Chapter III: Causes of Variation (1)

"It is not impossible," says a writer of the last century, "that such laws of Nature, and such a series of causes and effects, may have been originally designed, that not only general provisions may have been made for the several species of beings, but that even _particular cases_ (at least many of them) may have been provided for without innovations in the course of Nature[3]." And let us not suppose that this is a mere, wanton speculation, unsupported by evidence (if not actually circumstantial, at least) strongly presumptive; since the further we penetrate into the ramifications of the organic world, the less are we inclined to ignore the operation of those various modifying influences which our understanding tells us do everywhere exist.

To investigate the causes of things, and to endeavour to trace out by slow, inductive processes those secondary agents, by the assistance of which a large proportion of the phaenomena around us are gradually matured, is no insignificant task; yet how much animadversion from without have the students in such fields of research frequently to endure! A fact many times repeated, and which comes within our daily experience, is too often looked upon as a matter of course, and as therefore beneath the notice of an intelligent mind; yet the man who regards _truth_ as valuable, for its own sake, under whatever aspect it may come, and who can rise to the appreciation of _results_, whether they be of rare or constant occurrence, will have learnt to pronounce nothing as unimportant which may supply a single link in that chain of knowledge which would be broken and imperfect without it. A spirit of inquiry, however, is becoming, year by year, more evident; and we may confidently anticipate the period when such reproaches will have for ever died away. Natural history, in all its branches, will then advance more rapidly than heretofore, and each separate labourer, in his own peculiar province, will breathe a more genial atmosphere; whilst observation and reason, mutually dependent on each other, will work in concert more effectually. "Reason without _observation_," writes the author above quoted, "wants matter to act upon; and observations are neither to be justly made by ourselves, nor to be rightly chosen out of those collected by others, without the assistance of _reason_. Both together may support opinion and practice, in the absence of knowledge and certainty."

In the last chapter we offered a few passing remarks on insect-aberration generally, whether regarded as a _universal fact_ (which, however, even supposing such to be true, it is not the object of the present treatise to substantiate), or as an _occasional_ one,--that is to say, as existing at all times to that extent (as an hereditary principle), that it is _liable_ to manifest itself, or not, according as external agencies may favour or oppose its occurrence. In the latter case, which alone I propose to consider, this inherent tendency may be displayed, either through the expression of "varieties" well defined, or by a mere proneness to wander, irregularly and at large, from an assumed diagnostic type. In the following pages, the _former_ of these resultant conditions (namely, that in which "varieties," technically so called, though _more or less_ isolated in their character, are apparent) will be especially discussed; since my principal desire is, to point out the influence of _local disturbing causes_ in regulating, to a greater or less extent, though of course within certain specific limits, the outward contour of the insect tribes,--and it requires no argument to prove that, where those local elements (whatsoever they may be) prevail, the _same_ effects will, for the most part (in the same species), be produced; and that, therefore, modifications which are characteristic of countries and regions far removed from each other have an _a priori_ claim for stability, above those which circumstances less important than geographical ones, and which are consequently more fluctuating in their combinations, may from time to time (as it were, accidentally) shape out. Having then examined our premises, and prepared ourselves, with an unbiassed mind, for the reception of phaenomena which should be constant (and in some instances, also, conspicuous) _in proportion as_ the conditions which unite in bringing them about are significant; let us advert to a few of the more prominent cases in which our instinct would seem to warrant the belief that aberrations are to be usually anticipated. And since it will hardly be denied that, like the representatives of other departments of the animate world, insects _may_, in their outward configuration and development, be in some measure under the control of the external influences to which they are immediately exposed, we will take a rapid glance at a few of the circumstances and conditions which are known to have more or less of a qualifying effect on the members of large and opposite sections of the organic creation; and then see how far we are enabled, by means of facts, to trace out results for the Insecta, corresponding to those which are admitted to obtain in the other groups. And, since the existence of analogous results infers, to a certain extent, the similarity of the agents which have brought them about, our "causes of variation" (provided the effects can be shown) may be in reality almost demonstrated.

Amongst the numerous influences and conditions, in obedience to which the members of a large proportion of the animate world would appear, at times, in their outward aspect to be modified or fashioned, the following may be selected as perhaps of primary importance:--

1. Climatal causes _generally_ (whether dependent on latitude or upon altitude).

2. Temporary heat or cold, of an unusual degree.

3. Nature of the country and of the soil.

4. Isolation, and exposure to a stormy atmosphere.

Sec. I. _Climatal causes generally, whether dependent on latitude or altitude._

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 autochthones 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 phaenomena 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 phaenomena 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., _Laemophloe us pusillus_, Schoenh., _Dermestes vulpinus_, Fab., _Anobium striatum_, Oliv., _Rhizopertha pusilla_, Fab., _Sitophilus granarius_ and _Oryzae_, 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_, a 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_, Aube, of Madeira and Sardinia is very much paler than the same beetle when taken in Sicily. Specimens of _Pieris Brassicae_, 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 _Lycaena Phloeas_, 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 _a 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 Naturae_ 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 _Cicindelidae_ 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 _Chrysomelae_, 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_.

