Fossil skeleton of Phenacodus primavus.Fig. 80.—Fossil skeleton ofPhenacodus primavus. (After Cope.)
But now, as we have seen, for more rapid progression on hard uneven ground, a stronger and better jointed foot would be needed. Therefore we find the bones of the wrist and ankle beginning to interlock, both among themselves and also with those of the foot and hand immediately below them. Such a stage of evolution is still apparent in the now existing elephant. (See Fig. 81.)
Bones of the foot of four forms of the perissodactyl type.Fig. 81.—Bones of the foot of four different forms of the perissodactyl type, showing gradual reduction in the number of digits, coupled with a greater consolidation of the bones above the digits. The series reads from right to left. Drawn from nature (Brit. Mus.).
Fig. 81.—Bones of the foot of four different forms of the perissodactyl type, showing gradual reduction in the number of digits, coupled with a greater consolidation of the bones above the digits. The series reads from right to left. Drawn from nature (Brit. Mus.).
Next, however, a still stronger foot was made by the still further interlocking of the wrist and ankle bones, so that both the first and second rows of them were thus fitted into each other, as well as into the bones of the hand and foot beneath. This further modification is clearly traceable in some of the earlier perissodactyls, and occurs in the majority at the present time. Compare, for example, the greater interlocking and consolidation of these small bones in the Rhinoceros as contrasted with the Elephant (Fig. 81). Moreover, simultaneously with these consolidating improvements in the mechanism of the wrist and ankle joints, or possibly at a somewhat later period, a reduction in the number of digits began to take place. This was a continuation of the policy of consolidating the foot, analogous to the dropping out of the sixth row of small bones in the paddle ofBaptanodon. (Fig. 78.) In the pentadactyl plantigrade foot of the early mammals, the first digit, being the shortest, was thefirst to leave the ground, to dwindle, and finally to disappear. More work being thus thrown on the remaining four, they were strengthened by interlocking with the wrist (or ankle) bones above them, as just mentioned; and also by being brought closer together.
Bones of the foot of four forms of the artiodactyl type.Fig. 82.—Bones of the foot of four different forms of the artiodactyl type, showing gradual reduction of the number of digits, coupled with a greater consolidation of the bones above the digits. The series reads from right to left. Drawn from nature (Brit. Mus.).
Fig. 82.—Bones of the foot of four different forms of the artiodactyl type, showing gradual reduction of the number of digits, coupled with a greater consolidation of the bones above the digits. The series reads from right to left. Drawn from nature (Brit. Mus.).
The changes which followed I will render in the words of Professor Marsh.
Two kinds of reduction began. One leading to the existing perissodactyl foot, and the other, apparently later, resulting in the artiodactyl type. In the former the axis of the foot remained in the middle of the third digit, as in the pentadactyl foot. [See Fig. 81.] In the latter, it shifted to the outer side of this digit, or between the third and fourth toe. [See Fig. 82.]In the further reduction of the perissodactyl foot, the fifth digit, being shorter than the remaining three, next left the ground, and gradually disappeared. [Fig. 81 B.] Of the three remaining toes, the middle or axial one was the longest, and retaining its supremacy as greater strength and speed were required, finally assumed the chief support of the foot [Fig. 81 C], while the outer digits left the ground, ceased to be of use, and were lost, except as splint-bones [Fig. 81 D]. The feet of the existing horse shows the best example of this reduction in the Perissodactyls, as it is the most specialized known in the Ungulates [Fig. 81 D].In the artiodactyl foot, the reduction resulted in the gradual diminution of the two outer of the four remaining toes, the third and fourth doing all the work, and thus increasing in size and power. The fifth digit, for the same reasons as in the perissodactyl foot, first left the ground and became smaller. Next, the second soon followed, and these two gradually ceased to be functional, [and eventually disappeared altogether, as shown in the accompanying drawing of the feet of still existing animals, Fig. 82 B, C, D].The limb of the modern race-horse is a nearly perfect piece of machinery, especially adapted to great speed on dry, level ground. The limb of an antelope, or deer, is likewise well fittedfor rapid motion on a plain, but the foot itself is adapted to rough mountain work as well, and it is to this advantage, in part, that the Artiodactyls owe their present supremacy. The plantigrade pentadactyl foot of the primitive Ungulate—and even the perissodactyl foot that succeeded it—both belong to the past humid period of the world’s history. As the surface of the earth slowly dried up, in the gradual desiccation still in progress, new types of feet became a necessity, and the horse, antelope, and camel were gradually developed, to meet the altered conditions.
