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 assumed 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 classes are associated 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].
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 assumed 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 classes are associated 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].
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 constitute a peculiar genus;—and two species of snakes, both closely allied to South American forms. Of birds there are 57 species, of which noless 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 constitute peculiar genera. Moreover, while they are all unquestionably allied to South American forms, they present a beautiful series of gradations, “from perfect identity 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.
The insect fauna of the Galapagos islands is scanty, and chiefly composed of beetles. These number 35species, which are nearly all peculiar, and in some cases go to constitute peculiar genera. The same remarks apply to the twenty species of land-shells. 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 thedegreeof peculiarity on the partof 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 classes, 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.
There is a total absence of all indigenous mammals, reptiles, fresh-water fish, and true land-birds. There is, however, a species of plover, allied to onein South Africa; but it is specifically distinct, and therefore peculiar to the island. The insect life, on the other hand, is abundant. Of beetles no less than 129 species are believed to be aboriginal, and, with one single exception, the whole number are peculiar to the island. “But in addition to this large amount of specific peculiarity (perhaps unequalled anywhere else in the world), the beetles of this island are remarkable for their generic isolation, and for the altogether exceptional proportion in which the great divisions of the order are represented. The species belong to 39 genera, of which no less than 25 are peculiar to the island; and many of these are such isolated forms that it is impossible to find their allies in any particular country[24].” More than two-thirds of all the species belong to the group of weevils—a circumstance which serves to explain the great wealth of beetle-population, the weevils being beetles which live in wood, and St. Helena having been originally a densely wooded island. This circumstance is also in accordance with the view that the peculiar insect fauna has been in large part evolved from ancestors which reached the island by means of floating timber; for, of course, no explanation can be suggested why special creation of this highly peculiar insect fauna should have run so disproportionately into the production of weevils. About two-thirds of the whole number of beetles, or over 80 species, show no close affinity with any existing insects, while the remaining third have some relations, though often very remote, with European and African forms. That this high degree of peculiarityis due to high antiquity is further indicated, according to our theory, by the large number of species which some of the types comprise. Thus, the 54 species ofCossonidæmay be referred to three types; the 11 species ofBembidiumform a group by themselves; and theHeteromeraform two groups. “Now, each of these types may well be descended from a single species, which originally reached the island from some other land; and the great variety of generic and specific forms into which some of them have diverged is an indication, and to some extent a measure, of the remoteness of their origin[25].” But, on the counter-supposition that all these 128 peculiar species were separately created to occupy this particular island, it is surely unaccountable that they should thus present such an arborescence of natural affinities amongst themselves.
Passing over the rest of the insect fauna, which has not yet been sufficiently worked out, we next find that there are only 20 species of indigenous land-shells—which is not surprising when we remember by what enormous reaches of ocean the island is surrounded. Of these 20 species no less than 13 have become extinct, three are allied to European species, while the rest are so highly peculiar as to have no near allies in any other part of the globe. So that the land-shells tell exactly the same story as the insects.
Lastly, the plants likewise tell the same story. The truly indigenous flowering plants are about 50 in number, besides 26 ferns. Forty of the former and ten of the latter are peculiar to the island,and, as Sir Joseph Hooker tells us, “cannot be regarded as very close specific allies of any other plants at all” Seventeen of them belong to peculiar genera, and the others all differ so markedly as species from their congeners, that not one comes under the category of being an insular form of a continental species. So that with respect to its plants no less than with respect to its animals, we find that the island of St. Helena constitutes a little world of unique species, allied among themselves, but diverging so much from all other known forms that in many cases they constitute unique genera.
