From amongst the many results which I have been long accustomed to a.s.sociate (whether rightly so, or not, I leave it for others to decide) with certain special situations, I would draw attention to the singular inconstancy which numerous insects are liable to when existing on the coast,--and which frequently causes them to a.s.sume an aspect so permanently different from their inland types, that, without local knowledge to guide us, they might be supposed at first sight to be specifically distinct. Ten years ago I offered a few comments on this fact in the pages of the "Zoologist"; which, as I have seen no reason subsequently to modify them, I will transcribe at length:--
"The extraordinary changes which many insects are subject to when occurring near the sea, is a fact worthy of notice, and one which I do not remember to have seen recorded. The strictly maritime species must be left out of the question; for although many of them are exceedingly variable both in size and colour, still we have no means of ascertaining whether that variation is referable to the locality in which they are placed,--for, never being found inland, n.o.body can have an opportunity of a.s.serting that the same changes would not take place, were they to occur in positions far removed from the influence of the sea. When we find, however, the same insects in profusion both inland and on the coast, and observe also numerous and marked deviations from the typical forms peculiar to the latter situation; then, _a priori_, we have strong presumptive evidence that the changes in question are the result of local circ.u.mstances, and not referable to chance. The alteration in size I have almost always observed to be from large to small, and scarcely ever the reverse; whereas in colour the change takes place very nearly as much from light to dark as it does from dark to light: nevertheless the majority of instances I possess come under the latter department. It has been remarked that all the specimens of _Mesites Tardii_, which I captured in Devonshire, were much smaller than the original series taken by Mr.
Tardy at Powerscourt Waterfall, in the county of Wicklow; and so decided was the difference, that many of my friends, at first sight, concluded the two to be distinct species. This, however, I consider entirely owing to their locality, for my specimens were found only on the coast, and Mr. Tardy"s at a considerable distance inland. And, inasmuch as neither of these instances rested on mere individual examples, but on long and conspicuous series, the certainty of the change from large to small was the more apparent. Mr. Holme of Oxford mentions having taken _Olisthopus rotundatus_ in the Scilly Islands, in great profusion, none of the specimens of which exceeded two lines and a half in length. At Whitsand Bay in Cornwall I have captured _Gymnaetron Campanulae_, none of which exceeded three-quarters of a line,--the usual length being from a line to a line and three-quarters. _Anthonomus ater_, the average length of which is two lines, I have taken a series of in Lundy Island, none of which exceeded one. In the same locality, also, the common _Ceutorhynchus contractus_ scarcely ever reaches its natural size; and is, moreover, so variable in colour, that I was long before I could persuade myself that the species was not distinct. Instead of the bluish-black elytra which I had always considered invariable, they all possess a yellowish or bra.s.sy tinge; and the legs, instead of being black, are in most instances entirely of a light yellow,--and in all, more or less inclined to that colour. I have received from Mr. Hardy, of Gateshead, specimens of _Haltica rufipes_[22], captured by him on the coast, in which the entire insect is of a uniform brownish-red hue. Of the rare _Mantura Chrysanthemi_ I have taken beautiful varieties at Mount Edgc.u.mbe and in Lundy Island,--many of which inclined to a rich metallic-yellow, instead of the bra.s.sy-brown of the ordinary specimens: also, in the latter locality, particularly dark specimens of _Telephorus testaceus_. In like manner, I might enumerate other species equally remarkable; but I trust that those already mentioned are sufficient to verify my observations, of the extreme liability to change which, more or less, most insects possess when placed within the immediate influence of the sea. How to account for it, I know not.
I mention it as a mere fact, and leave it for others to a.s.sign a reason for its existence[23]."
