Читать книгу On the Variation of Species, with Especial Reference to the Insecta - Thomas Vernon Wollaston - Страница 9

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Perhaps, judging superficially, climatal causes generally would appear to have more effect on insect development than any with which we are acquainted; yet, powerful as they unquestionably are, experience teaches us that such is not the case. In combination with other modifying principles, hereafter to be noticed, they may be (and probably are) exceedingly important; yet, when taken singly and alone, we have no evidence to show that their consequences are of such primary significance as might be anticipated. Mr. Darwin, in describing the fauna (which includes many mundane forms) of the Galapagos Archipelago, situated immediately under the equator, remarks: "The birds, plants, and insects have a desert character, and are not more brilliantly coloured than those from Patagonia; we may therefore conclude, that the usual gaudy colouring of the intertropical productions is not related either to the heat or light of those zones, but to some other cause,—perhaps to the conditions of existence being generally favourable to life[4]."

Although it is true, in a broad sense, that the nearer we approach the Line the grander and more gorgeous are the animate beings which tenant the surface of our earth, there are at the same time so many exceptions to this law, that it cannot he regarded as by any means universal; and whatever, therefore, be our ideas on a subject which might perchance seem to be self-evident, we are compelled to infer that climatal causes, of themselves, will not suffice to account for the numerous cases of aberration which we so constantly meet with in representatives of the same species exposed, through a long series of centuries, to opposite conditions of atmosphere. We need not, however, go so far as the Galapagos to convince ourselves of this. The Madeiran Group is placed between the 32nd and 33rd parallels of north latitude, off the coast of Africa, and contains a Coleopterous fauna (as hitherto ascertained) of about 550 species. Now 240 of these, at least, occur also in Europe (many of them even in our own country); hence, if a more southern climate may be presumed, of itself, to exercise any very decided modifying influence on insect development, we have an amount of material for comparison which should surely afford us some definite and tangible result. My own experience in those islands would tend to prove, that, amongst the many aberrations from their northern types which are there everywhere displayed, comparatively few of them can be referred for explanation to causes strictly climatal. I do not say that none can be thus accounted for; yet I trust to make it obvious in the following pages that there are even greater agencies at work than climatal ones in regulating (albeit within prescribed limits, and by slow gradations) the outward contour of the insect tribes.

When viewed geographically, there are two heads under which the insects of every individual area may be classed: namely, those which were created within its bounds, and which constitute its true aborigines (in the strictest sense); and, secondly, those which have reached it, either by ordinary migration over an intervening land, or by accidental introduction through human or other agencies. Now it is to the members of the latter of these ideal divisions, that we principally look for any positive evidence, whilst discussing the causes of variation: since, by the nature of the case, we must have identical, or at any rate closely allied species to reason upon before any sound conclusions can be drawn concerning them from the circumstances and conditions to which they are severally exposed; and it is clear, that the fact of creatures being specifically coincident, and yet under influences remote, does, for the most part, actually imply a transportation of them (from their primeval centres) beyond the limits of a naturally acquired range. Moreover, the αὐτόχθονες of the soil (if we may be excused the idiom) are in all instances adjusted to the peculiarities of the region in which they were formed; and, consequently, where they have not (as very frequently happens) diffused themselves to a sufficient distance from the birthplace of their kind to be acted upon in two opposite manners from without, the date they supply, during our inquiry into specific modifications as dependent on external disturbing elements, cannot be very considerable.

In spite of this severe distinction, however, which I would urge between the insect aborigines of a country and those which (whether by compulsion or not) have colonized it, and of the preference which (as just stated) must be given to the latter whilst investigating the controlling principles of aberration, I would not wish to reject in toto the testimony which the former likewise may indirectly furnish,—especially under the present section, in which climatal causes on a large scale have to be taken into account. True it is that we cannot hope to descry physical results amongst phænomena which are due to the creative force alone; yet we may, in the contemplation of them, recognize such an amount of design, or a primary adaptation to conditions from without, as shall afford, through its permanence and method, fresh presumptive evidence that the "conditions" themselves may have some inherent modifying power of their own on the aggressors from other districts, in which a contrary influence may perchance prevail, and for the overspreading of which they were, in the beginning, more peculiarly constituted and ordained.