Sandwich Islands.—These are an extensive group of islands, larger than any we have hitherto considered—the largest of the group being about the size of Devonshire. The entire archipelago is volcanic, with mountains rising to a height of nearly 14,000 feet. The group is situated in the middle of the North Pacific, at a distance of considerably over 2,000 miles from any other land, and surrounded by enormous ocean depths. The only terrestrial vertebrata are two lizards, one of which constitutes a peculiar genus. There are 24 aquatic birds, five of which are peculiar; four birds of prey, two of which are peculiar; and 16 land-birds, all of which are peculiar. Moreover, these 16 land-birds constitute no less than 10 peculiar genera, and even one peculiar family of five genera. This is an amount of peculiarity far exceeding that of any other islands, and, of course, corresponds with the great isolation of this archipelago. The only other animals which have here been carefully studied are the land-shells, and these tell the same story as the birds. For there areno less than 400 species which are all, without any exception, peculiar; while about three-quarters of them go to constitute peculiar genera. Again, of the plants, 620 species are believed to be endemic; and of these 377 are peculiar, yielding no less than 39 peculiar genera.
Prejudice apart, I think we must all now agree that it is needless to continue further this line of proof. I have chosen the smallest and most isolated islands for the purposes of our present argument, first because these furnish the most crucial kind of test, and next because they best admit of being dealt with in a short space. But, if necessary, a vast amount of additional material could be furnished, not only from other small oceanic islands, but still more from the largest islands of the world, such as Australia and New Zealand. However, after the detailed inventories which have now been given in the case of some of the smaller islands most remote from mainlands, we may well be prepared to accept it as a general law, thatwhereverthere is evidence of land-areas having been for a long time separated from other land-areas, there we meet with a more or less extraordinary profusion of unique species, often running up into unique genera. And, in point of fact, so far as naturalists have hitherto been able to ascertain,there is no exception to this general law in any region of the globe. Moreover, there is everywhere a constant correlation between thedegreeof this peculiarity on the part of the fauna and flora, and thetimeduring which they have been isolated. Thus, for instance, among the islands whichI have called into evidence, those that are at once the most isolated and give independent proofs of the highest antiquity, are the Galapagos Islands, the Sandwich Islands, and St. Helena. Now, if we apply the method of tabular analysis to these three cases, we obtain the following most astonishing results. For the sake of simplicity I will omit the enumeration of peculiar genera, and confine attention to peculiar species. Moreover, I will consider only terrestrial animals; for, as we have already seen, aquatic animals are so much more likely to reach oceanic islands that they do not furnish nearly so fair a test of the evolutionary hypothesis.
PECULIAR SPECIES.Shells.Insects.Reptiles.Birds.Mammals.Sandwich400?2160Galapagos153510300St. Helena20128010Totals.43516312470
NON-PECULIAR SPECIES.Shells.Insects.Reptiles.Birds.Mammals.Sandwich0?000Galapagos??010St. Helena0?000Totals.0?010
From this synopsis we perceive that out of a total of 658 species of terrestrial animals known to inhabitthese three oceanic territories, all are peculiar, with the exception of a single land-bird which is found in the Galapagos Islands. This is the rice-bird, so very abundant on the American continent that its representatives must not unfrequently become the involuntary colonists of the Archipelago. There are, however, a few species of non-peculiar insects inhabiting the Sandwich and Galapagos Islands, the exact number of which is doubtful, and on this account are not here quoted. But at most they would be represented by units, and therefore do not affect the general result. Lastly, the remarkable fact will be noted, that there is no single representative of the mammalian class in any of these islands.
If we turn next to consider the case of plants, we obtain the following result:—
PeculiarSpecies.Non-peculiarSpecies.Sandwich377243Galapagos174158St. Helena5026Totals.601427
So that by adding together peculiar species both of land-animals and plants, we find that on these three limited areas alone there are 1258 forms of life which occur nowhere else upon the globe—not to speak of the peculiar aquatic species, nor of the presumably large number of peculiar species of all kinds not hitherto discovered in these imperfectly explored regions.
Now let us compare these facts with those which are presented by the faunas and floras of islands lessremote from continents, and known from independent geological evidence to be of comparatively recent origin—that is, to have been separated from their adjacent mainlands in comparatively recent times, and therefore as islands to be comparatively young. The British Isles furnish as good an instance as could be chosen, for they together comprise over 1000 islands of various sizes, which are nowhere separated from one another by deep seas, and in the opinion of geologists were all continuous with the European continent since the glacial period.
