On the Variation of Species, with Especial Reference to the Insecta Part 4

As regards the instability displayed by _colour_, in the insect tribes, when subjected to the action of certain conditions and influences from without, so much has been said in the fourth section of the preceding chapter, that it is unnecessary to repeat it here.

True it is that it was then my sole province to discuss the _causes_ which would appear to regulate, in a large measure, the external aspect of the Annulosa; yet the _existence_ of inconstancy, in the several organs and characters involved (with which alone we are now concerned), was, by the nature of the case, implied: so that if the _disturbing element_ was demonstrated, the mere fact that the thing (whatsoever it may have been) _was interfered with_, was surely proved _a fortiori_. I there pointed out the great p.r.o.neness to a change in hue which divers circ.u.mstances are apt to induce; and I particularly instanced proximity to the sea-sh.o.r.e, and other saline spots, as well as an attachment to calcareous districts, as amongst the most powerful of the deranging contingences. In case, however, that any further evidence should be looked for, on this immediate subject, I will quote the following,--relating to the _Bembidium Atlantic.u.m_ of the Madeira islands, which was but just touched upon in that chapter,--as a concluding example of the general effect of physical agents on the colour of these lower creatures. "Throughout all the Madeiran Coleoptera there is perhaps no insect which displays such an extraordinary range of colouring as the present one does; and although it is true that the section of _Bembidium_ to which it belongs is essentially a variable one, yet I am not acquainted with any _Peryphus_ in which the paler patches of the elytra are so remarkably unstable, or which appear to be so completely under the control of external circ.u.mstances, as are those of the _B. Atlantic.u.m_: and indeed unless viewed in the ma.s.s, we should scarcely be inclined to recognize the same species in the many aspects which it puts on between its extremes. The examination, however, of a very large number of examples, and a careful consideration of the several localities and alt.i.tudes in which they were taken, has convinced me that there is unquestionably but a single type of form amongst my entire series, since the whole are so intimately connected, by successive gradations both of outline and colour, that it is perfectly impossible to isolate even a single specimen, or to draw a line of specific demarcation between any two consecutive members of the chain. It will be perceived, by a reference to the diagnosis, that the insect in question pa.s.ses imperceptibly from nearly a pure green, through a well-defined spotted state, into one which has the elytra almost testaceous,--the paler portions being at last so largely developed as to become confluent, and almost to cover the entire surface. In Madeira proper the darker varieties would seem to be typical; whereas in Porto Santo the brightly coloured ones preponderate, and in fact are all but universal. Both extremes do nevertheless occur in both islands, the tendency being merely, in either case, to a.s.sume the particular modification characteristic of the spot[58]".

And so it is with the outline and sculpture (no less than with bulk and hue): they also are equally liable to disturbance from physical causes, as indeed has been already insisted upon. Like most of the minutiae of variation, however, to which we have called attention, it is more particularly on islands that this is to be observed,--isolation, during an interval sufficiently long, appearing to possess some especial control over the external contour and surface of the insect races. Thus, in the Madeiras, for instance, the _Caulotropis lucifugus_ has its prothorax more distinctly punctured, and its elytra more perceptibly striated, in the princ.i.p.al island, than on any of the smaller members of the group; in Porto Santo, indeed, it is almost free from sculpture of any kind; whilst its ally, the _C. conicollis_, apart from being somewhat larger, is, on the contrary, both more punctured and striated on the Dezerta Grande than it is in Madeira proper. The _Omias Waterhousei_, again (in addition to its slightly increased bulk and less shining envelope, in that locality), is more lightly impressed on the Dezerta than it is in Madeira and, not to mention other differences, the _Ellipsodes glabratus_ is densely beset with most minute granules on that same rock--whereas on the mountain slopes of the central ma.s.s, it is highly polished and glabrous. The _Helops confertus_, we have intimated at a previous page, is less coa.r.s.ely sculptured in the lofty regions of Madeira, than in the lower ones: and the _H. futilis_ has its elytral tubercles apparent in Madeira proper, but evanescent on the Dezerta Grande. The _Eurygnathus Latreillei_ a.s.sumes a permanent variety on the Dezerta, the insect having become modified through a long isolation on those weather-beaten heights,--here it not only attains a more gigantic stature than in Porto Santo, but is invariably also more parallel and opake, has the sides of its prothorax more recurved, with the punctures towards the lateral angles almost obsolete, and the striae of its elytra somewhat more evidently punctate[59].

