The Madeiras would seem to inherit, as it were, a more than usual control over the alary system of their insect population; for, out of about 550 species of Coleoptera which I have hitherto met with in that group, nearly 200 are either altogether apterous, or else have their organs of flight so imperfectly developed, that they may be practically regarded as such; so that, if our preceding conclusions (from the compensation-hypothesis) be correct, we should à priori anticipate an increase of bulk in those islands, rather than a decrease of it. Unfortunately the greater number of these 200 representatives are now, through the submergence of the once surrounding continent, endemic, so that we have no means of judging whether the obsoleteness of their wings is to be referred to the long action of Madeiran influences[37], or whether they were thus created severally in the beginning; and, for the same reason (that is to say, having no others of their kind to compare them with), we cannot pronounce, even if we might assume this partial organic decay to be the consequence of their isolation on these rocks, whether their general stature has been subsequently augmented or not. Still, there are some few, out of the 200 just alluded to, which are of common European distribution; and, as these would appear to have obeyed the principles to which we have been calling attention, it is not unreasonable to suppose, that many of the others (could we but behold them as they formerly were,—emigrants over a vast continuous land) would be found to have done so also.
I alluded, in a previous section, to the Dromius obscuroguttatus, Dufts., as presenting permanent characteristics in Madeira,—the combined result of latitude and isolation; and I also stated that it was not always possible, whilst dealing with physical agents which are necessarily obscure, to refer the respective phænomena (whatsoever they may be), which would seem to have departed from their types, to a single disturbing cause. Hence, whilst I there acknowledged latitude as in part answerable for the changes which that insect has undergone, I may here suggest that it is, in all probability, to isolation that we must mainly look, if we would understand those changes aright. But what are the distinctive features, it may be asked, which the D. obscuroguttatus has adopted, since its first arrival from more northern latitudes over an unbroken[38] continent? It has not altered much, after all: it is, however, the nature of the alterations, and their constancy, which give them their real importance. In a few words then, the insect is rather larger and more robust than its European analogue, and (to omit other minor differences) its wings are evanescent. But this, on our above hypothesis, is precisely what we should have expected: for, since it is self-evident that the species cannot have been naturalized accidentally on these mountains, and since geology informs us that a vast interval has elapsed since the Madeiran islands were portions of a continuous whole, we have at once a sufficient time assured us for the modifications to be completed, and to appear at length permanently adjusted in accordance with the conditions and influences which locally prevail.
There are other examples which might be quoted in support of my theory,—that isolation, when involving a sufficient period of time, has a direct tendency either to diminish the stature of the insect tribes, or else to neutralize their power of flight; but that, in the latter case, the creatures, when thus despoiled of a function, do, on the contrary (instead of deteriorating in size), often receive a compensation for their loss by an actual increase in their bulk. The common Bradycellus fulvus, Mshm, is another instance in point. From its occurrence in the almost inaccessible districts of the Madeiran group, far removed from cultivation, I am inclined to refer its entry into this southern region to that remote period when a connective land offered a natural passage to wanderers from the north. Hence our first stipulation, that of sufficient time, is satisfied; and what is the result? The insect is a trifle more robust than its ordinary European representatives, and it is invariably apterous. The Calathus fuscus, Fabr., is also, as is clear from its special attachment to the mountain tops, strictly indigenous in Madeira (that is to say, it must have arrived there during the migratory epoch); and the consequence is, that, although usually winged in our own country, it is permanently subapterous in that island. I think it far from unlikely that the Dromius negrita, Woll., may be the ultimate phasis (from isolation) of the common D. glabratus, Dufts.,—from which it may be distinguished by its somewhat larger bulk, more robust head and prothorax, and by the obsoleteness of its wings. True it is, that the latter species flourishes alongside it in Madeira; but, like the Vanessa Atalanta (when considered with respect to the V. Callirhoë), may it not be of more recent importation from the European continent, and as yet in a transition state?—an idea which the smallness of its wing, as compared with those of its British analogues, would seem rather to corroborate.
But, if this slight increase of stature would appear generally to accompany that gradual extinction of the powers of flight which isolation is apt to induce, it follows, on the other hand (as indeed I have lately intimated), that where wings are so essential to the continuance of a species that they cannot, without its positive destruction, be taken from it, the primary effect of isolation,—namely a diminution of bulk,—will for the most part happen instead. As this fact, however, has been already commented upon, we will not discuss it afresh.