BRITISH ISLES.NON-PECULIAR SPECIES.Plants.Land Shells.Insects.Reptiles and Amphibia.Land Birds.Land Mammals.14628312,5511313040PECULIAR SPECIES.Plants.Land Shells.Insects.Reptiles and Amphibia.Land Birds.Land Mammals.464149010
Total Peculiar Plants46Total Peculiar Animals154——Grand Total200
I have drawn up this table in the most liberal manner possible, including as peculiar species formswhich many naturalists regard as merely local varieties. But, even as thus interpreted, how wonderful is the contrast between the 1000 islands of Great Britain and the single volcanic rock of St. Helena, where almost all the animals and about half the plants are peculiar, instead of about 1/80 of the animals, and 1/30 of the plants. Of course, if no peculiar species of any kind had occurred in the British Isles, advocates of special creation might have argued that it was, so to speak, needless for the Divinity to have added any new species to those European forms which fully populated the islands at the time when they were separated from the continent. But, as the matter stands, advocates of special creation must face the fact that a certain small number of new and peculiar species have been formed on the British Isles; and, therefore, that creative activity has not been wholly suspended in their case. Why, then, has it been so meagre in this case of a thousand islands, when it has proved so profuse in the case of all single islands more remote from mainlands, and presenting a higher antiquity? Or why should the Divinity have thus appeared so uniformly to consult these merely accidental circumstances of space and time in the depositing of his unique specific types? Do not such facts rather speak with irresistible force in favour of the view, that while all ancient and solitary islands have had time enough, and separation enough, to admit of distinct histories of evolution having been written in their living inhabitants, no one of the thousand islands of Great Britain has had either time enough, or separation enough, to have admitted of more than some of the first pages of such a history having been commenced?
But this allusion to Great Britain introduces us to another point. It will have been observed that, unlike oceanic islands remote from mainlands, Great Britain is well furnished both with reptiles (including amphibia) and mammals. For there is no instance of any oceanic island situated at more than 300 miles from a continent where any single species of the whole class of mammals is to be found, excepting species of the only order which is able to fly—namely, the bats. And the same has to be said of frogs, toads, and newts, whose spawn is quickly killed by contact with sea-water, and therefore could never have reached remote islands in a living state. Hence, on evolutionary principles; it is quite intelligible why oceanic islands should not present any species of mammals or batrachians—peculiar or otherwise,—save such species of mammals as are able to fly. But on the theory of special creation we can assign no reason why, notwithstanding the extraordinary profusion of unique types of other kinds which we have seen to occur on oceanic islands, the Deity should have made this curious exception to the detriment of all frogs, toads, newts, and mammals, save only such as are able to fly. Or, if any one should go so far to save a desperate hypothesis as to maintain that there must have been some hidden reason why batrachians and quadrupeds were not specially created on oceanic islands, I may mention another small—but in this relation a most significant—fact. This is that on some of these islands there occur certain peculiar species of plants, the seeds of which are provided with numerous tiny hooks, obviously and beautifully adapted—like those on the seeds of allied plants elsewhere—to catch thewool or hair of moving quadrupeds, and so to further their own dissemination. But, as we have just seen, there are no quadrupeds in the islands to meet these beautiful adaptations on the part of the plants; so that special creationists must resort to the almost impious supposition that in these cases the Deity has only carried out half his plan, in that while he made an elaborate provision for these uniquely created species of plants, which depended for its efficiency on the presence of quadrupeds, he nevertheless neglected to place any quadrupeds on the islands where he had placed the plants. Such one-sided attempts at adaptation surely resolve the thesis of special creation to areductio ad absurdum; and hence the only reasonable interpretation of them is, that while the seeds of allied or ancestral plants were able to float to the islands, no quadrupeds were ever able over so great a distance to swim.
Although much more evidence might still be given under the head of geographical distribution, I must now close with a brief summary of the main points that have been adduced.
After certain preliminary considerations, I began by noticing that the theory of evolution has a much more intelligible account to give than has its rival of the facts of discontinuous distribution—the Alpine flora, for instance, being allied to the Arctic, not because the same species were separately created in both places, but because during the glacial period these species extended all over Europe, and were left behind on the Alps as the Arctic flora receded northwards—which was sufficiently long ago to explainwhy some of the Alpine species are unique, though closely allied to Arctic forms.