Such examples, however, might be multiplied _ad infinitum_; and I will not therefore devote further s.p.a.ce to the bringing together of facts which it is hardly possible will be disputed,--especially as it has been my wish, in the present chapter, merely to _enumerate_ what the organs and characters princ.i.p.ally are which are more peculiarly sensitive to change, throughout the Annulose tribes. This I may venture to hope, though briefly, I have in part done; and I will consequently pa.s.s on to other considerations, which, even if somewhat alien to the immediate question of insect instability, should scarcely be altogether omitted in a treatise like this.

FOOTNOTES:

[46] Insecta Maderensia, pp. 56, 57.

[47] Introduction to the Modern Cla.s.sification of Insects, ii. p. 466.

[48] _Id._ ii. p. 469.

[49] _Id._ ii. p. 454.

[50] Introduction to the Modern Cla.s.sification of Insects, ii. p. 480.

[51] Essai, p. 103.

[52] Introduction to the Modern Cla.s.sification of Insects, ii. p. 473.

[53] Trans. of the Ent. Soc. of London, ii. p. 60.

[54] _Id._ ii. p. 59.

[55] Insecta Maderensia, pp. 260, 261.

[56] Vide _supra_, p. 5.

[57] Although, in our ignorance of their real nature, we cannot cite them as actually a.n.a.logous to these separate phases in certain members of the Insecta, yet we are forcibly reminded by the latter of the distinct states which many of the Terrestrial Mollusca present (frequently in equal proportions) in the same localities. Thus, most of the _Pupae_ have at least two abruptly-marked forms,--a larger and smaller one. Many of the _Helices_ also exhibit this tendency in an eminent degree: I have indeed been shown specimens by Sir Charles Lyell of the _Helix hirsuta_, Say, from North America, one state of which is considerably more than double the dimensions of the other; and I believe it is a well-known fact that intermediate links _have_ not yet been observed to connect the extremes. May not therefore the gigantic _H. Lowei_ and _Bowdichiana_, which are now extinct in the Madeira Islands, have been but forms of the _H. Portosanctana_ and _punctulata_, respectively,--co-existent with them, though more sensitive to the great diminutions of alt.i.tude and area which were consequent on the breaking-up of a once continuous land? If such be the case, however, it is certain that they were far commoner at an early period than their smaller colleagues (which, now, in their proper districts, absolutely teem),--seeing that the _latter_ are extremely rare in the fossil deposits, whilst they themselves literally abound.

[58] Insecta Maderensia, p. 78.

[59] Insecta Maderensia, pp. 21, 22.

CHAPTER V.

GEOLOGICAL REFLECTIONS.

We frequently hear it a.s.serted, that, since the members of the Insecta are so numerous and minute, when compared with those of other departments of the organic world, the entomologist, whose province it is to collect and cla.s.sify them, can have but little time, if he attempt the real advancement of his particular science, for generalizations on a broad scale. Now, whilst there is necessarily some reason in this remark (for the investigation of species is a work of such labour and drudgery that it is apt to monopolize all the leisure hours which the greater number of us are able to command), we should recollect, on the other hand, that the soundest theorists have ever been the most patient and accurate observers; and have, many of them, spent whole years of their lives as humble students in Nature's domain. We need not be afraid that an occupation amongst what is microscopically small is liable to cramp the mind, and render it unfit for wider processes of induction, since the very opposite of this would seem to come nearer to the truth. The understanding which has been well tutored by a system of close and steady observation, which has been trained to seize upon differences amongst the objects of our common experience, to balance the importance of generic and specific characters, as tested in the acquisitions of our daily walks; and which has been gradually brightened and matured by the habitual exercise of its judgment on the most trifling phaenomena around us, has usually gained strength enough to form conclusions from such data, which will not only stand the test of a.n.a.lysis, but will be free from those eccentricities of genius which too often mar the speculations of less practical naturalists. The mind, moreover, having been chained and fettered for a season to the mere detail of facts, breaks forth, under such circ.u.mstances, with all the vigour with which the contemplation of truth has gifted it, and takes its flight as it were to a clearer sky; and, though a reaction may at times set in, hurrying it away into regions beyond its sphere, it will a.s.suredly return at length, fraught with the soberness which its vocation has inspired, and commence to build up its hypotheses, step by step, in harmony with the material which it has ama.s.sed.