Apparently dependent, in a large measure, on the same circ.u.mstance (namely proximity to the coast), the _Bembidium saxatile_, Gyll., so common at the edges of the mountain streams in the north of England, in Scotland, and throughout a portion of Ireland, presents itself along our southern sh.o.r.es in the form of a permanent variety; being, as the Rev. J. F. Dawson remarks, "more depressed, never narrower in front (the sides therefore more parallel), whilst the colour is always much paler and the spots larger,--that before the apex being round and very conspicuous, and the anterior one occasionally expanding over the surface very considerably[24]." I have taken it in profusion on the coasts of the Isle of Wight, Dorsetshire, and Devon. And so with the _Cistela sulphurea_, Linn., which in certain maritime localities (as I have particularly noticed on the sand-hills at Deal) is liable to become so dark in colouring, that, without the intermediate shades to judge from (which however may usually be obtained _in situ_), it might stand a fair chance, occasionally, of being mistaken for a separate species. A _Psylliodes_ in Lundy Island, allied to (if not identical with) the _chrysocephala_, Linn., found in abundance on a _Bra.s.sica_ along the ascent from the eastern landing-place, varies "in every consecutive shade between the limits of light yellow and dark metallic-green[25]," the former of which states (the normal one on that rock) might have been fairly set down as specifically distinct from the latter, did not observation on the spot decide the question for us without doubt.
Another curious example of the effect of local influences (amongst which proximity to the sh.o.r.e 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 circ.u.mstance 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 _Gymnaetron_, 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. Veronicae_, 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. Veronicae_ 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 a.s.sume 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 circ.u.mstances 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 Skrimshira.n.u.s_, 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 Ferte observes: "Il y a sculement lieu de remarquer que les individus du bord de l"ocean sont generalement plus pales que ceux des contrees orientales de l"Europe, et que ceux des cotes de France et de Belgique sent entierement depourvus de tache discodale[27]." And bearing, in much the same manner, on the subject of variations, the _Anthicus humilis_, Germ., "est une des especes le plus generalement repandues en Europe; mais il lui faut le voisinage de l"eau salee. Aussi on le rencontre non-seulement sur les rivages de toutes les mers, meme de la Baltique, mais encore aux bords des lacs sales, tels que celui de Mannsfeld, en Saxe. _Ceux de cette derniere localite sont generalement noirs_; ceux que j"ai pris a Perpignan sont d"un rouge tres-clair, ce qui me porte a croire que cette espece est dans le meme cas que quelques autres _Anthicus_, dont les varietes les plus foncees appartiennent au nord de l"Europe, et les plus pales 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 bra.s.sy 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 phaenomena 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_, _Aepus 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 a.s.sociate 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 a.s.suredly 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 ill.u.s.tration of their action, must suffice for our present purpose. The _Tarus lineatus_, Schonh., is slightly shorter in Madeira, as also somewhat darker on its head and prothoracic disk (and with its elytral striae 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 a.n.a.logues; as are also, in the latter respect, those of the _Clypeaster pusillus_, Gyll. The _Scydmaenus 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 lat.i.tudes; 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 sh.o.r.e, 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 a.s.sisted, 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 Atlantic.u.m_, 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, a.s.sumes in Porto Santo (which is not only more calcareous than the central ma.s.s; 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 fasciae, 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 sh.o.r.e 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 circ.u.mference, at another to the deep-wooded ravines of intermediate alt.i.tudes, 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, _a 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 sh.o.r.e 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 alt.i.tudes. On the contrary, it would seem to pa.s.s 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 pa.s.sing 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 alt.i.tude) 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 antennae 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) a.s.sumed the features which characterized it on the sh.o.r.es 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 phaenomena 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 antic.i.p.ating them, will not be slow to perceive, that, in the small deviations which we are so often accustomed under such circ.u.mstances 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 a.n.a.lyse even the general bearings of a question thus necessarily comprehensive.
Having thought it desirable to defer to a subsequent chapter of this treatise the few geological reflections which our subject may give rise to, it will not be my aim to allude to them in the present section more than is absolutely requisite. I propose rather to consider some of the ordinary effects of isolation, as mere matters of experience; and to allow geology to tell its own tale when we come to examine the problem of _self-dispersion, as occasionally interrupted by subsidence_.