Next we saw that, although living things are always adapted to the climates under which they live (since otherwise they could not live there at all), it is equally true that, as a rule, besides the area on which they do live, there are many other areas in different parts of the globe where they might have lived equally well. Consequently we must conclude that, if all species were separately created, many species were severally created on only one among a number of areas where they might equally well have thrived. Now, although this conclusion in itself may not seem opposed to the theory of special creation, a most serious difficulty is raised when it is taken in connexion with another fact of an equally general kind. This is, that on every biological region we encounter chains of allied species constituting allied genera, families, and so on; while we scarcely ever meet with allied species in different biological regions, notwithstanding that their climates may be similar, and, consequently, just as well suited to maintain some of the allied species. Hence we must further conclude, if all species were separately created, that in the work of creation some unaccountable regard was paid to making areas of distribution correspond to degrees of structural affinity. A great many species of the rat genus were created in the Old World, and a great many species of another, though allied, genus were created in the New World: yet no reason can be assigned why no one species of the Old World series should not just as well have been deposited in the New World, andvice versa. On the other hand, the theory of evolutionmay claim as direct evidence in its support all the innumerable cases such as these—cases, indeed, so innumerable that, as Mr. Wallace remarks, it may be taken as a law of nature that “every species has come into existence coincident both in space and time with a pre-existing and closely allied species.” A general law which, while in itself most strongly suggestive of evolution, is surely impossible to reconcile with any reasonable theory of special creation. Furthermore, this law extends backwards through all geological time, with the result that the extinct species which now occur only as fossils on any given geological area, resemble the species still living upon that area, as we should expect that they must, if the former were the natural progenitors of the latter. On the other hand, if they were not the natural progenitors, but all the species, both living and extinct, were the supernatural and therefore independent creations which the rival theory would suppose, then no reason can be given why the extinct species should thus resemble the living—any more than why the living species should resemble one another. For, as we have seen, there are almost always many other habitats on other parts of the globe, where any members of any given group of species might equally well have been deposited; and this, of course, applies to geological no less than to historical time. Yet throughout all time we meet with this most suggestive correlation between continuity of a geographical area and structural affinity between the forms of life which have lived, or are still living, upon that area.
Similarly, we find the further, and no less suggestive,correlation between the birth of new species and the immediate pre-existence of closely allied species on the same area—or, at most, on closely contiguous areas.
Where a continuous area has long been circumscribed by barriers of any kind, which prevent the animals from wandering beyond it, then we find that all the species, both extinct and living, constitute more or less a world of their own; while, on the other hand, where the animals are free to migrate from one area to another, the course of their migrations is marked by the origination of new species springing upen route, and serving to connect the older, or metropolitan, forms with the younger, or colonising, forms in the way of a graduated series. This principle, however, admits of being traced only in certain cases of species belonging to the same genus, of genera belonging to the same family, or, at most, of families belonging to the same order. In other words, the more general the structural affinity, the more general is the geographical extension—as we should expect to be the case on the theory of descent with branching modifications, seeing that the larger, the older, and the more diverse the group of organisms compared, the greater must be their chances of dispersal.
