Another curious example of the effect of local influences (amongst which proximity to the shore plays, in all probability, an important part) on the external aspect of insects exists in the Aphodius plagiatus, Linn.,—which in this country is generally deep black. "It is a circumstance worth noticing," I remarked in the 'Zoologist,' in 1846, "that the form which is looked upon by the continental naturalists as the variety, is in England evidently the typical one,—for out of about sixty specimens which I captured [at Tenby in South Wales], only two possess the conspicuous red dashes on the elytra which are considered abroad as the almost invariable accompaniment." I have observed the same peculiarity in the flat and damp spots between the sand-hills at Deal, where I have never detected a single individual which is not perfectly dark; and I believe that the greater number of the specimens which were originally taken at Wisbeach, in Cambridgeshire, offered the same geographical characteristics; whilst those which were found near the more inland towns of Peterborough and Norwich present a larger proportion of the ordinary European state. The blood-red dashes, however, with which the elytra of numerous insects are adorned, I have constantly remarked possess a singular tendency to become evanescent. It is indeed almost diagnostic of the genus Gymnaëtron, either that its representatives should be thus ornamented typically, or else that those which are normally black should, when they vary, keep in view, as it were, this principle for their wanderers to subscribe to. Thus, I have no doubt that the G. Veronicæ, Germ., is but a variety of the G. niger,—an opinion which I expressed in the 'Zoologist' nine years ago. Whilst commenting on the Coleoptera of Dorsetshire, I then stated, that "for my own part I must confess I should have doubted the G. Veronicæ being really distinct from the G. niger, for red dashes on the elytra seem naturally peculiar, more or less, to the whole genus; and I should therefore have suspected that, had occasional aberrations from a black type existed (which is not unlikely), those aberrations would probably assume a form which is so common in the other species of the generic group[26]."
The Bembidium bistriatum, Dufts., is usually much paler when found in saline districts (under which circumstances it was described as a distinct species by Mr. Stephens) than when occurring in more inland positions. The Blemus areolatus, Creutz., I have frequently remarked is similarly affected in brackish places: and I think it far from improbable that the Stenolophus Skrimshiranus, Steph., is but a local modification (though not altogether, perhaps, through marine influences) of the S. Teutonus, Schr. The Dromius fasciatus, Gyll., not being exclusively littoral, may be quoted as another case in point,—the specimens which are collected near the coast being for the most part singularly pale. In speaking of the Anthicus bimaculatus, Illig., M. de la Ferté observes: "Il y a sculement lieu de remarquer que les individus du bord de l'océan sont généralement plus pâles que ceux des contrées orientales de l'Europe, et que ceux des côtes de France et de Belgique sent entièrement dépourvus de tache discoïdale[27]." And bearing, in much the same manner, on the subject of variations, the Anthicus humilis, Germ., "est une des espèces le plus généralement répandues en Europe; mais il lui faut le voisinage de l'eau salée. Aussi on le rencontre non-seulement sur les rivages de toutes les mers, même de la Baltique, mais encore aux bords des lacs salés, tels que celui de Mannsfeld, en Saxe. Ceux de cette dernière localité sont généralement noirs; ceux que j'ai pris à Perpignan sont d'un rouge très-clair, ce qui me porte à croire que cette espèce est dans le même cas que quelques autres Anthicus, dont les variétés les plus foncées appartiennent au nord de l'Europe, et les plus pâles au midi[28]."
Whilst touching on this immediate question of variability as dependent to a great extent, in numerous cases, on proximity to the sea, we may just notice the marked tendency which even the insects peculiar to saline spots would seem in a large measure to possess, of converging, more or less obviously, to a lurid-testaceous, or pale brassy hue, in their colouring. True it is that we cannot (as above suggested) deduce any evidence of direct physical modifications from amongst species which are strictly maritime,—seeing that we have no means of judging in such instances whether similar phænomena would or would not be produced in central districts also: nevertheless we may perhaps detect in this general law some slight indication of the effects which an atmosphere and soil constantly impregnated with salt would be likely to bring about in the external aspect of those members of the insect tribes whose range is sufficiently extensive to expose them to its operation. The bare mention of such names as Nebria complanata and livida, Calathus mollis, Pogonus luridipennis, Trechus lapidosus, Aëpus marinus and Robinii, Cillenum laterale, Bembidium scutellare, ephippium and pallidipenne, Ochthebius marinus, Psylliodes marcida, Phaleria cadaverina, Helops testaceus, and Anthicus instabilis, so eminently characteristic as they are of briny situations, will at once appeal to our native entomologists; whilst the acknowledgement of the same principle is no less conspicuous in a host of other species which are not included in the British fauna.