If we except a few of the _Heteromera_ and apterous _Curculionidae_, which appear to be influenced in a different manner, the power of isolation over insect form is perhaps more especially to be detected in a deterioration of stature. Whether this princ.i.p.ally emanates from the constant irritation of a stormy atmosphere, such as small islands are of course exposed to, and which would seem to have stunted the development (during a long series of ages) of the animal and vegetable worlds, or from a diminution of area consequent on the breaking up of a continuous land, it is difficult to p.r.o.nounce: nevertheless, it is most consistent with both reason and a.n.a.logy to suppose that each of those causes has operated to induce a similar result; and that we must therefore view them as working in concert, if we would appreciate their action aright.
It is a law to which a large proportion of the organic creation would appear to be subject, that the exuberance of life (not so much, however, as regards the number of individuals which the various species may present, as in the grandeur of their size) has reference to the magnitude of the spot over which it is permitted to range. The unnatural breeding-in of a single race, which must of necessity happen unless the intercourse with other varieties of its kind be possible, has always been attended with effects more or less pernicious; and in the Annulose tribes I believe that the reduction of s.p.a.ce which geological convulsions have at various epochs brought about, has been commonly succeeded (_inter alia_) by a reduction of stature in those species which have been cut off from their fellows. I do not a.s.sert that there are no exceptions to this rule; for counter-influences may at times prevail (as we shall shortly see), to neutralize the above tendency. I hold it, however, as an absolute truism, in physics, that a law without an exception is an anomaly. If, therefore, we were once to admit the latter to negative the former, no such thing as a law could exist. Hence it follows, as a corollary (unless, indeed, we are prepared to endorse that conclusion), _that_ _where there is a law there must be an exception to it_; and that, consequently, exceptional cases, if not exceedingly numerous, should never pervert our belief from an otherwise presumptive truth.
This dwindling-down of size has seldom failed to attract my attention, more or less, in almost every island which I have hitherto had an opportunity of exploring: s.p.a.ce, however, will not permit me to dwell upon many instances. I have already adverted to the diminished stature of _Anthonomus ater_, Mshm, and _Ceutorhynchus contractus_, Mshm, in Lundy Island,--the first of which scarcely ever reaches, on that rock, more than half its natural bulk. The late Mr. Holme, of Corpus Christi College, Oxford, in like manner, captured the common _Calathus melanocephalus_, Linn., and _Olisthopus rotundatus_, Payk., in Scilly,--the former of which seldom exceeded two lines, and the latter two and a half, in length: and he also recorded, that the _Bolitochara a.s.similis_, Kby, is invariably smaller in those islands than it is in the neighbourhood of Penzance[31]. The _Vanessa Callirhoe_, Fabr. (a geographical a.n.a.logue of the Red Admiral b.u.t.terfly[32], so common in our own country), is permanently smaller in Porto Santo than it is on the larger, more luxuriant and varied, and therefore more protected, island of Madeira proper. And, as regards the _Ptini_ of that group, so completely are some of them "affected by isolation, and by exposure to a perpetually stormy atmosphere, that they do not attain half the bulk on many of the adjacent rocks that they do in the more sheltered districts of the central ma.s.s; and so marvellously is this verified in a particular instance, that I have but little doubt that five or six _species_ (so called) might have been recorded out of one, had only a few stray specimens been brought home for identification, without any regard having been paid to the respective circ.u.mstances under which they were found[33]." That "one," Protean, representative is the _Ptinus albopictus_, Woll.; and it is so eminently a case in point, that it may be admissible to quote, _in extenso_, a few of the observations which I have already published concerning it:--
"The _P. albopictus_ is the commonest of the Madeiran _Ptini_, and by far the most variable, having a separate radiating-form for almost every island of the group,--whilst, at the same time, the whole are so intimately connected together (and merge into each other) by innumerable intermediate links, that it is impossible to regard them, in spite of the opposite contour of the _extremes_, in any other light than as different aspects of a single species, according as circ.u.mstances may favour, r.e.t.a.r.d, or otherwise regulate its development. Instability in fact (in its broadest sense) may be considered to be one of its most prominent characteristics, since it appears to be more sensitive to isolation and alt.i.tude than any of the other members of the genus with which we have here to do,--as may be proved to a demonstration by a careful study of its habits on the spot, where the influences of position and exposure are, in nearly all instances, more than sufficient to account for the successive phases a.s.sumed. Thus, commencing with _var._ alpha, which reaches its maximum in the sheltered ravines of the central ma.s.s, the bulk is usually large, and the tints comparatively intense. _Var._ beta.