Two kinds of reduction began. One leading to the existing perissodactyl foot, and the other, apparently later, resulting in the artiodactyl type. In the former the axis of the foot remained in the middle of the third digit, as in the pentadactyl foot. [See Fig. 81.] In the latter, it shifted to the outer side of this digit, or between the third and fourth toe. [See Fig. 82.]
In the further reduction of the perissodactyl foot, the fifth digit, being shorter than the remaining three, next left the ground, and gradually disappeared. [Fig. 81 B.] Of the three remaining toes, the middle or axial one was the longest, and retaining its supremacy as greater strength and speed were required, finally assumed the chief support of the foot [Fig. 81 C], while the outer digits left the ground, ceased to be of use, and were lost, except as splint-bones [Fig. 81 D]. The feet of the existing horse shows the best example of this reduction in the Perissodactyls, as it is the most specialized known in the Ungulates [Fig. 81 D].
In the artiodactyl foot, the reduction resulted in the gradual diminution of the two outer of the four remaining toes, the third and fourth doing all the work, and thus increasing in size and power. The fifth digit, for the same reasons as in the perissodactyl foot, first left the ground and became smaller. Next, the second soon followed, and these two gradually ceased to be functional, [and eventually disappeared altogether, as shown in the accompanying drawing of the feet of still existing animals, Fig. 82 B, C, D].
The limb of the modern race-horse is a nearly perfect piece of machinery, especially adapted to great speed on dry, level ground. The limb of an antelope, or deer, is likewise well fittedfor rapid motion on a plain, but the foot itself is adapted to rough mountain work as well, and it is to this advantage, in part, that the Artiodactyls owe their present supremacy. The plantigrade pentadactyl foot of the primitive Ungulate—and even the perissodactyl foot that succeeded it—both belong to the past humid period of the world’s history. As the surface of the earth slowly dried up, in the gradual desiccation still in progress, new types of feet became a necessity, and the horse, antelope, and camel were gradually developed, to meet the altered conditions.
The best instance of such progressive modifications in the case of perissodactyl feet is furnished by the fossil pedigree of the existing horse, because here, within the limits of the same continuous family line, we have presented the entire series of modifications.
Feet and teeth in fossil pedigree of the Horse.Fig. 83.—Feet and teeth in fossil pedigree of the Horse. (After Marsh.)a, bones of the fore-foot;b, bones of the hind-foot;c, radius and ulna;d, tibia and fibula;e, roots of a tooth;fandg, crowns of upper and lower molar teeth.
Fig. 83.—Feet and teeth in fossil pedigree of the Horse. (After Marsh.)a, bones of the fore-foot;b, bones of the hind-foot;c, radius and ulna;d, tibia and fibula;e, roots of a tooth;fandg, crowns of upper and lower molar teeth.
Palæotherium.Fig. 84.—Palæotherium. (Lower Tertiary of Paris Basin.)