Yet though entomologists may be in reality as well qualified as any other natural historians for drawing general conclusions from the result of their researches, it is impossible to conceal the fact, that, as a body, they have not ordinarily done so. Whether this has happened through an accidental disinclination on their part to occupy themselves in such matters, or (which is more probable) from their whole time having been engrossed by the dry routine of their science, I do not pretend to determine: be the solution, however, what it may, the inference is practically the same,--that the Annulosa have not hitherto been sufficiently regarded, in the great questions of zoological geography. But especially have they been ignored during that most significant of considerations which has been so ably brought forward of late years by some of our keenest observers,--namely, the distribution of animals, as affected by geological changes, on the earth's surface.

It would be well if the collector of insects would devote at least a t.i.the of his energies to the speculative branch of his subject.

Certain it is that much would probably be advanced, at first, on slender premises; and would, as a consequence, fall to the ground, leaving no record behind it. Yet such must inevitably be the case, at the outset, in every region of inquiry; and we are prepared to expect it. It does not however follow that _good_ would not be developed also; whilst we are confident of the fact, that unless the trial be made, it cannot possibly arise. No question has ever yet been mooted without beneficial results: it has either been shown to be absurd, and has received its death-blow on the spot, or else truth has been elicited (indirectly perhaps), which has at once shed a new ray of light on some of its obscurest bearings. And so, a.s.suredly, it would be in the present instance. We cannot doubt that there is much to be discovered in the past history of insect dissemination, which would tend, when rightly interpreted, to explain many of the occult phaenomena of the present day; and we may be equally satisfied that this cannot by any possibility be attempted without the a.s.sistance of geology. Let us therefore glance hastily at a few of those more undeniable convulsions which we are aware have, at various epochs, taken place; and endeavour to catch a glimpse of how, in the common course of things, that portion of the insect world would be affected which was exposed to their influence.

First and foremost, perhaps, in importance, of all the changes which it is self-evident have happened, may be mentioned _subsidence_.

Including, as it does, both the general lowering of some countries, and the actual isolation of others, there are, I believe, no physical crises to which we could point, through the instrumentality of which the very _existence_ of the insect races (not to allude to their diffusion) has been, by the nature of the case, more seriously interfered with. We know that there are certain species of an alpine and boreal character, which cannot live except in a climate of low temperature,--guaranteed to them either by _elevation_ in one land, or by a higher lat.i.tude in another: and let us picture the consequences of the gradual sinking of a mountain chain, even to a small extent, the _summits_ of which only just afforded the conditions of atmosphere necessary for the continuance of creatures like these. Now this is an example by no means far-fetched, and such as _must_ have occurred in instances innumerable. But, what would be the many results of a diminution in the level of our imaginary range? It needs no argument to prove, that _one_ at least would be manifest in the total extinction of those forms which could not adapt themselves to the increased heat. Others, which were able with difficulty to endure the alteration, would in all probability, even though they had now emigrated to the loftiest peaks, flourish less vigorously than before; and it is not unlikely, moreover, that they would become _somewhat modified from their normal states_,--states which, be it recollected (for this is an instructive lesson), would still exist in more northern zones.

