selection, as will be shown later on; but it is not so easy for special creationists to explain why this enormous number of peculiar species of beetles should have been deposited on Madeira, all allied to beetles on the nearest continent, and nearly all deprived of the use of their wings. And similarly, of course, with all the peculiar species of the Bermudas and the Azores. For who will explain, on the theory of independent creation, why all the peculiar species, both of animals and plants, which occur on the Bermudas should so unmistakably present American affinities, while those which occur on the Azores no less unmistakably present European affinities? But to proceed to other, and still more remarkable, cases.

_The Galapagos Islands._--This archipelago is of volcanic origin, situated under the equator between 500 and 600 miles from the West Coast of South America. The depth of the ocean around them varies from 2,000 to 3,000 fathoms or more. This group is of particular interest, from the fact that it was the study of its fauna which first suggested to Darwin's mind the theory of evolution. I will, therefore, begin by quoting a short pa.s.sage from his writings upon the zoological relations of this particular fauna.

Here almost every product of the land and of the water bears the unmistakeable stamp of the American continent. There are twenty-six land birds; of these, twenty-one, or perhaps twenty-three, are ranked as distinct species, and would commonly be a.s.sumed to have been here created; yet the close affinity of most of these birds to American species is manifest in every character, in their habits, gestures, and tones of voice. So it is with the other animals, and with a large proportion of the plants, as shown by Dr. Hooker in his admirable Flora of this archipelago. The naturalist, looking at the inhabitants of these volcanic islands in the Pacific, distant several hundred miles from the continent, feels that he is standing on American land. Why should this be so? Why should the species which are supposed to have been created in the Galapagos Archipelago, and nowhere else, bear so plainly the stamp of affinity to those created in America? There is nothing in the conditions of life, in the geological nature of the islands, in their height or climate, or in the proportions in which the several cla.s.ses are a.s.sociated together, which closely resembles the conditions of the South American coast; in fact, there is a considerable dissimilarity in all these respects. On the other hand, there is a considerable degree of resemblance in the volcanic nature of the soil, in the climate, height, and size of the islands, between the Galapagos and Cape de Verde Archipelagoes; but what an entire and absolute difference in their inhabitants! The inhabitants of the Cape de Verde Islands are related to those of Africa, like those of the Galapagos to America. Facts such as these admit of no sort of explanation on the ordinary view of independent creation; whereas on the view here maintained, it is obvious that the Galapagos Islands would be likely to receive colonists from America, and the Cape de Verde Islands from Africa; such colonists would be liable to modification--the principle of inheritance still betraying their original birthplace[22].

[22] _origin of species_, pp. 353-4.

The following is a synopsis of the fauna and flora of this archipelago, so far as at present known. The only terrestrial vertebrates are two peculiar species of land-tortoise, and one extinct species; five species of lizards, all peculiar--two of them so much so as to const.i.tute a peculiar genus;--and two species of snakes, both closely allied to South American forms. Of birds there are 57 species, of which no less than 38 are peculiar; and all the non-peculiar species, except one, belong to aquatic tribes. The true land birds are represented by 31 species, of which all, except one, are peculiar; while more than half of them go to const.i.tute peculiar genera. Moreover, while they are all unquestionably allied to South American forms, they present a beautiful series of gradations, "from perfect ident.i.ty with the continental species, to genera so distinct that it is difficult to determine with what forms they are most nearly allied; and it is interesting to note that this diversity bears a distinct relation to the probabilities of, and facilities for, migration to the islands. The excessively abundant rice-bird, which breeds in Canada, and swarms over the whole United States, migrating to the West Indies and South America, visiting the distant Bermudas almost every year, and extending its range as far as Paraquay, is the only species of land-bird which remains completely unchanged in the Galapagos; and we may therefore conclude that some stragglers of the migrating host reach the islands sufficiently often to keep up the purity of the breed[23]." Again, of the thirty peculiar land-birds, it is observable that the more they differ from any other species or genera on the South American continent, the more certainly are they found to have their nearest relations among those South American forms which have the more restricted range, and are therefore the least likely to have found their way to the islands with any frequency.

[23] Wallace, _Island Life_, pp. 271-2.

The insect fauna of the Galapagos islands is scanty, and chiefly composed of beetles. These number 35 species, which are nearly all peculiar, and in some cases go to const.i.tute peculiar genera. The same remarks apply to the twenty species of land-sh.e.l.ls. Lastly, of the total number of flowering plants (332 species) more than one half (174 species) are peculiar. It is observable in the case of these peculiar species of plants--as also of the peculiar species of birds--that many of them are restricted to single islands. It is also observable that, with regard both to the fauna and flora, the Galapagos Islands as a whole are very much richer in peculiar species than either the Azores or Bermudas, notwithstanding that both the latter are considerably more remote from their nearest continents. This difference, which at first sight appears to make against the evolutionary interpretation, really tends to confirm it. For the Galapagos Islands are situated in a calm region of the globe, unvisited by those periodic storms and hurricanes which sweep over the North Atlantic, and which every year convey some straggling birds, insects, seeds, &c., to the Azores and Bermudas.

Notwithstanding their somewhat greater isolation geographically, therefore, the Azores and Bermudas are really less isolated biologically than are the Galapagos Islands; and hence the less degree of peculiarity on the part of their endemic species. But, on the theory of special creation, it is impossible to understand why there should be any such correlation between the prevalence of gales and a comparative inertness of creative activity. And, as we have seen, it is equally impossible on this theory to understand why there should be a further correlation between the _degree_ of peculiarity on the part of the isolated species, and the degree in which their nearest allies on the mainland are there confined to narrow ranges, and therefore less likely to keep up any biological communication with the islands.

_St. Helena._--A small volcanic island, ten miles long by eight wide, situated in mid-ocean, 1100 miles from Africa, and 1800 from South America. It is very mountainous and rugged, bounded for the most part by precipices, rising from ocean depths of 17,000 feet, to a height above the sea-level of nearly 3,000. When first discovered it was richly clothed with forests; but these were all destroyed by human agency during the 16th, 17th and 18th centuries. The records of civilization present no more lamentable instance of this kind of destruction. From a merely pecuniary point of view the abolition of these primeval forests has proved an irreparable loss; but from a scientific point of view the loss is incalculable. These forests served to harbour countless forms of life, which extended at least from the Miocene age, and which, having found there an ocean refuge, survived as the last remnants of a remote geological epoch. In those days, as Mr. Wallace observes, St. Helena must have formed a kind of natural museum or vivarium of archaic species of all cla.s.ses, the interest of which we can now only surmise from the few remnants of those remnants, which are still left among the more inaccessible portions of the mountain peaks and crater edges. These remnants of remnants are as follows.