There are now known over thirty species of horse-like creatures, beginning from the size of a fox, then progressively increasing in bulk, and all standing inlinear series in structure as in time. Confining attention to the teeth and feet, it will be seen from the wood-cut on page 189 that the former grow progressively longer in their sockets, and also more complex in the patterns of their crowns. On the other hand, the latter exhibit a gradual diminution of their lateral toes, together with a gradual strengthening of the middle one. (See Fig. 83.) So that in the particular case of the horse-ancestry we have a practically complete chain of what only a few years ago were “missing links.” And this now practically completed chain shows us the entire history of what happens to be the most peculiar, or highly specialized, limb in the whole mammalian class—namely, that of the existing horse. Of the other two wood-cuts, the former (Fig. 84) shows the skeleton of a very early and highly generalized ancestor, while the other is a partial restoration of a much more recent and specialized one. (Fig. 85.)
Hipparion.Fig. 85.—Hipparion. (New World Pliocene.)
On the other hand, progressive modifications of the artiodactyl feet may be traced geologically up to the different stages presented by living ruminants, in some of which it has proceeded further than in others. For instance, if we compare the pig, the deer, and the camel (Fig. 82), we immediately perceive that the dwindling of the two rudimentary digits has proceeded much further in the case of the deer than in that of the pig, and yet not so far as in that of the camel, seeing that here they have wholly disappeared. Moreover, complementary differences are to be observed in the degree of consolidation presented by the two useful digits. For while in the pig the two foot-bones are still clearly distinguishablethroughout their entire length, in the deer, and still more in the camel, their union is more complete, so that they go to constitute a single bone, whose double or compound character is indicated externally only by a slight bifurcation at the base. Nevertheless, if we examine the state of matters in the unborn young of these animals, we find that the two bones in question are still separated throughout their length. and thus precisely resemble what used to be their permanent condition in some of the now fossil species of hoofed mammalia.
Turning next from bones of the limb to other parts of the mammalian skeleton, let us briefly consider the evidence of evolution that is here likewise presented by the vertebral column, the skull, and the teeth.
As regards the vertebral column, if we examine thisstructure in any of the existing hoofed animals, we find that the bony processes called zygapophyses, which belong to each of the constituent vertebræ, are so arranged that the anterior pair belonging to each vertebra interlocks with the posterior pair belonging to the next vertebra. In this way the whole series of vertebræ are connected together in the form of a chain, which, while admitting of considerable movement laterally, is everywhere guarded against dislocation. But if we examine the skeletons of any ungulates from the lower Eocene deposits, we find that in no case is there any such arrangement to secure interlocking. In all the hoofed mammals of this period the zygapophyses are flat. Now, from this flat condition to the present condition of full interlocking we obtain a complete series of connecting links. In the middle Miocene period we find a group of hoofed animals in which the articulation begins by a slight rounding of the previously flat surfaces: later on this rounding progressively increases, until eventually we get the complete interlocking of the present time.
As regards teeth, and still confining attention to the hoofed mammals, we find that low down in the geological series the teeth present on their grinding surfaces only three simple tubercles. Later on a fourth tubercle is added, and later still there is developed that complicated system of ridges and furrows which is characteristic of these teeth at the present time, and which was produced by manifold and various involutions of the three or four simple tubercles of Eocene and lower Miocene times. In other words, the principle of gradual improvement in theConstruction of teeth, which has already been depicted as regards the particular case of the horse-family (Fig. 83), is no less apparent in the pedigree of all the other mammalia, wherever the palæontological history is sufficiently intact to serve as a record at all.
Comparative series of Brains.Fig. 86.—Comparative series of Brains. (After Le Conte.) The series reads from above downwards, and represents diagrammatically the brain of a Fish, a Reptile, a Bird, a Mammal, and a Man. In each case the letter A marks a side view, and the letter B a top view. The small italics throughout signify the following homologous parts:m, medulla;cb, cerebellum;op, optic lobes;cr, cerebrum and thalamus;ol, olfactory lobes. The series shows a progressive consolidation and enlargement of the brain in general, and of the cerebrum and cerebellum in particular, which likewise exhibit continually advancing structure in respect of convolution. In the case of Man, these two parts of the brain have grown to so great a size that they conceal all the other parts from the superficial points of view represented in the diagram.