During my researches in mountain tracts, I have usually remarked, that the highest points of land either teem with life, or else are perfectly barren. My own experience would certainly tend to prove, that, in a general sense, one or the other of these extremes does almost constantly obtain. And, although I would not wish to dogmatize on phaenomena which may in reality be explicable on other hypotheses, it would perhaps be worth while to inquire whether the geological movements of subsidence and elevation will not afford some clew to the right interpretation of them. Be this, however, as it may, I can answer, that in many countries, where there are strong indications of the former, the alpine summits harbour an insect population to a singular extent; whilst in others, where the latter is as distinctly traceable, the upland ridges are comparatively untenanted. Now we have already shown, that where the gradual lowering of a region has taken place, there will be, of necessity, an undue acc.u.mulation of life on its loftiest pinnacles,--for, even allowing a certain number of species (which _even formerly_ were only just able to find a sufficient alt.i.tude for their development) to have perished, we shall have concentrated at that single elevation the residue of all those which have survived _from the ancient elevations above it_. But, if, on the other hand, an area, already peopled, be in parts greatly upheaved, there will be _either_ a universal dying-out, from the cold, of a large proportion of its inhabitants, or else an instinctive striving amongst them to desert the higher grounds on which they have been lifted up, and to descend to their normal alt.i.tudes: in both cases, however, the present summits will display the same feature,--namely, utter desolation.

Such are a few of the effects which elevation and subsidence, even on a small scale, would seem (when tested by theory and practice) to produce. It yet remains for us to suggest, that the latter, when carried to its maximum, so as to cause the actual separation by the sea of one district from another, is a contingency of immense significance in regulating the distribution of the Annulose tribes.

Their outward contour and aspect we have shown in a previous chapter to be very largely beneath the control of isolation, provided a sufficient _time_ can be granted for the change: but their ultimate absence from any particular place, through the impediment which it offers to their migratory progress, we have not yet touched upon. Let us conceive, therefore, an extensive continent; and, since the insects which at present inhabit our earth must, if the doctrine of specific centres be true, have been originally created in certain definite spots, let us suppose a limited proportion of them to have been first produced upon this tract. Self-dissemination, we will a.s.sume, has been going on for centuries: those species which were gifted with quick diffusive powers have become pretty evenly dispersed over its surface; whilst those of naturally slow or sedentary habits have peopled, comparatively, but small areas around the respective localities of their birth. Such may have been the case, at some fixed period, amongst the aboriginal beings of any country which we choose to select as an ill.u.s.tration. But there is another element to be considered. If this region be not insular, it will have received colonists from foci of radiation situated beyond its bounds; and these, therefore, according to their several capabilities for progression, will have, likewise, in parts, overspread, or tenanted, it. Now it is impossible to cite a more simple example than this. But let us endeavour to realize what would be the necessary consequence of the breaking up of such a district as that which we have imagined. If a _general_ sinking should take place, causing its higher points to be alone visible above the ocean, or merely a _partial_ one, so as to admit of the sea encompa.s.sing portions of it which would remain unaffected in their alt.i.tude; the result practically would be the same,--namely, the const.i.tution of a group of islands out of a once continuous land. Then, as regards the animal population of this tract, the main phaenomena are almost self-evident. Should any of its isolated fragments chance to contain a portion of one of _those limited areas_ which a species of slow progressive powers had succeeded in colonizing, it would of course harbour (provided that the other portion has disappeared) what would now be defined as _endemic_.

Numbers of these small areas, or, in other words, of the species which had overspread them, would in all probability be lost for ever; whilst the occurrence of any of the surviving ones in more than a single island would manifestly depend on the proximity of the islands _inter se_. Those forms which had diffused themselves over the whole original continent would now be found in all the detachments of the cl.u.s.ter; whilst others, which had wandered over the greater portion of it only, might be traceable perhaps in every island _except a few_.

Such are the primary facts which suggest themselves, whilst discussing the question of isolation as regulating the _distribution_ of the Annulose tribes. Its _after effects_, on their external configuration and development, we have examined in a preceding chapter of this treatise; and we have also lately intimated what might be a few of the presumptive consequences of a subsidence (in a general sense), _apart from_ the still more important principle of isolation. Before, however, we dismiss these brief and elementary reflexions on the upward and downward movements which geology testifies to have occurred, at various epochs, on the earth's surface, I shall perhaps be pardoned if I digress so far from my immediate subject as to trace out some of the actual results of isolation in the diffusion of the Insecta (especially recognizable in the stoppage of a former migratory progress) in a few of the northern Atlantic groups. I should premise, however, that it is from the Coleoptera alone that I shall attempt to draw my inferences; nevertheless, since that order is more extensive than any of the others, and has moreover been closely investigated in most of those islands, it may possibly afford us data of sufficient comprehensiveness and accuracy for practical purposes.