These general considerations led us to contemplate more in detail the correlation between structural affinity and barriers to free migration. Such barriers, of course, differ in the cases of different organisms. Marine organisms are stopped by land, unsuitable temperature, or unsuitable depths; fresh-water organisms by sea and by mountain-chains; terrestrial organisms chiefly by water. Now it is a matter offact which admits of no dispute, that in each of these cases we meet with a direct correlation between the kind of barrier and the kind of organisms whose structural affinities are affected thereby. Where we have to do with marine organisms, barriers such as the Isthmus of Panama and the varying depth of the Western Pacific determine three very distinct faunas, ranging north and south in closely parallel lines, and under corresponding climates. Where we have to do with fresh-water organisms, we find that a mountain-chain only a few miles wide has more influence in determining differences of organic type on either side of it than is exercised by even thousands of miles of a continuous land-area, if this be uninterrupted by any mountains high enough to prevent water-fowl, whirlwinds, &c., from dispersing the ova. Again, where we have to do with terrestrial organisms, the most effectual barriers are wide reaches of ocean; and, accordingly, we find that these exercise an enormous influence on the modification of terrestrial types. Moreover, we find that themoreterrestrial an organism, or thegreaterthe difficulty it has in traversing a wide reach of ocean, thegreateris the modifying influence of such a barrier upon that type. In oceanic islands, for example, many of the plants and aquatic birds usually belong to the same species as those which occur on the nearest mainlands, and where there are any specific differences, these but rarely run up to generic differences. But the land-birds, insects, and reptiles which are found on such islands are nearly always specifically, and very often generically, distinct from those on the nearest mainland—although invariably allied with sufficient closenessto leave no manner of doubt as to their affinities with the fauna of that mainland. Lastly, no amphibians and no mammals (except bats) are ever found on any oceanic islands. Yet, as we have seen, on the theory of special creation, these islands must all be taken to have been the theatres of the most extraordinary creative activity, so that on only three of them we found no less than 1258 unique species, whereof 657 were unique species of land animals, to be set against one single species known to occur elsewhere. Nevertheless, notwithstanding this prodigious expenditure of creative energy in the case of land-birds, land-shells, insects, and reptiles, no single new amphibian, or no single new mammal, has been created on any single oceanic island, if we except the only kind of mammal that is able to fly, and the ancestors of which, like those of the land-birds and insects, might therefore have reached the islands ages ago. Moreover, with regard to mammals, even in cases where allied forms occur on either side of a sea-channel, it is found to be a general rule that if the channel is shallow, the species on either side of it are much more closely related than if it be deep—and this irrespective of its width. Therefore we can only conclude, in the words of Darwin—"As the amount of modification which animals of all kinds undergo partly depends on lapse of time, and as the islands which are separated from each other or from the mainland by shallow channels are more likely to have been continuously united within a recent period than islands separated by deeper channels, we can understand how it is that a relation exists between the depth of the sea separating two mammalianfaunas, and the degree of their affinity—a relation which is quite inexplicable on the theory of independent acts of creation.”
Looking to all these general principles of geographical distribution, and remembering the sundry points of smaller detail relating to oceanic islands which I will not wait to recapitulate, to my mind it seems that there is no escape from the following conclusion, with which I will bring my brief epitome of the evidence to a close. The conclusion to which, I submit, all the evidence leads is, that if the doctrine of special creation is taken to be true, then it must be further taken that the one and only principle which has been consistently followed in the geographical deposition of species, is that of so depositing them as to make it everywhere appear that they were not thus deposited at all, but came into existence where they now occur by way of genetic descent with perpetual migration and correlative modification. On no other principle, so far as I can see, would it be possible to account for the fact that “every species has come into existence coincident both in space and time with a pre-existing and closely allied species,” together with the carefully graduated regard to physical barriers which the Creator must have displayed while depositing his newly formed species on either sides of them—everywhere makingdegreesof structural affinity correspond todegreesof geographical continuity, anddegreesof structural difference correspond todegreesof geographical separation, whether by mountain-chains in the case of fresh-water faunas, by land and by deep sea in the case of marinefaunas, or by reaches of ocean in the case of terrestrial faunas—stocking oceanic islands with an enormous profusion of peculiar species all allied to those on the nearest mainlands, yet everywhere avoiding the creation upon them of any amphibian or mammal, except an occasional bat. We are familiar with the doctrine that God is a God who hideth himself; here, however, it seems to me, we should have but a thinly-veiled insinuation, not merely that in his works he is hidden, but that in these works he is untrue. Than which I cannot conceive a stronger condemnation of the theory which it has been my object fairly to represent and dispassionately to criticise.
Thus far we have been considering the main evidences of organic evolution considered as a fact. We now enter a new field, namely, the evidences which thus far have been brought to light touching the causes of organic evolution considered as a process.