Fig. 86.—Comparative series of Brains. (After Le Conte.) The series reads from above downwards, and represents diagrammatically the brain of a Fish, a Reptile, a Bird, a Mammal, and a Man. In each case the letter A marks a side view, and the letter B a top view. The small italics throughout signify the following homologous parts:m, medulla;cb, cerebellum;op, optic lobes;cr, cerebrum and thalamus;ol, olfactory lobes. The series shows a progressive consolidation and enlargement of the brain in general, and of the cerebrum and cerebellum in particular, which likewise exhibit continually advancing structure in respect of convolution. In the case of Man, these two parts of the brain have grown to so great a size that they conceal all the other parts from the superficial points of view represented in the diagram.
Ideal section through all the above stages.Fig. 87.—Ideal section through all the above stages. (After Le Conte.)
Lastly, as regards the skull, casts of the interior show that all the earlier mammals had small brains with comparatively smooth or unconvoluted surfaces; and that as time went on the mammalian brain gradually advanced in size and complexity. Indeed so small were the cerebral hemispheres of the primitive mammals that they did not overlap the cerebellum, while their smoothness must have been such as in this respect to have resembled the brain of a bird or reptile. This, of course, is just as it ought to be, if the brain, which the skull has to accommodate, has been graduallyevolved into larger and larger proportions in respect of its cerebral hemispheres, or the upper masses of it which constitute the seat of intelligence. Thus, if we look at the above series of wood-cuts, which represents the comparative structure of the brain in the existing classes of the Vertebrata, we can immediately understand why the fossil skulls of Mammalia should present a gradual increase in size and furrowing, so as to accommodate the general increase of the brain in both these respects between the level marked “maml” and that marked “man,” in the last of the diagrams. (Fig. 87.)
The tabular statement on the following diagram, which I borrow from Prof. Cope, will serve at a glance to reveal the combined significance of so many lines of evidence, united within the limits of the same group of animals.
To give only one special illustration of the principle of evolution as regards the skull, here is one of the most recent instances that has occurred of the discovery of a missing link, or connecting form (see Fig. 88). The fossil (B), which was found in New Jersey, stands in an intermediate position between the stag and the elk. In the stag (A) the skull is high, showing but little of that anterior attenuation which is such a distinctive feature of the skull of the elk (C). The nasal bones (N) of the former, again, are remarkably long when compared with the similar bones of the latter, and the premaxillaries (PMX), instead of being projected forward along the horizontal plane of the base of the skull, are deflected sharply downward. In all these points, it will be seen, the newly discovered form (Cervalces) holds an intermediateposition (B). “The skull exhibits a partial attenuation anteriorly, the premaxillaries are directed about equally downward and forward, and the nasal bones are measurably contracted in size. The horns likewisefurnish characters which further serve to establish this dual relationship[18].”
Skulls of Canadian Stag, Cervalces Americanus and Elk.Fig.88.—Skulls of—A, Canadian Stag; B,Cervalces Americanus; and C, Elk. (After Heilprin.)
Formation.No. of toes.Feet.Astragalus.Carpus and tarsus.Ulno-radiusSuperior molars.Zygapophyses.Brain.Pliocene.1-12-2Digitigrade.(Plantigrade.)Grooved.(Flat.)Interlocking.(Opposite.)Faceted.4-tubercles, crested and cementedDoubly involute.Singly involute.Hemispheres larger, convoluted.Upper Miocene.(Loup Fork.)3-34-4(5-5)Middle.(John Day.)2-23-34-4Digitigrade.Grooved.Interlocking.Faceted.Smooth.4-tubercles, and crested.Singly involute.Doubly involute.Hemispheres larger, convoluted.Lower(White River.)3-34-34-4Digitigrade.Plantigrade.Grooved.Interlocking.Smooth.Faceted.4-tubercles, and crested.? Singly involute.Hemispheres small, and larger.Eocene.Upper(bridger.)3-34-34-55-5(Digitigrade.)Plantigrade.Grooved.(Flat.)Opposite. Interlocking.Smooth.4-tubercles.3-tubercles, and crested.Singly involute.Plane.Hemispheres smallMiddle.(Wasatch.)4-34-55-5Plantigrade.(Digitigrade.)Flat.(Grooved.)Opposite. Interlocking.Smooth.4-tubercles.3-tubercles, a few crested.Plane.Singly involute.Hemispheres small; mesencehpalon sometimes exposed.Lower(Puerco.)5-5Plantigrade.Flat.Opposite.Smooth.3-tubercles.(4-tubercles), none crested.Plane.Mesencephalon exposed; hemisphere small and smoother.