To commence, then, with the Madeiras and Canaries; the first facts which isolation discloses to us, concerning the statistics of a region which was once continuous throughout that portion of the Atlantic, are the _slowness_ and the _direction_ of the ancient migratory movements.

The former of these is rendered evident from the vast number of endemic species which are at present contained, not merely in the two groups combined, but in the several islands of which each of them is composed. True it is, that these peculiar forms are, most of them, apterous, and of naturally sluggish self-disseminating powers; yet, still the circ.u.mstance remains, that these various creatures had not overrun areas of any extent before the land of pa.s.sage was destroyed,--for otherwise they must have occurred, now, on islands and rocks but slightly removed from each other, _which they do not_. The latter of the above conclusions, namely, the _direction_ of the migratory current, will become apparent in the sequel. We may premise however, that, so far as the aborigines of this province are concerned, their course will be found, upon the whole, to have been a _northerly_ one.

As regards the slowness, and the direction, of the _quondam_ migration (questions which can scarcely be treated apart from each other), some light may be thrown on the subject from considerations like the following. The Canaries are the head-quarters of the genus _Hegeter_; Teneriffe may indeed be called the land of Hegeters. No less than thirteen or fourteen species have been recorded as indigenous to those islands; and there can be no reasonable doubt whatsoever that that ancient region (when continuous and entire) was the primaeval centre, or range, of that Heteromerous group. The Hegeters are an apterous race, and of a sedentary temperament; hence, when the area (whether by general or partial subsidence, it signifies not) was broken up, it is not surprising that those local fragments of it should have become the nucleus of reception, as it were, for the members of that genus.

Nevertheless, a few of these many representatives (of more discursive capabilities perhaps than the rest) had found their way, before the period of dissolution, to a considerable distance from their original haunts. Thus, one of them (the _H. latebricola_, Woll.) had arrived at what now const.i.tutes the rocks of the Salvages; another (the _H.

elongatus_, Oliv.), at least, if not two, had colonized the Madeiras, and is said (though I believe incorrectly) to have even reached the present coast of Portugal. This latter species is clearly of a more adaptive nature than its allies, inasmuch as it has, also, naturalized itself (though this may be a more recent, and accidental, circ.u.mstance) on the opposite sh.o.r.es of Africa. One thing, however, is at any rate manifest,--that the Hegeters attain their maximum in the Canaries, and that a few members only have been sent off, in a northerly, or north-easterly, direction, from thence.

In like manner, the genus _Tarphius_ is distinctively Madeiran. I have detected nearly twenty well-defined species of it in that group; yet, out of so large a number, two only have occurred beyond the central island. Now the _Tarphii_ are, also, wingless; and creatures of very sluggish propensities,--scarcely ever stirring from the ma.s.ses of loose rotting timber which they so a.s.similate in hue, and to the under sides of which they affix themselves, day and night. Although difficult to investigate in their precise economy, it is extremely probable (may I not say, certain?) that some important and peculiar office is a.s.signed to them in the remote upland districts to which they exclusively belong: and there cannot be any question, to a person who has studied them carefully on the spot, but that the region which they now inhabit is the actual area of their primaeval appearance on this earth. Many kindred species may of course have been lost, during those gigantic subsidences which caused the Madeiras to be shaped out, and to tell their tale above the waves as ruins of an ancient land; yet our existing cl.u.s.ter of forms could not have wandered far at that early period, from the Serras and ridges of their birth,--perhaps not _so_ far indeed (considering the limited bounds within which they are now confined, and that time should in reality have increased their range rather than diminished it) as they have succeeded in doing at the present day. Hence we may reasonably conclude, that Madeira proper is an example of what we have alluded to in a preceding page,--namely, of the accidental retention, during a vast downward movement, of a nucleus of small specific areas of colonization, the colonizers of which _had not extended elsewhere_. But I stated, that two of the above-mentioned _Tarphii_ have occurred beyond the central ma.s.s. It is in Porto Santo that they make their appearance; nevertheless, since one of them is apparently peculiar to that island, it is only the _T.

