Although it is not my province in this volume to draw inferences from data which are not strictly entomological, I shall perhaps be pardoned for adding a few words on the testimony which the Land Mollusca of the Madeiras would seem to afford, in support of the general slowness of the animal migrations over that primæval continent. The researches of the Rev. R. T. Lowe, and of myself, on every rock and island of the group, have, it appears, so nearly exhausted the whole number of species which lately remained to be found, that the conchological statistics are perhaps, at the present time, more accurate than those of any other department of the fauna: and, independently of the modifications which have been manifestly brought about, in some few instances, by isolation, since the periods of subsidence, it is truly singular to remark how every detached portion of the entire cluster harbours real species, which are now peculiarly its own. Thus (to select an illustration from amongst the most anomalous of the endemic forms), we have in Madeira proper, Porto Santo, and on the Southern Dezerta, respectively, true representatives, in the Helix tiarella, coronata, and coronula,—which in all probability still occupy the positions (or nearly so) of their original début upon this earth. Considering the sluggish, or sedentary, nature of the Terrestrial Mollusks, it is extremely likely (nay, almost certain) that many intermediate links, radiating from the same type, were lost for ever, when the gigantic movements which rent this ancient region were in course of operation: so that, if such were in reality the case, we need not be surprised that one at least of this small geographical nucleus should have been preserved on three of the existing islands of the group. That these are actual species (saved alive from their fellows, after the wholesale destructions in this Atlantic province had been completed), and no results of insular development, is demonstrated by the fact that two of them (for the third has apparently become extinct) have not altered one iota since the fossil period, which, in the opinion of Sir Charles Lyell, is anterior to the dissolution of the intermediate land;—whereas, had they been mere modifications of each other, induced by the local conditions and influences to which they have been, through a long series of ages, severally exposed, the difference between their recent contour and that of their fossil homologues would have been doubtless at once conspicuous. I gather, therefore, that like the Tarphii, to which we have lately drawn attention, they are veritable surviving members of an esoteric assemblage which found its birth-place on this post-miocene (?) tract.
In a similar manner, the H. undata in Madeira proper, the H. Vulcania on the Dezertas, and the H. Porto-sanctana in Porto Santo, are representative species,—each occupying the same position, and being equally abundant, on their respective islands: and, although it may be a problem whether the second of these is not an insular modification of the first (or vice versâ); yet, with the analogy of the three already mentioned before us, I am inclined à priori to view it as distinct. These, also, occur in a subfossil state; and no alteration appears to have been brought about, by either circumstances or time. And so it is with numerous others (as the H. latens in Madeira, and the H. obtecta in Porto Santo; the H. squalida in Madeira, and the H. depauperata in Porto Santo; the H. Delphinula in Madeira, and the H. tectiformis in Porto Santo), which are no less representative inter se. From which we are driven to conclude;—first, that this quondam continent was densely stocked at the beginning with foci of radiation created expressly for itself[63]; and, secondly, that the areas which these various creatures had overspread, before the land of passage was broken up, was extremely limited,—or, which amounts to the same thing, that their migratory progress was unusually slow.
Touching the two-fold question, of the local engagement of this Atlantic district with specific centres of diffusion, and the extreme slowness of their diffusive progress, much instruction may be derived from a contemplation of the conchological statistics. Porto Santo, for instance, is a very small island (not more than seven miles in length), yet the number of endemic species which it includes is so perfectly astounding that it may be appropriately termed a generic area of radiation. Nor does this primæval excess of its aboriginal beings strike us more forcibly than does the utter quiescence (if I may so express it) which has been going on amongst them since the remote era of their birth. Although a few have apparently died out[64] since that epoch, consequent perhaps on the change of level and diminished range which took place during the process of subsidence; we are amazed to find that certain species which are now limited to particular spots (even whilst unopposed by physical barriers) have been absolutely peculiar to them from the first,—or, in other words, that, whilst the fossil deposits extend throughout the lower regions of the island, far and wide, it is only in those respective portions of the beds which join on to the present "habitats" that the fossil homologues of several of the species are to be met with. The H. Wollastoni is eminently a case in point. That most interesting of the Madeiran mollusks was first detected by myself on the southern ascent of the Pico de Conseilho, of Porto Santo, April 22, 1849; and the subsequent explorations of the Rev. R. T. Lowe, in conjunction with my own, have, I think, satisfactorily proved that it occurs nowhere else except upon that single slope. Throughout the large expanse of calcareous incrustations which are spread over the island elsewhere, and on the adjoining Ilheo de Baixo, all of which teem with shells, I think I may assert, without fear of contradiction, that the H. Wollastoni does not so much as exist. Yet at the Zimbral d'Areia, which the Pico de Conseilho directly overhangs,—a rich tract for these fossil remains,—as well as in the muddy composition of a cliff near at hand, it literally abounds.