Why it is, in the Insecta, that islands[39] should predispose to an apterous state more than continents, it is not easy to speculate. Mr. Darwin has indeed suggested, and with much apparent reason, that, were wings fully developed, the indiscriminate use of them might lead to unhappy results, by tempting the creatures to venture too far from their native rocks; and that, therefore, this partial decay is, under such circumstances, a wise provision in their favour: whilst it has been urged, on the other hand, that since insular species are at all times liable during heavy gales to be blown out to sea, they should in reality be gifted with stronger powers of flight (rather than weaker ones), to fortify them against such disasters; and that, consequently, the above phænomena are not explicable on Mr. Darwin's hypothesis. For my own part, I am inclined to accept that theory, in all its fullness; and, furthermore, I do not believe that the latter consideration (though it unquestionably contains much presumptive truth) does at all interfere with the admission of it,—seeing that either requirement may be fulfilled, according to the nature of the several species which are destined to be acted upon. Thus, if flight is absolutely indispensable, as in the greater number of the Lepidoptera, and beetles of a flower-infesting tendency, we shall find that the wings remain unaltered (if indeed they be not actually increased in capacity, of which I am by no means certain), and that the effect of isolation is more particularly evident in a diminution of stature. But if, on the contrary, the creatures are less dependent on aërial progression for their sustenance, as in the predacious tribes generally, especially those of nocturnal habits, the reduced area in which they are confined, in conjunction, it may be, with the danger to which they would constantly expose themselves by the promiscuous employment of organs which their modes of life do not positively need, would seem to render the presence of wings unnecessary; and they are accordingly, by degrees, removed:—in which case, however, a compensation for the loss is not unfrequently granted by an increase (more or less perceptible) in bulk.
In the Madeiras, this diminution and enlargement of stature, accompanied for the most part respectively by the retention and annihilation of the powers of flight, is singularly traceable on the selfsame rocks, particularly the smaller ones of the group. Thus, on the Flat Deserta, or Ilheo Chão, the Scarites abbreviatus, Koll., Laparocerus morio, Schön., and the Helops Vulcanus, Woll., attain a gigantic size; yet it is on that very island that the Ptinus albopictus, Woll., finds its minimum of development,—scarcely exceeding in dimensions some of the larger members of the Trichopterygia. The Deserta Grande has some special modifying capability of its own,—the Eurygnathus Latreillei, Lap., Notiophilus geminatus, Dej., Zargus pellucidus, Woll., Calathus complanatus, Koll., Olisthopus Maderensis, Woll., Caulotrupis conicollis, Woll., Laparocerus morio, Schön., Omias Waterhousei, Woll., Helops Vulcanus, Woll., and the Ellipsodes glabratus, Fab., being also larger on that rock than is typical: all of them, however, with the exception of Notiophilus geminatus, are there, as elsewhere, apterous.
Other qualifying results, from isolation, are equally apparent. Take colour, for instance; and we shall perceive that in the Dromius sigma, Rossi, it is sensibly affected. The normal state of that insect "does not occur at all in Madeira proper, but only in Porto Santo. True it is that the modifications in the several islands present but slight differences inter se; nevertheless, being constant, I would lay particular stress upon them, since they go very materially to prove that the effects of isolation on external insect form are even more important, if possible, than those of latitude. That this is the case in the present instance, appears clear from facts so minute as these. For, out of the many specimens which have come under my observation from various countries of Europe, if there is one point more constant than another in this otherwise variable species, it is, I believe, to all circumstances, its immaculate prothorax. Now, whilst this (we may almost say essential) character obtains in Porto Santo, in Madeira it does not hold good: the prothorax there is invariably infuscate in the centre; and on a small adjacent rock (the Ilheo de Fora) it is entirely dark. Nor let anyone suppose that details apparently so trivial are beneath our notice, or the mere result of chance, since it is by the observation of such-like points, and by marking their development according to the circumstances of the several localities in which they obtain, that we are alone able to appreciate their importance, and so to form, in a wider and geographical sense, a correct estimate of their value[40]." The Olisthopus Maderensis, Woll., is much paler, larger, and more opake, on the Dezerta Grande than it is in Madeira proper. So great indeed is the change which it has undergone through a long isolation on that rock, "that, had the case been a solitary one, I should not have hesitated in regarding the specimens obtained from thence as specifically distinct; nevertheless, with the knowledge both of the modifying effects of isolation, and also of the kind of modification essentially peculiar to that island, I