It has been already mentioned (and, despite the exceptional cases which are to be found, it is in a general sense true), that the splendour and extravagance of the insect world attain their maximum within the tropics; and that the nearer we approach the central heat, the more and more unmistakeable is the existence of this law. It has been also hinted, that when viewed on a very extensive scale, we shall not derive much direct assistance (whilst examining insect-variation, with reference to climate) from the consideration of a fact thus seemingly important,—since there are but few species whose range is so comprehensive as to embrace, at the same time, the equatorial and temperate regions of the earth; and since, as lately suggested, it is not from a comparison of the aborigines of countries far removed that we can hope to derive much positive information during our present inquiry. It may be useful however to speculate, why the creative energy should have been thus lavished, as it were, in the torrid zone, whilst the fauna of the cold north is so unpretending and sombre. I believe that in the actual number, both of individuals and species, which they contain, the difference is not so great, between the two latitudes, as might be imagined; and that, were the minims of Scandinavia to be suddenly magnified into the giants of Brazil, the Laplanders and Swedes might stand a fair chance of being temporarily alarmed: nevertheless, as regards the multitude and eccentricity of her forms, there can be no question in which field it is that Nature has ever delighted more particularly to sport.

Laying aside, therefore, the numerical statistics from our account, is not the exuberance of the tropics at once responsive to the conditions imposed upon them? Do we ask why it is that the insect population is there moulded upon a type comparatively so colossal?—let the redundancy of the vegetation reply. Have not, also, more rapid laws of putrefaction and decay been prescribed than in our cooler clime; and can we imagine that it was not in obedience to this decree, that larger and more active scavengers were framed? The gaudy wings that float idly on the breeze, and the coats of mail which glitter in the light, have they nothing to tell of the local circumstances around them; or, is it too much to infer, that a more glorious and stimulating sun required creatures of superior brilliancy to bask in its rays? A moderate degree of heat, and that only during a certain portion of the year, may suffice in quiescent regions to keep up the equilibrium of the organic world, the various members of which, whether animals or plants, are ensured, in such countries, their alternate seasons of activity and rest; but within the tropics, life, in all its aspects, is ever vigorous; and, though the several species may have their appointed times of partial repose, there is no such thing as tranquillity for the mass. Hence it is, that to meet the requirements of a Flora[5] such as there obtains, a less magnificent Fauna would have been inadequate; and we cannot but recognize, that, in the wonderful and almost endless modifications of the insect tribes which people those zones, a special provision has been made to check the overgrowth of other created things.

But how, it may be asked, does this primary adaptation to external conditions affect the question of specific development? Perhaps not much: nevertheless, as lately urged, it is well that such adaptations should be borne in mind, not merely that due importance may be given to influences in conformity with which the creative act was at the first expressly regulated; but also that we may be prepared, if any qualifying power be admitted to reside in those influences themselves, for the kind of aberration which reason and experience would seem alike to imply that we should, in the various instances, anticipate.

We have already stated, that climate, when taken alone, does not appear to produce any very decided modifying effect on insect form, seeing that there are vast numbers of species of a wide geographical range which do not display, on their northern and southern limits, differences sufficiently constant to be regarded as purely climatal ones; and it is clear that, if climatal causes of themselves were of real primary significance, we should probably seldom fail to trace out, from their long-continued operation, some steady and positive result. Yet when combined with other principles, there is evidence that a considerable amount of influence must be conceded to the action of mere heat and cold, working permanently and according to fixed laws, on the members of the insect world. Such being the case, it is perhaps not surprising that a slight difficulty should arise, through our employment of separate sections under which to examine the causes of variation; for, since it is ordinarily by the union of several disturbing influences that aberrations are brought about, it is for the most part impossible, to refer the results, however conspicuous they may be, to a solitary controlling element. And hence, though we may be able at times to point out perchance the single reason for certain phænomena with comparative precision, it will generally happen that two or three agents must be appealed to before we can arrive at a conclusion by any means satisfactory. I would desire, therefore, that the examples hereafter to be noticed may be judged of in the mass; and may not be considered as severally assigned, of necessity, to an isolated deranging cause, through the fact of their being placed, for the sake of convenience, and because of the predominance which special controlling principles have had in maturing them, under sections, both, as it were, exclusive and particular.