As was pointed out in the opening chapter, this is obviously the methodical course to follow: we must have some reasonable assurance that a fact is a fact before we endeavour to explain it. Nevertheless, it is not necessary that we should actually demonstrate a fact to be a fact before we endeavour to explain it. Even if we have but a reasonable presumption as to its probability, we may find it well worth while to consider its explanation; for by so doing we may obtain additional evidence of the fact itself. And this because, if it really is a fact, and if we hit upon the right explanation of it, by proving the explanation probable, we may thereby greatly increase our evidence of the fact. In the very case before us, for example, the evidence of evolution as a fact has from the first been largely derived from testing Darwin’s theory concerning its method. It was this theoretical explanation of itsmethod which first set him seriously to enquire into the evidences of evolution as a fact; and ever since he published his results, the evidences which he adduced in favour of natural selection as a method have constituted some of the strongest reasons which scientific men have felt for accepting evolution as a fact. Of course the evidence in favour of this fact has gone on steadily growing, quite independently of the assistance which was thus so largely lent to it by the distinctively Darwinian theory of its method; and, indeed, so much has this been the case, that in the present treatise we have been able to consider such direct evidence of the fact itself, without any reference at all to the indirect or accessory evidence which is derived from that of natural selection as a method. From which it follows that in most of what I am about to say in subsequent chapters on the evidences of natural selection as a method, there will be furnished a large addition to the evidences which have already been detailed of evolution as a fact. But, as a matter of systematic treatment, I have thought it desirable to keep these two branches of our subject separate. Which means that I have made the evidences of evolution as a fact to stand independently on their own feet—feet which in my opinion are amply strong enough to bear any weight of adverse criticism that can be placed upon them.
Our position, then, is this. On the foundation of the previous chapters, I will henceforth assume that we all accept organic evolution as a fact, without requiring any of the accessory evidence which is gained by independent proof of natural selection as a method. But in making this assumption—namely, that we are all now firmly persuaded of the fact of evolution—I donot imagine that such is really the case. I make the assumption for the purposes of systematic exposition, and in order that different parts of the subject may be kept distinct. I confess it does appear to me remarkable that there should still be a doubt in any educated mind touching the general fact of evolution; while it becomes to me unaccountable that such should be the case with a few still living men of science, who cannot be accused of being ignorant of the evidences which have now been accumulated. But in whatever measure we may severally have been convinced—or remained unconvinced—on this matter, for the purposes of exposition I must hereafter assume that we are all agreed to the extent of regarding the process of evolution as, at least, sufficiently probable to justify enquiry touching its causes on supposition of its truth.
Now, the causes of evolution have been set forth in a variety of different hypotheses, only the chief of which need be mentioned here. Historically speaking the first of these was that which was put forward by Erasmus Darwin, Lamarck, and Herbert Spencer. It consists in putting together the following facts and inferences.
We know that, in the lifetime of the individual, increased use of structures leads to an increase of their functional efficiency; while, on the other hand, disuse leads to atrophy. The arms of a blacksmith, and the legs of a mountaineer, are familiar illustrations of the first principle: our hospital wards are full of illustrations of the second. Again, we know that the characters of parents are transmitted to their progeny by means of heredity. Now the hypothesisin question consists in supposing that if any particular organs in a species are habitually used for performing any particular action, they must undergo a structural improvement which would more and more adapt them to the performance of that action; for in each generation constant use would better and better adapt the structures to the discharge of their functions, and they would then be bequeathed to the next generation in this their improved form by heredity. So that, for instance, if there had been a thousand generations of blacksmiths, we might expect the sons of the last of them to inherit unusually strong arms, even if these young men had themselves taken to some other trade not requiring any special use of their arms. Similarly, if there had been a thousand generations of men who used their arms but slightly, we should expect their descendants to show but a puny development of the upper extremities. Now let us apply all this to the animal kingdom in general. The giraffe, for instance, is a ruminant whose entire frame has been adapted to support an enormously long neck, which is of use to the animal in reaching the foliage of trees. The ancestors of the giraffe, having had ordinary necks, were supposed by Lamarck to have gradually increased the length of them, through many successive generations, by constantly stretching to reach high foliage; and he further supposed that, when the neck became so long as to require for its support special changes in the general form of the animal as a whole, these special changes would have brought about the dwindling of other parts from which so much activity was no longer required—the general result being that the whole organization of the animal became more and moreadapted to browsing on high foliage. And so in the cases of other animals, Lamarck believed that the adaptation of their forms to their habits could be explained by this simple hypothesis that the habits created the forms, through the effects of use and disuse, coupled with heredity.