am perfectly satisfied that it is a mere local state, although a very remarkable one, and has no claim whatsoever to be otherwise considered[41]." The Pecteropus Maderensis, Woll., is of a greenish-brassy tinge in Porto Santo, and much acuminated in front; whereas on the Dezerta Grande it is almost invariably coppery, and less narrowed anteriorly. The Caulotrupis lucifugus, Woll., although ranging through no very opposite phases, either of outline or sculpture, "appears to possess a slight modification for every island of the Madeiran Group: and hence small shades of difference, which might otherwise be regarded as trifling, become directly important, and cannot be ignored in a local fauna,—even though a general collector may deem it unnecessary to recognize them. In real fact, however, such distinctions, when viewed geographically, are of the greatest interest, as serving to illustrate what we have so often had occasion to comment upon, namely the influence of isolation and other circumstances on external insect form[42]." The Psylliodes vehemens, Woll., is permanently paler in Porto Santo than it is in Madeira proper, being almost entirely testaceous. "That the species is identical, however, with the Madeiran one I have not the slightest doubt,—the sculpture and colour, as I conceive, having merely undergone a change since the remote period of its isolation on a comparatively calcareous soil[43]." The Scarites abbreviatus, Koll., occupies the loftiest peaks of nearly all the Madeiran islands, and was probably once abundant over the entire ancient continent, whatsoever its limits may have been, of which the present group forms but an isolated part. "There are traces of it in the Canaries, from whence occasional specimens have been brought, and which, from the want of local data and of sufficient numbers to reason upon, have in their turn been severally regarded as distinct. The fact however is, that the species in question is an extremely variable one, assuming differences of size according to the altitude at which it lives, and differences of sculpture according to the circumstances of the spot on which it is isolated. That such is actually the case, a careful observation of the many minute changes which the insect has undergone in the various islands and altitudes of the Madeiran group will, I think, prove to a demonstration. For it is impossible to suppose that every rock contains its own species, that is to say, has had a separate creation expressly for itself,—a conclusion at which we must assuredly arrive, if small and even constant differences are of necessity specific. Rejecting therefore this hypothesis as utterly untenable, and as contrary to all experience, we are driven to acknowledge that isolation does, in nearly every instance, in the course of time, affect, more or less sensibly, external insect form;—which being admitted, we have at once an intelligible principle whereby to account for modifications innumerable, each of which, when viewed simply as a difference, independently of the circumstances producing it, might have been regarded as sufficient to erect a 'species' upon, had the desire for multiplying them overbalanced the love of truth[44]."
Such are a few of the circumstances, influences, and conditions, by which the outward aspect of the insect tribes is liable, within definite limits, to be more or less regulated: and it is impossible to view them with an unbiassed mind and not arrive at the conclusion, that physical agents generally have a very decided control over the external contour of these lower creatures. In selecting the examples which we have lately discussed, I have avoided as much as possible those startling instances of variation which distant quarters of the globe will readily supply, because there are vast numbers of our naturalists who will not acknowledge the validity of any evidence which would tend to amalgamate, in a broad sense, the species of the Old and New Worlds. I have therefore contented myself with such data as must fall within our common experience, feeling satisfied that if the principle be allowed in the one case, it cannot long be objected to in the other. There are few entomologists who would not recognize, in the abstract, a legitimate capacity for adaptation in every insect with which they have to do; yet I believe there are not many, who, if modifications were to be shown them as the fixed result of disturbances from without, would be prepared at once practically to accept them as such. The collectors of the present day are so prone to regard every permanent difference as a specific one, that a large proportion of them do not sufficiently realize, that well-marked races, or states, are no longer matters of hypothesis, but of fact; and that, therefore, a sensible amount of aberration should not only be conceded to the action of certain physical combinations and elements, but even anticipated and looked for. Such however ought not to be; and earnestly therefore would I advocate a greater latitude for geographical influences than has been hitherto admitted by many of us. Especially would I urge the necessity for a more careful study of insular phænomena, for I am convinced that a due allowance is seldom, if ever, made for the qualifying power of isolation, per se,—the most significant perhaps of all the conditions which we have attempted in the preceding pages to examine.