is likewise brightly variegated, but it is smaller. Now, if our premises be correct, that locality and the action of the external elements have much to do with the changes in question, we might have expected, _a priori_, that this state, from its peculiarity to the Dezerta Grande, would not only have reduced in dimensions (which it is), but in colour also (which it is not). Here, therefore, observation, _in situ_, becomes extremely important; since such does at once convince us that its almost exclusive attachment to the interior of the stalks of the _Silyb.u.m Marianum_, Grtn. (the _Holy Thistle_ of the ancients), with which the more protected portions of that island everywhere abound, affords it ample conditions, even on so bleak a rock, for its completion. Nevertheless, its _stature_ (as already stated) is slightly diminished in spite of this: and when we come to examine the individuals which infest the lichen of more open situations (aberrant however on the Dezerta Grande, and answering to the _var._ gamma. of the diagnosis), we immediately perceive that _both_ of our required results are indicated,--the reduction not being limited to size, but extended also to hue. In Porto Santo this modification is the normal one,--where the insect likewise displays the same lichenophagous tendency, and where the districts in which it exists are equally barren. But, if its maximum be attained in Madeira proper, and a certain number of minor deviations range throughout Porto Santo and the Dezerta Grande, it still remains for us to show where its _minimum_ is to be obtained:--which, true to the _modus operandi_ by which we have conjectured its divers degrees of abortion to have been brought about, would seem to be centred on the Northern Dezerta, or Ilheo Cho. When we bear in mind the minute dimensions of that flattened rock, which does not include so much as a single valley, or depression, within its bounds, and is consequently seldom free from the violence of the winds (which sweep across it incessantly, from whatever quarter they may arise); it could hardly be supposed that an insect which is so obviously subservient to atmospheric control should not have become materially affected, in its outward guise, through long seclusion on such a spot:--and accordingly we are not astonished to find the race which has been thus cut off for ages on this extraordinary little island, itself _as_ extraordinary. It is indeed very remarkable to trace out how clearly the agencies we are discussing have here operated on the species under consideration,--for both s.e.xes (though especially the male) descend on the Ilheo Cho to somewhat less than half a line in length, being literally of scarcely greater magnitude than some of the larger representatives of the _Ptiliadae_!"[34]
I stated above, that, although this diminution of stature is a very general accompaniment of isolation, amongst insects which have been _long_ cut off from the rest of their kind, there is no rule without an exception to it; and that, therefore, we must not always antic.i.p.ate the result which has been described. We should remember that _immense_ periods of time are apparently necessary before any perceptible change can come over creatures from the stoppage of their migratory progress, and the unnatural in-breeding of their several tribes; so that in islands geologically recent (which often implies, however, their existence through epochs which would sound vast indeed to ears unscientific) we must not invariably expect to discover evidences of this law. On the contrary, we must first of all take into account the age of their formation, before we can judge _a priori_ as to the probability of its operation through a sufficient interval of time to have become conspicuous in its effects. I say "through a sufficient interval of time," because the process of deterioration may be silently going on, even now, in many an island, _which has not yet shown any matured traces of its action_, except perhaps in the case of a few species which appear to be more particularly susceptible to contingencies from without. We should then call to mind, that an enormous proportion of nearly every insular fauna is composed of accidental colonists during the last few centuries, in which civilization and commerce have been unintentionally at work in the cause of animal diffusion; and that, therefore, if modifications in outward contour have not necessarily resulted during a positive _geological_ interval, it would be absurd to look for them in the mere settlers (as it were) of yesterday.