Such is what is ordinarily known as Lamarck’s theory of evolution. We may as well remember, however, that it really constitutes only one part of his theory; for besides this hypothesis of the cumulative inheritance of functionally-produced modifications—to which we may add the inherited effects of any direct action exercised by surrounding conditions of life,—Lamarck believed in some transcendental principle tending to produce gradual improvement in pre-determined lines of advance. Therefore it would really be more correct to designate the former hypothesis by the name either of Erasmus Darwin, or, still better, of Herbert Spencer. Nevertheless, in order to avoid confusion, I will follow established custom, and subsequently speak of this hypothesis as the Lamarckian hypothesis—understanding, however, that in employing this designation I am not referring to any part or factor of Lamarck’s general theory of evolution other than the one which has just been described—namely, the hypothesis of the cumulative transmission of functionally-produced, or otherwise “acquired,” modifications.
This, then, was the earliest hypothesis touching the causes of organic evolution. But we may at once perceive that it is insufficient to explain all that stands to be explained. In the first place, it refers in chief part only to the higher animals, which are actuated toeffort by intelligence. Its explanatory power in the case of most invertebrata—as well as in that of all plants—is extremely limited, inasmuch as these organisms can never be moved to a greater or less use of their several parts by any discriminating volition, such as that which leads to the continued straining of a giraffe’s neck for the purpose of reaching foliage. In the second place, even among the higher animals there are numberless tissues and organs which unquestionably present a high degree of adaptive evolution, but which nevertheless cannot be supposed to have fallen within the influence of Lamarckian principles. Of such are the shells of crustacea, tortoises, &c., which although undoubtedly of great use to the animals presenting them, cannot ever have beenusedin the sense required by Lamarck’s hypothesis, i. e. actively exercised, so as to increase a flow of nutrition to the part. Lastly, in the third place, the validity of Lamarck’s hypothesis in any case whatsoever has of late years become a matter of serious question, as will be fully shown and discussed in the next volume. Meanwhile it is enough to observe that, on account of all these reasons, the theory of Lamarck, even if it be supposed to present any truth at all, is clearly insufficient as a full or complete theory of organic evolution.
In historical order the next theory that was arrived at was the theory of natural selection, simultaneously published by Darwin and Wallace on July 1st, 1858.
If we may estimate the importance of an idea by the change of thought which it effects, this idea of natural selection is unquestionably the most importantidea that has ever been conceived by the mind of man. Yet the wonder is that it should not have been hit upon long before. Or rather, I should say, the wonder is that its immense and immeasurable importance should not have been previously recognised. For, since the publication of this idea by Darwin and Wallace, it has been found that its main features had already occurred to at least two other minds—namely, Dr. Wells in 1813, and Mr. Patrick Matthew in 1831. But neither of these writers perceived that in the few scattered sentences which they had written upon the subject they had struck the key-note of organic nature, and resolved one of the principal chords of the universe. Still more remarkable is the fact that Mr. Herbert Spencer—notwithstanding his great powers of abstract thought and his great devotion of those powers to the theory of evolution, when as yet this theory was scorned by science—still more remarkable, I say, is the fact that Mr. Herbert Spencer should have missed what now appears so obvious an idea. But most remarkable of all is the fact that Dr. Whewell, with all his stores of information on the history of the inductive sciences, and with all his acumen on the matter of scientific method, should not only have conceived the idea of natural selection, but expressly stated it as a logically possible explanation of the origin of species, and yet have so stated it merely for the purpose of dismissing it with contempt[26]. This, I think, is most remarkable, because it serves to prove how very far men’s minds at that time must have been from entertaining, as in any way antecedently probable, the doctrine of transmutation. In order to show this I will here quote onepassage from the writings of Whewell, and another from a distinguished French naturalist referred to by him.
In 1846 Whewell wrote:—