That climate of itself possesses but a limited modifying power on insect development, is evident from the consideration (just alluded to), that numerous species of comparatively wide distribution are totally unaffected by it. Thus, for instance, the Pissodes notatus, Fab., a weevil which occurs in pine forests from Lapland to Barbary, and which has been naturalized even in the Madeira Islands, passes through the alternations to which it is specifically subject, irrespective of country. In like manner, the Lixus angustatus, Fab., so abundant in Central and Southern Europe, the north of Africa, Malta, Madeira, and the Canaries, and which has been detected in Persia, would seem to be perfectly free from atmospheric control. The Coccinella 7-punctata, Linn., which exists in nearly every portion of the Old World, is apparently unacted upon geographically. Numberless beetles which follow in the track of man, or at any rate are liable to do so, almost everywhere (such as Carpophilus hemipterus, Linn., Trogosita mauritanica, Linn., Læmophlœus pusillus, Schönh., Dermestes vulpinus, Fab., Anobium striatum, Oliv., Rhizopertha pusilla, Fab., Sitophilus granarius and Oryzæ, Linn., and Tribolium ferrugineum, Fab.), show little or no tendency to variation. Nor is this independence of climate to be observed less frequently in the aquatic forms, than in the terrestrial ones: the Agabus bipustulatus, Linn., common in the streams and pools of the whole of Europe, the north of Africa, and in Madeira, although naturally somewhat inconstant, offers no aberration, the result of latitude; as is equally the case with the Hydroporus confluens, Fab., which is found from Sweden to the Canaries, and the Eunectes sticticus, Linn.,—an insect literally cosmopolitan. The Swallow-Tail Butterfly (Papilio Machaon, Linn.), the Clouded Yellow (Colias Edusa, Fab.) and the Painted Lady (Cynthia Cardui, Linn.),—the first and second of which occur throughout Europe, in Siberia, Syria, Egypt, Barbary, Nepaul, and Cashmere; whilst the third (so general in our own country) has been recorded from India, North America, the Brazils, Africa, Java, and New South Wales,—however irregular they may be, afford no indications[6] of undoubted geographical instability.

We need not however multiply examples, since our space will scarcely admit of it, and numbers of them will be at once suggested to the entomologist: what it mainly concerns us here to corroborate, is the thesis, that climatal operation, although by no means invested with a universal qualifying power, has an amount of influence on certain species, even whilst unconnected with other elements,—and therefore, á; fortiori, when in combination with them.

The two principal conditions on which climatal causes generally may be said to rest, are latitude and altitude. As regards the former of these, however, whilst the equatorial and arctic regions of the earth will of course give us the extremes of heat and cold, we shall often perceive differences of temperature (the result perhaps of local circumstances) in areas but slightly removed from each other, sufficient to affect very materially, though by what means it is difficult to understand, the outward contour of the insect tribes. Thus, to go no further than Ireland, we find that the specimens of Silpha atrata, Linn., so abundant throughout England and the whole of Europe, have put on (it may be from the moisture of the atmosphere, or from some other obscure influence) the appearance of a distinct race,—so distinct indeed as to have long received another name, S. subrotundata, from British naturalists. I think it far from improbable that the Tachyporus nitidicollis, Steph., an insect eminently characteristic of that country (and one on which I have lately offered some remarks[7]), is but a darker climatal modification of the common T. obtusus: and it is well known that the examples of Pelophila borealis, Payk., from Killarney and Loch Neagh are permanently larger, and much more metallic, than those from the Orkneys. The Nebria complanata, Linn., assumes a more pallid hue in the neighbourhood of Bordeaux than it does on the sandy coasts of Devonshire and Wales: and I have but little doubt that the Omaseus nigerrimus, Dej., of Spain, the north of Africa, and Madeira, is a geographical state of the O. aterrimus of Central Europe. The Sitona gressoria, Illig., so universal throughout the Mediterranean districts, Madeira and the Canaries, may be but the subaustral form of S. grisea. The Bembidium obtusum, Sturm, is shorter and less parallel in our own latitude than it is in the Madeiran group and along the Mediterranean shores: whilst the Holoparamecus niger, Aubé, of Madeira and Sardinia is very much paler than the same beetle when taken in Sicily. Specimens of Pieris Brassicæ, Linn. (the White Cabbage-Butterfly,—an insect of widely acquired range), from Nepaul and Japan, are recorded[8] to have differed so strongly from the ordinary European type as to have been referred, by Boisduval, in doubt to that species. Mr. Westwood has received the Vanessa Atalanta, Linn., from North America, receding slightly from its British analogue; but which he, nevertheless, does not regard as specifically distinct: and such also (he adds) was the opinion of Mr. Kirby, who has described his American examples under that name. The common Hipparchia of Madeira I believe to be a fixed geographical modification of the H. Semele, Linn., of our own country,—in which the paler bars of the upper surface are evanescent;—there are, however, I imagine, but few entomologists who would concur with me in this hypothesis. The Madeiran specimens of Lycæna Phlœas, Linn. (the Small Copper Butterfly), are invariably darker, and more suffused, than the English ones: and Mr. Westwood remarks that he possesses examples from North America which "differ in the decided black spotting of the under side of the hind wings, in the bright red streak near their hind margin, and in wanting the minute spot on the costa of the fore wings; but that these characters can scarcely be held to constitute a distinct species[9]."