Thus, it will be perceived, how necessary it is to take every element and contingency into account before we venture to p.r.o.nounce dogmatically on either the existence or non-existence of any physical law; and how cautious we should be of denying the legitimate operation of external influences in one region, because they would seem, _prima facie_, to be contradicted in another. It is surely more philosophical to endeavour to reconcile the two, by tracing out (as may frequently be done) some opposing principle in the latter, which shall enable us to understand the discrepancy, and to believe that the same action may be going on in both cases, but that in one of them it is either overruled by a greater controlling power than itself, or else has not had sufficient time to bring its fruits to maturity. If a proposition be true, we should recollect that it is _always_ so (under all the circ.u.mstances and conditions to which it is applicable); for, otherwise, it would be both true and false,--which is absurd: hence, _if_ my premises be true, that the general tendency of isolation is to diminish the stature of those insects which have become isolated; it follows that that tendency must remain, so long as there are no other special disturbing influences to absorb or neutralize it. "When any observation," says a writer of the last century, "hath hitherto constantly held true, or hath _most commonly_ proved to be so, it has by this acquired an established credit: the cause may be presumed to retain its former force; and the effect may be taken as probable, _if in the example before us there doth not appear something particular,--some reason for exception_[35]." Hence it is, that, even amongst the _opposite_ phaenomena which one island may occasionally present from those of another, I have often been able to recognize the working of a selfsame law; and clearly to detect, that it is not from _its failure_, in either instance, that contending results are brought about, but simply that some counteracting agent has been exerting its energy in the one case, so as to nullify what would have otherwise come to pa.s.s.
The main object however of the present section being to show that a considerable amount of power is due to isolation itself, in regulating (after a long series of ages) the outward aspect of the insect tribes, it is not strictly necessary that we should so rigidly insist on deterioration of size as one of its primary consequences,--since (whether it be so or not) we are merely concerned here to demonstrate, that its influence, _in some shape or other_, is absolute and real.
After the above remarks, we shall not be surprised that the phaenomena displayed in certain islands, as regards size, are sometimes (though I believe it to be an exception to the ordinary rule) the exact opposite of what we have been describing. Let us not however be alarmed at this fact, on the bare statement of it,--as though the proposition which we have been lately advancing were at once disproved; since we shall find, on inquiry, that the case is not so desperate as might be imagined; and that in many islands where even this principle is to be detected, we may recognize traces of the other also. But how, it will be asked, can this be? for, since the influences are the same, creatures similarly exposed to them must be similarly affected. Now, although, on a broad scale, such a notion contains much presumptive truth; on a narrower one it does not always apply; for species are differently const.i.tuted _ab ovo_, and will sometimes give a different result from the operation of causes which are identical. Moreover, there is a curious tendency which I have remarked in most islands, that the wings (especially the metathoracic ones) of their insect inhabitants are liable to be r.e.t.a.r.ded in their development,--often indeed to such an extent as to become actually evanescent: and I believe it to be a law of Nature, that when any particular organ is either stunted or taken away, the creature receives a compensation for its loss either by the undue enlargement of some other one[36], or else in a general increase of its bulk. If such be the case, the presence of two apparently conflicting effects in a single island is rendered somewhat more intelligible; nevertheless, on the above hypothesis, the specimens which increase in dimensions should undoubtedly have their organs of flight more or less enfeebled, whilst those which diminish should be regularly winged. And hence we arrive at the question, is this so? My own experience would certainly tend to prove that it is; and I suspect that future observations will confirm the fact. Meanwhile, I must content myself with simply advancing the subject for consideration, and with recording such few examples, in support of the theory, as s.p.a.ce will permit, and which occur to me almost spontaneously.
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 _a priori_ antic.i.p.ate 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 p.r.o.nounce, even if we might a.s.sume 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 lat.i.tude 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 phaenomena (whatsoever they may be), which would seem to have departed from their types, to a single disturbing cause. Hence, whilst I there acknowledged lat.i.tude 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 lat.i.tudes 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 a.n.a.logue, 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_ a.s.sured 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 pa.s.sage 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.