In like manner, we might recall many others which are peculiar, recent and fossil, to the self-same precincts. Such, for example, are the H. calculus and commixta, which swarm on the summit of the Ilheo de Baixo, in both states. The H. attrita, again, is the Pico d'Anna Ferreira modification of the H. polymorpha; and it is only in the beds towards the base of that mountain that its fossil homologue is found. But what do these facts indicate? Surely they tell us plainly of what we have already so often insisted upon,—namely, the redundancy of this once continuous land with specific foci of its own, and the sluggish or sedentary nature of those primæval radiating forms.
We must not however omit to notice, that some few of these endemic Helices appear to have been gifted (as we should à priori anticipate) with more rapid capabilities for diffusion than the rest. Thus, the H. erubescens and paupercula seem not only to have colonized the entire province of which the Madeiras are detached fragments, but to have even found their way to that distant portion of it which now constitutes the Azores. The H. polymorpha has also penetrated the Madeiran region throughout; and being, like the H. erubescens, peculiarly sensitive to the action of external influences, we perceive, in consequence, that almost every island and rock has now its own especial phasis of it. So greatly indeed is that species beneath the control of local circumstances, that the very districts of an island as insignificant as Porto Santo have each their separate races to boast of. On the Pico d'Anna Ferreira it assumes a form to which the name of H. attrita has been applied; when on the Ilheo de Baixo, it is the H. papilio; at the Zimbra d'Areia, on the Pico de Conseilho, and in the Ribeira da Coxinha, it is the H. pulvinata; and, in many other situations widely removed inter se, it puts on the shape (variable, both in size and hue) to which the title of H. discina has been given. But, if we leave Porto Santo, and follow this Protean Helix into the other divisions of the group; we meet with it on the Dezertas as the H. senilis (those moreover from the central island having a much more open umbilicus than is the case in the northern and southern ones), whilst in Madeira proper it constitutes the H. lincta (with an additional pale variety for the calcareous district of Caniçal),—and the H. saccharata, from the São Lourenço promontory.
In the same way we might pursue the H. erubescens, and show that in the sylvan regions, and on the low barren Ponta São Lourenço of Madeira, on the Pico de Facho of Porto Santo, on the Ilheo Chão, on the Central Dezerta, and on the Bugio (where it attains a gigantic size), it has its distinct and permanent phases,—the evident results of isolation, and other topographical influences, since the subsidence of the intervening tracts. And in like manner, the Clausilia deltostoma is universal throughout the Madeiran Archipelago,—displaying, however, in Porto Santo a fixed and strongly ribbed state, peculiar to that island. Thus, if the examples which we previously cited tend to establish the extreme slowness of the migratory movements of the terrestrial mollusca across this former continent, the present ones (which refer to a few exceptional species of quicker self-diffusive powers) will show, no less than the insects to which I have lately called attention, that where sufficient areas had been overspread (before the periods of subsidence) for the creatures to have reached what now constitute the various islands of the cluster, we at once detect traces of this fact, through their more or less altered aspects,—the result of isolation, and diminished range, during the enormous interval which has elapsed since the successive convulsions which caused the partial destruction of this Atlantic province were brought to a close.
To return, however, to the insects, after this long conchological digression,—I need not multiply evidence, in corroboration of my theory. Enough has been said to render intelligible the idea which I wished to convey, concerning the general direction of the migratory current over that ancient tract, and the extreme slowness of its progress,—the former of which I consider probable from the north-easterly course in which creatures generically identical were, if we may so express it, "given-off;" whilst the circumstance of their being for the most part specifically dissimilar (or, in other words, of the islands harbouring, many of them, species which are endemic) would seem as it were to establish the latter.
We must not however forget, that it is only to the aborigines of this quondam land that the above speculations apply. Assuming the region not to have been insular, that is to say, to have been connected, on its outer limits, with a European, or Mediterranean, continent; it would necessarily follow, that a certain number of colonists must have found their way over its area, and moreover in an opposite direction to the living stream (if we may so call it) which had been long flowing in a north-easterly course across its surface. Whatever be the length of the periods, however, during which these counter migrations were going on, I think it sufficient to state that I would refer them to epochs altogether different,—so that, accompanied as they may have been by special geological phænomena, which, if known, would in all probability become at once explanatory, we should be the less inclined to regard as absurd what might appear at first sight difficult to understand. In the case of the British Isles indeed, no less than five of these distinct migratory eras have been assumed, and specified[65], by Professor Edward Forbes; therefore (whatever value be attached to his able and interesting theory) I do not consider it necessary to apologize for requiring at least two in behalf of this ancient Atlantic province. Not to insist upon those of his faunas and floras which are of a less evident, or more questionable, character, he has at any rate proved, I think, almost to a demonstration, the westward progress of the great mass of our British animals and plants, over a then unbroken land (the upheaved bed of the glacial sea), from the central Germanic plains; whilst the accurate calculations of the late Mr. Thompson of Belfast, concerning the reptile statistics of Ireland, England, and Belgium, respectively, have succeeded in showing, with much presumptive reason, how the formation of St. George's Channel, before that of the German Ocean, interrupted the march of these wanderers to the far West, and debarred an immense proportion of them from an entry into Ireland,—which would otherwise have colonized that country equally with our own.