Hence, when we see the tendencies of coloration (not to mention other particulars, often readily apparent) essentially the same, both in insects which are peculiar to, and in those which have overspread (from without) certain regions or localities, it is impossible not to associate some inherent controlling power with the regions themselves; and we are driven to the conclusion, that either well-defined races have been gradually shaped out, by means of the physical influences to which they have been exposed, or else that the species themselves (as witnessed by the intermediate geographical links, which, although sometimes rare, are in all instances to be found) do assuredly merge into each other.
In addition to those which we have been just discussing, there are other influences (equally independent of mere heat and cold) by which insect modifications may be brought about,—modifications moreover of that precise character which must be referred, in general terms, to the nature of the country and of the soil in which they severally obtain: a very few examples, however, in illustration of their action, must suffice for our present purpose. The Tarus lineatus, Schönh., is slightly shorter in Madeira, as also somewhat darker on its head and prothoracic disk (and with its elytral striæ less deeply impressed), than it is in Algeria and Spain. The Madeiran specimens of the Aphodius nitidulus, Fabr., are usually a little paler, and more distinctly punctulated, than their northern analogues; as are also, in the latter respect, those of the Clypeaster pusillus, Gyll. The Scydmænus Helferi, Schaum, is permanently smaller in the Madeiran group than it is in Sicily; and I believe that the Achenium Hartungii, Heer, of those islands, is but a local state of the A. depressum, Grav., of Central Europe. The Bembidium tabellatum and Schmidtii, Woll., may be in reality but geographical modifications of the B. tibiale and callosum of higher latitudes; and the Malthodes Kiesenwetteri, Woll., of the common European M. brevicollis. Calcareous deposits would appear, ever and anon, to have considerable efficacy in regulating the outward aspect of such species as are able to adapt themselves to different geological districts; and when in juxtaposition with the shore, their effects are often very conspicuous. The Dromius arenicola, Woll., is the Portosantan representative of the D. obscuroguttatus, Dufts.; and distinct as it is in colouring from that insect (as evinced both in Madeira proper and throughout Europe), I believe it to be in reality but a local condition of it, occasioned by a residence through a long series of ages on a calcareous soil. For the same reason perhaps (though assisted, in all probability, by the qualifying power of isolation), the Hadrus illotus, Woll., may be specifically identical with the Madeiran H. cinerascens. In like manner, the Bembidium Atlanticum, Woll., which in Madeira proper is frequently so dark that its elytral patches are sub-obsolete, and which is but seldom brightly arrayed in that island, assumes in Porto Santo (which is not only more calcareous than the central mass; but is strongly impregnated, as its streams and rills everywhere testify, with muriate of soda) a permanently paler hue,—being at times almost testaceous. Some districts seem to be more prolific in varieties, generally, than others. The neighbourhood of Ipswich, in our own country, has been cited by Mr. Curtis[29] as possessing this peculiarity; and I have remarked a similar tendency in certain parts of Ireland. The common Haliplus obliquus, indeed, of the Blackwater river, in the county of Cork, is usually so dark and suffused in colouring, that it might be almost taken for a distinct species,—its fasciæ, especially the hinder ones, being occasionally evanescent.