Callirhoe_), 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 a.n.a.logues, 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 circ.u.mstances, 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 phaenomena 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 aerial 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 Cho, the _Scarites abbreviatus_, Koll., _Laparocerus morio_, Schon., and the _Helops Vulca.n.u.s_, 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_, Schon., _Omias Waterhousei_, Woll., _Helops Vulca.n.u.s_, 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 lat.i.tude.
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 circ.u.mstances, 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 circ.u.mstances 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-bra.s.sy tinge in Porto Santo, and much ac.u.minated 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 ill.u.s.trate what we have so often had occasion to comment upon, namely the influence of isolation and other circ.u.mstances 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, a.s.suming differences of size according to the alt.i.tude at which it lives, and differences of sculpture according to the circ.u.mstances 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 alt.i.tudes 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 a.s.suredly 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 circ.u.mstances 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 circ.u.mstances, 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 unbia.s.sed 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 p.r.o.ne 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 antic.i.p.ated and looked for. Such however ought not to be; and earnestly therefore would I advocate a greater lat.i.tude 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_ phaenomena, 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.
"Felix qui potuit rerum cognoscere causas" is a motto which the student of Nature should keep constantly in view; for it is undoubtedly a more honourable task to discover the _reasons_ for what we see, than the mere appearances themselves. He who has dived deeply into the everyday circ.u.mstances around him will be reluctant to ascribe so much as a single item of all that comes within his ken, to chance; for to him the whole system of created things is, from first to last, replete with design. _Natura nil agit sine causa_ is as true now as it ever was, and it will be so to the end. Let us not therefore be discouraged at the apparent smallness of the data from which many of our conclusions have to be drawn, for nothing is in reality trivial which is the effect of a wisely appointed law; and, even were such the case, it would not be thereby proved that the investigation of the law itself (however liable it may be to exceptions) is unimportant. Nor ought we, on the other hand, to be discouraged if we cannot always reconcile conflicting phaenomena, and detect in each a primary controlling cause. We should rather bear in mind, that the elements with which we have to deal are obscure, and subject to permutations from which various results must of necessity arise; and that it is only, therefore, on a broad scale that we can look for uniformity of action, even from conditions which may appear to be identical. "Nature is not irregular, or without method, because there are some _seeming_ deviations from the common rule. These are generally the effects of that influence which free agents, and various circ.u.mstances, have upon natural productions[45]."
FOOTNOTES:
[3] Religion of Nature Delineated, p. 103.
[4] Journal of Researches (London, 1852), p. 381.
[5] The great preponderance of the phytophagous over the predacious tribes, in the hotter regions of the earth, is a remarkable fact, and strongly suggestive of the relation which the insect and vegetable worlds (both of which attain their maximum in those zones) bear to each other. "The carnivorous beetles, or _Carabidae_," says Mr. Darwin, "appear in extremely few numbers within the tropics. The carrion-feeders and _Brachelytra_ are very uncommon; on the other hand, the _Rhynchophora_ and _Chrysomelidae_, all of which depend on the vegetable world for subsistence, are present in astonishing numbers. The orders _Orthoptera_ and _Hemiptera_ are peculiarly numerous; as is, likewise, the stinging division of the _Hymenoptera_, the bees, perhaps, being excepted."--Journal of Researches, p. 34.
[6] Mr. Westwood states that he possesses an individual of the _Papilio Machaon_ from the Himalayan Mountains, captured by Professor Royle, "which scarcely exhibits the slightest differences when compared with English specimens."--_The b.u.t.terflies of Great Britain_, p. 4.
[7] Zoologist, xiii. p. 4655.
[8] The b.u.t.terflies of Great Britain (London, 1855), p. 17.
[9] _Id._ p. 94.
[10] Insecta Maderensia (London, 1854), pp. 7, 8, 9.
[11] Insecta Maderensia, p. 516.
[12] Insecta Maderensia, p. 17.
[13] I possess specimens of this insect captured on the summit of Mount Olympus by my friend E. Armitage, Esq., who is also of opinion that it may be but a mountain state of the _C. sylvatica_, Linn.