Few observers can have failed to remark, that increased altitude frequently corresponds, both in its fauna and flora, to a higher latitude; and that, consequently, if we ascend the mountains of a southern land, we shall be struck, at times, by the presence of a host of species which obtain at a lower level in more temperate zones. This is peculiarly traceable in the Madeira Islands,—which, from their subaustral position, and height (the loftiest peak of the central mass exceeding 6000 feet above the sea), afford a rich field to the student of zoological geography. Yet, though the degrees of mere heat and cold are such as to allow, in the two cases, species positively identical to flourish; we should surely anticipate some slight change from the different atmospheric conditions (especially when in union with other circumstances) to which they have been, through a lapse of ages, respectively exposed: it may be well therefore to inquire, whether experience does at all tend to strengthen what our reason has an à priori inclination to endorse. It must be recollected however that, in the instances to which we would draw attention, small aberrations are all that can be usually looked for, since climate of itself does not appear to be very potent in its action. We should remember, also, that the boundaries of insect instability are restricted; and, although we would advocate freedom of development within limits which are more or less comprehensive according to the species, to pass beyond them would be confusion, and such as could result from a lapsus Naturæ only, rather than from a power of legitimate variation.

In exact conformity with what the above remarks will have prepared us for, we find that the Dromius obscuroguttatus, Dufts., of Central Europe, has undergone on the mountain summits of Madeira changes precisely to that extent which we should have calculated upon; and although they would seem in reality to be referable to climate and isolation combined, yet, since it is not always possible (as lately stated) to treat the elements of disturbance separately, and it is my object in this short treatise to bring forward a few prominent examples in support of the considerations proposed, rather than to accumulate a mass of material for the registry of which my space would be inadequate, I will quote in extenso the reflections which, during the compilation of the 'Insecta Maderensia,' suggested themselves to me. "The Dromius obscuroguttatus is a common European insect, and the Madeiran specimens recede from the ordinary ones in being slightly larger, and in having their elytra more obscurely striated, with the humeral patch less distinct: their entire surface, moreover, is of a deeper black, a difference which is especially perceptible on the legs. It occurs in the greatest profusion in Madeira proper, though only from about 5000 to 6000 feet above the sea. Although so common throughout Europe, it is perhaps, when geographically considered, one of the most interesting of the Madeiran Coleoptera, as affording a striking example, not only of the modification of form in a normally northern insect when on its southern limit, but as showing likewise how a species, abundant on the low sandy shores and sheltered sea-cliffs of more temperate regions, finds its position here only on the summits of the loftiest mountains. It is true that the aberration from the typical state is not in the present instance very considerable; yet when the circumstances producing it are taken into account, I am persuaded that the difference is exactly of that nature on which too great stress cannot possibly be placed, when discussing the general question of geographical distribution as having a tendency, more or less directly, to affect both colour and form. It is well known to naturalists that a multitude of insects from the New World, receding from their European analogues merely in certain excessively minute characters, have usually been pronounced at once as new to science, first because those differences are constant, and secondly because the specimens have been received from the other side of the Atlantic. And yet in instances like the present one,—in an island which, while it belongs artificially to Europe, is yet naturally sufficiently distinct from it as to form at any rate a stepping-stone to the coast of Africa and the mountains of Barbary,—species similarly circumstanced are not necessarily received as new (and rightly so, I apprehend), though in every respect affording differences not only analogous to those already mentioned, but in many instances positively identical with them. If, however, a specific line of demarcation does of necessity exist between the creatures of the Old and New Worlds, the problem yet remains unsolved, so long as intermediate islands present parallel modifications, where that line is to be drawn. Meanwhile, how far geographical varieties of this kind, concerning the non-specific claims of which confessedly but little doubt can exist, may lead to the explanation of the Transatlantic ones just referred to, I will not venture to suggest. Yet certain it is, that the one case bears directly on the other; and that, if we can prove that common European insects, when isolated in the ocean, become in nearly all cases more or less modified externally in form, there is at least presumptive evidence that the law will hold good on a wider scale, and may be extended, not only to the Atlantic itself, but even to countries beyond. The differences of the present Dromius from its more northern representatives are, as just stated, small; nevertheless, since they are fixed, those naturalists who do not believe in geographical influence might choose to consider them of sufficient importance to erect a new species upon. But after a careful comparison of this with other insects similarly circumstanced, I am convinced that the modifications in question are merely local ones, and such as may be reasonably accounted for by the combined agencies of latitude and isolation, and the consequently altered habits of the creature, which is thus compelled to seek alpine localities in lieu of its natural ones[10]."