The evidence, then, which is furnished by all parts of the vertebral skeleton—whether we have regard to Fishes, Reptiles, Birds, or Mammals—is cumulative and consistent. Nowhere do we meet with any deviation or ambiguity, while everywhere we encounter similar proofs of continuous transformation—proofs which vary only with the varying amount of material which happens to be at our disposal, being most numerous and detailed in those cases where the greatest number of fossil forms has been preserved by the geological record. Here, therefore, we may leave the vertebral skeleton; and, having presented a sample of the evidence as yielded by horns and bones, I will conclude by glancing with similar brevity at the case of shells—which, as before remarked, constitute the only other sufficiently hard or permanent material to yield unbroken evidence touching the fossil ancestry of animals.
Of course it will be understood that I am everywhere giving merely samples of the now superabundant evidence which is yielded by palæontology; and, as this chapter is already a long one, I must content myself with citing only the case of mollusk-shells, although shells of other classes might be made to yield highly important additions to the testimony. Moreover, even as regards the one division of mollusk-shells, I can afford to quote only a very few cases. These, however, are in my opinion the strongest single pieces of evidence in favour of transmutation which have thus far been brought to light.
Transmutations of Planorbis.Fig.89.—Transmutations ofPlanorbis. (After Hyatt.)
Near the village of Steinheim, in Würtemberg, there is an ancient lake-basin, dating from Tertiary times. The lake has long ago dried up; but its aqueous deposits are extraordinarily rich in fossil shells, especially of different species of the genusPlanorbis. The following is an authoritativerésuméof the facts.
As the deposits seem to have been continuous for ages, and the fossil shells very abundant, this seemed to be an excellent opportunity to test the theory of derivation. With this end in view, they have been made the subject of exhaustive study by Hilgendorf in 1866, and by Hyatt in 1880. In passing from the lowest to the highest strata the species change greatly and many times, the extreme forms being so different that, were it not for the intermediate forms, they would be called not only different species, but different genera. And yet the gradations are so insensible that the whole series is nothing less than a demonstration, in this case at least, of origin of species by derivation with modifications. The accompanying plate of successive forms (Fig. 89), which we take from Prof. Hyatt’s admirable memoir, will show this better than any mere verbal explanation. It will be observed that, commencing with four slight varieties—probably sexually isolated varieties—of one species, each series shows a gradual transformation as we go upward in the strata—i. e. onward in time. Series I branches into three sub-series, in two of which the change of form is extreme. Series IV is remarkable for great increase in size as well as change in form. In the plate we give only selected stages, but in the fuller plates of the memoir, and still more in the shells themselves, the subtilest gradations are found[19].
As the deposits seem to have been continuous for ages, and the fossil shells very abundant, this seemed to be an excellent opportunity to test the theory of derivation. With this end in view, they have been made the subject of exhaustive study by Hilgendorf in 1866, and by Hyatt in 1880. In passing from the lowest to the highest strata the species change greatly and many times, the extreme forms being so different that, were it not for the intermediate forms, they would be called not only different species, but different genera. And yet the gradations are so insensible that the whole series is nothing less than a demonstration, in this case at least, of origin of species by derivation with modifications. The accompanying plate of successive forms (Fig. 89), which we take from Prof. Hyatt’s admirable memoir, will show this better than any mere verbal explanation. It will be observed that, commencing with four slight varieties—probably sexually isolated varieties—of one species, each series shows a gradual transformation as we go upward in the strata—i. e. onward in time. Series I branches into three sub-series, in two of which the change of form is extreme. Series IV is remarkable for great increase in size as well as change in form. In the plate we give only selected stages, but in the fuller plates of the memoir, and still more in the shells themselves, the subtilest gradations are found[19].