One more example must satisfy us under this section,—namely, the Harpalus vividus, Dej., of the Madeiran group. So curiously is that insect affected by the nature of the areas through which it successively ascends, and that too irrespectively of heat and cold (as may be gathered from the fact that its phases on the shore and upland heights are well nigh coincident), that it may be appropriately singled out as a concluding instance of the effects of those obscure local influences to which we have been drawing attention. "Ranging from the beach to the extreme summits of the loftiest mountains, accommodating itself at one time to a low barren rock of 20 yards circumference, at another to the deep-wooded ravines of intermediate altitudes, around which the clouds perpetually cling, and where vegetation and decay are ever rampant, or harbouring beneath the rough basaltic blocks of the weather-beaten peaks (6000 feet above the sea); we should naturally expect, à priori, to discover some slight modifications of outward structure, according as the respective localities differed in condition. And such we find to be everywhere the case. I am satisfied, moreover, that it is only by a careful observation on the spot that an insect like the present one can be properly understood; for, to anybody acquainted with it practically in all its phases, it is but too evident how many 'species' (so called) might be established on undoubted varieties, where there exists a desire for creating them, and where our sole knowledge is gathered from a few stray specimens collected by another person, and unaccompanied by local information to render the aberrations intelligible. For it must be tracked from the shore to an elevation of more than 6000 feet before we are enabled to discern the causes by which its development is controlled, or even to connect by slow and easy gradations its opposite extremes of form. And it is an interesting fact, that the distance between its variations does not increase in proportion to the distance between its altitudes. On the contrary, it would seem to pass through its minimum of size and maximum of sculpture at about the elevation of from 3000 to 4000 feet; both above and below which,—that is to say, as it recedes from the upper and lower limits of the sylvan districts,—it becomes gradually modified, and almost in a similar manner. Thus, to a person who had visited Madeira and had picked up specimens on the coast, and to another who had perchance penetrated into the interior, as passing visitors from the vessels are accustomed to do, and had brought away examples from the wooded mountain-slopes, the two insects would appear altogether distinct. For, commencing on the level of the beach, the usual type is broad, flat, more or less opake, with the prothorax almost impunctate, and the elytra soldered together. As we ascend higher, the breadth invariably diminishes, the brightness, and depth of sculpture, seem (up to a certain altitude) to increase, and the elytra are seldom, or but very imperfectly united; until, on entering the lower limits of the forest region, at an elevation perhaps, ore rotundo, of 3000 feet, we find that it has gradually put on a very different aspect,—being small, narrow, bright, convex, comparatively ovate and deeply striated; the legs and antennæ have become exceedingly pale; the prothorax has altered considerably in shape, being much narrowed behind and punctured; and the elytra are nearly always free. In this state it continues for about 1500 feet; when again emerging into the broad daylight of the open hills, it recommences to mould itself as it did below; until, having reached the summits of the loftiest peaks, more than 6000 feet above the sea, it has almost (though not entirely) assumed the features which characterized it on the shores beneath[30]."
§ IV. Isolation; and exposure to a stormy atmosphere.
Having in the preceding pages touched upon the subject of insect variability, as the occasional result, to a greater or less extent, of climatal and other influences; let us now proceed to consider the importance of a certain physical condition, which will be found, I believe, on inquiry, to be accompanied by a more decided modifying power than any which we have yet discussed.
Every one who has examined the natural history of islands, both in theory and practice, must be aware of the many difficulties which have constantly to be encountered, before the several phænomena can be satisfactorily explained. Laying aside those forms which are manifestly endemic (the numerical proportion of which usually accords with the distance from the nearest mainland), again and again are we baffled by the near resemblance of the various creatures to continental types,—whilst the minute differences which they display, from them, are at the same time so permanently fixed, that we are almost precluded, under the ordinary acceptation of a "species," from regarding the two as undoubted descendants of a common stock: and thus it is that insular faunas have frequently been magnified, in the novelties which they are supposed to contain, far beyond what is right. A person however who looks to the causes of things, and is prepared to recognize effects where there are fair grounds for anticipating them, will not be slow to perceive, that, in the small deviations which we are so often accustomed under such circumstances to behold, the results of isolation itself (as an active controlling principle) may be traced out; whilst geology, ever ready to lend a helping hand when appealed to, will seldom fail to supply those intermediate links of probability which the believer in specific centres of creation must needs subscribe to, before he can draw any deductions on a broad scale, or be competent to analyse even the general bearings of a question thus necessarily comprehensive.