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 tithe 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, assuredly, 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 phænomena of the present day; and we may be equally satisfied that this cannot by any possibility be attempted without the assistance 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 latitude 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 phænomena 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 accumulation 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 altitude 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 altitudes: 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 assume, 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 illustration. 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 encompassing portions of it which would remain unaffected in their altitude; the result practically would be the same,—namely, the constitution of a group of islands out of a once continuous land. Then, as regards the animal population of this tract, the main phænomena 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 cluster; 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 circumstance remains, that these various creatures had not overrun areas of any extent before the land of passage 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 primæval 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 constitutes 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, circumstance) on the opposite shores 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 masses of loose rotting timber which they so assimilate 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 assigned 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 primæval 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 cluster 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 mass. 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 faggots 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 altitudes lower than their normal ones: so that there are many chances, even à 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, Schön., 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 cluster. 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. oceanicum, 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 minutiæ, yet agreeing to an identity 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 constituted 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 circumstances, 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 primæval 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]."