As regards Professor Forbes's views of the creation of a vast continent (reaching far into the Atlantic[66]) at the close of the miocene epoch, through the upheaved bed of a shallow miocene sea,—a region moreover of such an extent as to have connected the various island groups between the Fucus bank and the shores of the Old World, not only with each other, but with a Mediterranean province, Asturias, and even the south-west of Ireland,—I must be content to pass them by, hazarding only a few crude and desultory remarks. So large a question, indeed, cannot be safely handled without a corresponding amount of data, in all departments of natural science, to reason from,—which I do not possess: still, if a speculation from entomological premises, per se, be not altogether worthless, I would point to the conclusions (lately adverted to) which my Madeiran researches have forced upon me, concerning the direction of the former insect migrations,—inferences which are, from first to last, of necessity erroneous, if the requisite medium for transit (into South-European latitudes, at all events) be a mere conjecture or romance. Such a notion, however, I would not for a moment entertain,—for there is too much direct evidence in support of distinct epochs of diffusion, to allow of any hypothesis, when endeavouring to account for the phænomena which we now behold, to supersede the assumption of a once continuous tract. No matter if we be compelled to suppose, whilst attempting to interpret what we see, that the disseminating current has flowed in exactly opposite courses, at different and remote periods, over the surface of that ancient land,—seeing that the fact (if such in reality it be) remains untouched, that the land itself is at any rate there. I am not, however, prepared to assert that the opinion at which I had independently arrived, from the insect statistics, does positively require a northerly prolongation of that area beyond the line of the central Mediterranean districts; yet, after making every possible allowance for accidental introductions since the subsidences have taken place, there is still left a large residuum which I am convinced can never be explained (unless the doctrine of specific centres be a myth) except through the means of ordinary and regular migration over an unbroken continent. Nevertheless, though I would not presume, from insufficient material, to insist upon an extension of this Atlantic region into higher latitudes than those which I have just referred to, I must express my individual belief that, the more the subject is examined, with reference to the distribution of the Annulosa, the less will Professor Forbes's idea suffer from the inquiry. In the 'Insecta Maderensia,' I have already thrown out a few scattered hints which bear on this immediate consideration; and, since no subsequent reason has induced me either to withdraw or modify them (but rather the reverse), I will select the following,—extracted from my preface to that work.
"Taking a cursory view of the Coleoptera here described, the fauna may perhaps be pronounced as having a greater affinity with that of Sicily than of any other country which has been hitherto properly investigated. Apart from the large number of our genera (and even species) which are diffused over more or less of the entire Mediterranean basin, this is especially evinced in some of the most characteristic forms,—such as Apotomus, Xenostrongylus, Tarphius, Cholovocera, Holoparamecus, Berginus, Litargus, Thorictus, and Boromorphus. There is, moreover, strange though it may appear to be, some slight (though decided) collective assimilation with what we observe in the south-western extremity of our own country and of Ireland,—nearly all the species which are common to Madeira and the British Isles being found in those particular regions; whilst one point of coincidence at any rate, and of a very remarkable nature, has been fully discussed under Mesites. Whether or not this partial parallelism may be employed to further Professor E. Forbes's theory of the quondam approximation, by means of a continuous land, of the Kerry and Gallician hills, and of a huge miocene continent extending beyond the Azores, and including all these Atlantic clusters within its embrace, I will not venture to suggest: nevertheless, it is impossible to deny that, so far as the Madeiras betoken, everything would go to favour this grand and comprehensive idea. Partaking in the main of a Mediterranean fauna, the northern tendency of which is in the evident direction of the south-western portions of England and Ireland, and with a profusion of endemic modifications of its own (bearing witness to the engorgement of this ancient tract with centres of radiation created expressly for itself), whilst geology proclaims the fact that subsidences on a stupendous scale have taken place, by which means the ocean's groups were constituted; we seem to trace out on every side records of the past, and to catch the glimpses, as it were, of a veritable Atlantis from beneath the waves of time[67]."