Lowei_, Woll. (an insect of a different, and more active, nature than the rest) which has violated that _local exclusiveness_ which would seem to be almost a generic character, as it were, of its allies. That species, however, both in its manners and aspect, recedes materially from the remainder. Although, like them, nocturnal in its habits, it is able to run with considerable velocity; and, instead of attaching itself to the blocks of putrefying wood, which both fall and decay _in situ_ on those elevated tracts, it hides within the bunches of _Evernia scopulorum_ and _prunastri_ which clothe the trunks of living trees, and fill up the crevices of the weather-beaten peaks. Hence, when contrasted with its comrades, we can easily understand how the varied processes of accidental transportation would operate to increase the range of a creature which differs so essentially, in many respects, from them. It is indeed, not unfrequently, brought down, at the present day, by _human_ agencies from the mountain-slopes; for, since the cutting of f.a.ggots is one of the few sources of livelihood to a large proportion of the poor of Funchal, numerous insects of subcortical and lichen-infesting tendencies are subject to be naturalized (provided they can adapt themselves to the change) in alt.i.tudes lower than their normal ones: so that there are many chances, even _a priori_, in favour of the _T. Lowei_ having overspread, whether by natural or artificial means, a wider area than its congeners. I believe that there is no such thing as a _Tarphius_ in the Canarian Group: nevertheless, singularly enough, a representative, which is more akin to the _T. Lowei_ than to any other hitherto discovered (and which was imagined until lately to have been the sole exponent of the genus), namely, the _T. gibbulus_, Germ., occurs in Sicily. From which data we arrive at this significant fact: that, whilst Madeira proper is, without doubt, the original centre of the _Tarphii_, two species (one of which is, likewise, Madeiran) are found in Porto Santo, to the north-east of it; whilst a third makes its appearance in an island of the Mediterranean.

The genus _Acalles_ presents a nucleus of species in the Canaries, moulded on a very large pattern. A closely allied member, the _A.

Neptunus_, Woll. (which may perhaps be in reality but an insular modification of the _A. argillosus_, Schon., from Teneriffe), has been detected on the rocks of the Salvages, to the north of them; whilst on the Dezerta Grande, one of the most southern stations of the Madeiran Group, we have a third, which displays far more in common with the Canarian type than it does with that which obtains in Madeira proper;--which last is gradually, in its turn, merged into the ordinary European form. The genus _Pecteropus_, Woll., is another instance in point. I possess three or four species from the Grand Canary, Fuertaventura, and Teneriffe; and I believe it will be found, on inquiry, to attain its maximum in that cl.u.s.ter. Unlike the others, however, which we have just cited, it is powerfully winged; and we should consequently expect to trace the evidences of its northward progression with comparative perspicuity. Can we therefore do so? Yes: in Madeira proper it has two representatives, and in Porto Santo (to the north of it) one. And so with _Xenostrongylus_, Woll. (which is likewise winged), we have two species, at least, in the Canaries; one in the Madeiras; and a third, unless I am mistaken, in Sicily. The genus _Ditylus_ is shadowed forth in the Canary Islands by two or three singular representatives of a pallid, testaceous hue; and, although the group is entirely absent in Madeira, a species (the _D.