In like manner the Calathus fuscus, Fab., the Anchomenus marginatus, Linn., and the Anthicus fenestratus, Schmidt, which occur almost exclusively in the lower regions of northern latitudes, are found in Madeira on the mountain tops; each, moreover, possessing characters which are just sufficient (although slight) to distinguish them from their European representatives.

And if we inquire, on the other hand, into the aboriginal species of those islands,—or, at any rate, into such of them whose naturally acquired range embraces the opposite extremes of atmosphere,—we shall detect no less surely (albeit within a narrower space) the result of climatal action on insect form. The Helops confertus, Woll., "varies according to the altitude at which it is found; being usually deeply striated and rugose on its lower, but subpicescent and much more lightly sculptured on its upper limits. I have taken specimens indeed on Pico Ruivo, and on the mountain-plain of the Fateiras, which are so far diminished in roughness as almost to resemble, at first sight, the H. Pluto[11]." The Pecteropus Maderensis, Woll., which ranges from about 2500 feet above the sea to the summits of the loftiest hills, although usually with pale legs, is distinguished by having its femora almost invariably dusky when on its highest elevation; and, following out the analogy with that beetle, the Trechus alticola, Woll., should perhaps be regarded as an alpine state of the T. custos. The Calathus complanatus, Koll., assumes along the upland heights a very different aspect to what it does in the regions below, being generally more piceous and convex, altogether broader (in proportion) and shorter, and with both sexes (though, of course, especially the male) shining.

Nor is this principle of topographical variability (the result of climate) less apparent in other countries also. The Notiophili, for instance, "are extremely unstable, both in their sculpture and hue, being subject to considerable local modifications, though more particularly affected, it would appear, by altitude. Thus, in our own country, the N. semipunctatus, Fab., one of the common representatives of the plains, is found likewise on the summits of the mountains; but at that elevation it becomes liable to great alternations of colour, ranging from pale brassy-brown, with the apex testaceous, into deep black. The sculpture, however, perhaps is nearly as much dependent on other circumstances for its modification as upon altitude, since it seems tolerably clear that proximity to the sea-shore, especially where the localities are saline, will frequently produce a more faintly impressed surface[12]." It has indeed been lately suggested, that the Helobia nivalis, Payk., may be perhaps, after all, but a mountain variety of the H. brevicollis; the Leistus montanus, Steph., of the L. fulvibarbis, and the Patrobus septentrionis, Dej., of the P. excavatus; but of this I think further proof is needed, seeing that certain species do appear to exist which are strictly alpine (that is to say, which have not been, severally, detected in the lower regions of more northern zones); and, in most instances, where aberrations are to be met with from the effect of altitude, we have a right to inquire (provided the types from which they are supposed to have originally sprung obtain in the less-elevated portions of the same country), where are the intermediate links? Now I am not aware that any such links have, in the examples above cited, ever been observed; whilst I can vouch that in at any rate many districts where the quasi variety is found, the descendants of its assumed progenitor do occur in the plains beneath. I have remarked that the Cicindelidæ often become inconstant in colouring as they approach their maximum of height above the sea; and I have but little doubt that the C. fasciatopunctata[13], Germ., from Asia Minor and Turkey, is the C. sylvatica modified by a long residence in elevated regions. And so it is with the Chrysomelæ, many of which become, in the loftiest altitudes to which they ascend (as I have noticed at the head of the St. Gotthard Pass of the Swiss Alps), subject to unusual changes, both in lustre and hue.

The above examples, although few and indiscriminately selected, will serve to illustrate the principle which we have been contending for,—that climatal influences generally, may (and in most instances do) tend to affect, more or less directly, the outward contour of the insect tribes. It will be remarked that, in the cases hitherto cited no great disturbing power has been made evident,—the aberrations to which we have appealed being, most of them, comparatively minute. This, however, is simply in harmony with the belief which we have already expressed, that climatal causes, when taken singly and alone, are not of primary importance whilst discussing the question of specific modification. It remains for us, in the following sections, to inquire, whether there are any other elements at work from which greater results are to be expected. Meanwhile, let us not forget that differences may be, in the strictest sense, significant, even whilst small; and that it is their constancy, rather than their magnitude, which more particularly concerns us in the present treatise, seeing that it is with reference to those distinctions which are less conspicuous that the greatest amount of misunderstanding (through the fact of their being fixed) usually prevails; whilst it is our main object to show that dissimilarities do not necessarily imply the specific isolation of the creatures which display them, merely because they are, in their several localities, permanent.