Here is another and more recently observed case of transmutation in the case of mollusks.
The recent species,Strombus accipitrinus, still inhabits the coasts of Florida. Its extinct prototype,S. Leidy, was discovered a few years ago by Prof. Heilprin in the Pliocene formations of the interior of Florida. The peculiar shape of the wing, and tuberculation of the whorl, are thus proved to have grown but of a previously more conical form of shell.
Transformation of Strombus.Fig.90.—Transformation ofStrombus. (After Heilprin.) 1, 1a,Strombus Leidy(1, typical), Pliocene; 2, 2a,Strombus accipitrinus(2atypical) Recent.
Lastly, attention may here again be directed to the very instructive series of shells which has already been shown in a previous chapter, and which serves to illustrate the successive geological forms ofPaludinafrom the Tertiary beds of Slavonia, as depicted by Prof. Neumayr of Vienna. (Fig. 1, p. 19.)
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 space—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, thatthe 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 anticipations 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 illustration. 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 betteradmit 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 transmutation, 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 flourishing 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 geographicaldistribution 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 atone 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 latitudes 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 assume 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, thatspecies 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 someplace, 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 constituent 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 constitute 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 theirOld World cousins in form and habits; but differ from them in dentition 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 dentition, 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 classification.
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 constituent 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 thefaunas 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 classes 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 thepolicy 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 circumstance 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 classes. 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 andNatural affinity among inhabitants of the same areas; second, another of a more detailed character betweendegreesof geographical restriction anddegreesof 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 class 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 classes 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 tocreate his species[20]. but now we see that he must be held to have neglected this inscrutable reason (whatever it was) when he passed beyond the range of genera—and this always in proportion to the remoteness of systematic affinity on the part of the species concerned.
I cannot well conceive areductio ad absurdummore 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 illustrations of them in detail, for in this way I think that their overwhelming weight may become yet more abundantly apparent.
It will assist us in dealing with these detailed illustrations 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 pass them by more exceptional and accidental means. The principal barriers of a geographical kind are oceans, rivers, mountain-chains, and desert-tracts, in the case ofterrestrial 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 constitute 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 constitutes 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 shores. 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 alsoproved 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 illustrations 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 shores of South America are very distinct; with extremely few shells, crustacea, or echinodermata in common.” Again, westward of the shores of America, a wide space 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 passed, we meet in the eastern islands of the Pacific with another and totally distinct fauna. “So that three marinefaunas range northward and southward in parallel lines not far from each other, under corresponding climates": they are, however, “separated from each other by impassable 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 shores of South America were formed with one set of natural affinities, while species deposited on the Western shore 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 specifictypes? 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.
Although the geographical distribution of fresh-water fish and fresh-water shells 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 assisted 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 pass, 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 specifictypes, 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 impassable 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 space 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—class 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 wouldagree 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 anticipations. 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 ships upon some of these islands, and even already it is observed that their descendants have undergone a slight change of appearance, so as to constitute them what naturalists call local varieties. The change, of course, is butslight, 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 pass 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 analysis 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-matterwould be too extensive to be dealt with on the present occasion, were we to attempt an exhaustive analysis 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 analysis 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 butterflies, 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-shells, of which 37 are European, and 32 peculiar, though all allied to Europeanforms. 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-shells 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-shells. 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 constitute a peculiar species. But it is more easy for large flying animalsto 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-shells present a much greater proportion of peculiar species—namely, one half and one fourth of the land-shells 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 passage from his writings upon the zoological relations of this particular fauna.