fulvus_, Woll.) is found on the 'Great Piton' of the Salvages, so nearly resembling, except in its smaller size, one of those from the Canaries that I think it far from improbable that it is a fixed insular state of that insect. _Deucalion_, also, may be quoted in support of this twofold hypothesis, of the direction, and the slowness, of the former migratory movements. It is an apterous genus, and of eminently sluggish habits; and what is the consequence?--we have a very remarkable species (the _D. oceanic.u.m_, Woll.) on one of the rocks of the Salvages, whilst another (the _D. Desertarum_, Woll.) has been isolated on the two southernmost islands of the Madeiran Group; and of so sedentary a nature is this last, that, although physically unimpeded, it has not, even to this day, overrun the diminutive areas on which, when the surrounding region was submerged, it was originally saved from destruction. So strongly indeed was this fact impressed upon me, when I first detected it, that I shall perhaps be excused for recapitulating _in extenso_ the few reflexions which then suggested themselves to my mind. "There is no genus, perhaps, throughout all the Madeiran Coleoptera, more truly indigenous than _Deucalion_. Confined apparently, so far as these islands are concerned, to the remote and almost inaccessible ridges of the two southern Dezertas, it would seem to bid defiance to the most enthusiastic adventurer who would scale those dangerous heights. Its excessive rarity, moreover, even when the localities are attained, must ever impart to it a peculiar value in the eyes of a naturalist; whilst its anomalous structure and sedentary[60] mode of life give it an additional interest in connexion with that ancient continent, of which these ocean ruins, on which for so many ages it has been cut off, are the undoubted witnesses. Approximating in affinity to _Parmena_ and _Dorcadion_, yet presenting a modification essentially its own, it becomes doubly important in a geographical point of view; and it was therefore with the greater pleasure that I lately received a second representative, from the distant rocks of the Salvages,--midway between Madeira and the Canaries. Differing widely in specific minutiae, yet agreeing to an ident.i.ty in everything generic, they offer conjointly the strongest presumptive evidence to the _quondam_ existence of many subsidiary links (long since lost, and radiating in all probability from some intermediate type) during the period when the whole of these islands were portions, and perhaps very elevated ones, of a vast continuous land. * * * * * The _Deucalion Desertarum_ is of the utmost rarity, the only two[61] specimens which I have seen having been captured (the first by myself, in 1849; and the second by the Rev. R. T. Lowe, in 1850) on the respective summits of the Middle and Southern Dezertas. So local indeed does it seem to be, that it, apparently, has not extended itself even over the Dezerta Grande (where there are no external obstacles to bar its progress); but retains the very position which in all probability const.i.tuted its original centre of dissemination at the remote period of time when this ancient continent received its allotted forms. Judging from the slowness with which creatures of such habits must necessarily, under any circ.u.mstances, be diffused, it is at least unlikely that the present one could have circulated far, when the now submerged portions of that region began to give way; and hence it is not impossible that the Southern Dezerta, with the adjacent part (then united to it) of the Central one, may have embraced the _whole area_ of its actual primaeval range,--the remains of which (though they be now separated by a channel) it still continues to occupy, and from which, even when physically unimpeded, it has never roamed[62]."

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 primaeval 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 cl.u.s.ter harbours real species, which are now peculiarly its own. Thus (to select an ill.u.s.tration 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 _debut_ 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 h.o.m.ologues 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 a.s.semblage 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 versa_); yet, with the a.n.a.logy of the three already mentioned before us, I am inclined _a priori_ to view it as distinct. These, also, occur in a subfossil state; and no alteration appears to have been brought about, by either circ.u.mstances 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 pa.s.sage 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 primaeval 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 h.o.m.ologues 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 sh.e.l.ls, I think I may a.s.sert, 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 h.o.m.ologue 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 primaeval radiating forms.

We must not however omit to notice, that some few of these endemic _Helices_ appear to have been gifted (as we should _a priori_ antic.i.p.ate) 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 const.i.tutes 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 circ.u.mstances, 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 a.s.sumes 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 c.o.xinha, 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 t.i.tle 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 const.i.tutes the _H. lincta_ (with an additional pale variety for the calcareous district of Canical),--and the _H. saccharata_, from the So Lourenco promontory.

In the same way we might pursue the _H. erubescens_, and show that in the sylvan regions, and on the low barren Ponta So Lourenco of Madeira, on the Pico de Facho of Porto Santo, on the Ilheo Cho, 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 const.i.tute the various islands of the cl.u.s.ter, 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 circ.u.mstance 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.