Sec. II. _Temporary heat or cold, of an unusual degree._

It is perhaps unnecessary that the action of temporary heat and cold, of an unusual degree, should be considered under a separate head from that of climatal causes generally; nevertheless, since the latter are, in a certain sense, permanent in their operation, it may be thought desirable that I should offer a few words on the effect of sudden exceptions to the ordinary routine of things, such as, for instance, seasons of peculiar intensity. It does not however appear that any very important modifications do often occur from conditions thus abnormal, and as it were _accidentally_ brought about: on the contrary, indeed, it is a well-known fact, that the members of the insect world are singularly independent of such contingencies; and that, in the same manner as their times of maturation are neither hastened nor retarded by them, their external development is for the most part free from their control. Yet, in spite of this, specific results _are_ wont to happen, ever and anon, from such circumstances, as though it were a fundamental axiom, that every agent which Nature can press (regularly or irregularly) into her service should have, though it may not always exercise its privilege, some qualifying voice.

I believe that almost the only deviation from the typical state, in insect form, which has been observed to originate, _par excellence_, from the occasional continuance of undue heat or cold, is curiously enough an organic one,--having reference to the enlargement of the wings. Every entomologist must be aware that a vast proportion of the Coleoptera (especially the _Carabidae_) are subject to great inconstancy in their metathoracic organs of flight. Many species, as the common _Calathus mollis_ of our own country (to which my attention has been more particularly drawn by the Rev. J. F. Dawson), have the hind wings at one time ample, at another rudimentary, and at a third nearly obsolete. Now, although other causes, hereafter to be noticed, would seem to have far greater power than climatal ones in _permanently_ regulating the size and capacity of these appendages; I think it will be found on examination (and I may add that Mr. Westwood is of the same opinion[14]), that the greater or less development of them may be frequently explained by the unusual severity of the seasons. My own researches would certainly tend to prove, that _heat_ does (in the main) favour, and _cold_ retard, their presence. Exceptions (often rendered intelligible from the evident working of counter influences) will of course arise in abundance to this hypothesis; yet my impression is that, upon a broad scale, it will stand the ordeal of a rigid inquiry.

Speaking of certain representatives of the Hymenoptera (_Chalcididae_), Mr. Westwood observes: "A curious peculiarity exists in one at least of these apterous species, which has been noticed by no previous author, namely, _Choreius ineptus_, Westw., which, although ordinarily found in an apterous state, was discovered by me in considerable numbers during the hot summer of 1835, with wings[15]". And, touching the irregularity of the alary organs in the Homopterous _Fulgoridae_, he remarks: "Other instances, in which the wings undergo a deficiency of development, occur in the genus _Delphax_, the majority of which, in our English species, have the upper wings not covering more than one half of the abdomen,--the terminal membrane being deficient, _as well as the hind wings_. In certain seasons, however, especially hot ones, the wings are fully developed[16]". Mr. Curtis has indeed formed the undeveloped specimens into a different genus, _Criomorphus_.

Although the result of a more stimulating sun may be often neutralized by that of _isolation_ (which, as we shall hereafter see, is a resistless agent, amongst a host of species, in weakening, and frequently rendering abortive, the powers of flight); yet _heat_, when freed from counter influences, may be traced in its _permanent_ effect on the alary system of insects, no less than when temporarily applied. The consideration of this, however, belongs strictly to the preceding pages, and we will not therefore discuss it here. The common Bed-bug (_Cimex lectularius_, Linn.) is almost invariably apterous, or with very short rudimental hemelytra; yet Scopoli (_Ent. Carn._ p. 354) mentions its occurrence with perfect wings. Fallen, also, and Latreille, state that it has been found winged; whilst Westwood remarks that it has been reported as occasionally winged in the East Indies; and it would seem extremely probable that, in these examples, as in numerous others which are on record, we may detect the consequences of heat; either as temporarily applied (in an unusual degree), or through the accidental transportation of the insect into a naturally warmer atmosphere.

Sec. III. _Nature of the country and of the soil._

Before we proceed to inquire to what extent the outward aspect of insects is liable to be controlled by the physical state of the areas in which they severally obtain, it may not be altogether out of place to offer a few reflections on the superiority which some regions possess intrinsically over others, both for the _increase_ and _diffusion_ of the animal tribes. To suppose that all countries within the same parallels of latitude are equally favourable for the development of life (not to mention the after-dispersion of it), is contrary to experience; for although (as we have already pointed out) the organic world does certainly, when viewed in the mass, approach its maximum as we near the tropics, there are at the same time so many violations of this law, that we cannot admit its operation except in a broad and general sense.

In a former section of this chapter, I drew attention to the fact, that certain islands, equatorial and subaustral, are anything but suggestive of their actual positions with respect to the line of central heat on the surface of the earth. It was with regard to _climate alone_, however, that I wished them to be understood: and it is not until now that I have ventured to urge the necessity of taking other influences into account also, if we would desire to recognize anything like design and adaptation (I will hardly call it cause and effect) between the continent and the thing contained. It is almost needless to add, that there are _many_ elements to be considered, such as local atmospheric conditions, excess or deficiency of electricity, superabundant moisture, diminished light, and the geological composition of the soil, before we can hope either to appreciate zoological phaenomena as a whole, or to reconcile the apparent inconsistencies which they are accustomed to display.

Mr. Darwin, to whom we are indebted for so much valuable information concerning the natural history of various portions of the world, in his notes on Tierra del Fuego, observes: "Beetles occur in very small numbers; it was long before I could believe that a country as large as Scotland, covered with vegetable productions and with a variety of stations, could be so unproductive. The few which I found were alpine species of _Harpalidae_ and _Heteromera_, living beneath stones. The vegetable-feeding _Chrysomelidae_, so eminently characteristic of the tropics, are here almost entirely absent. I saw very few flies, butterflies, or bees, and no crickets or Orthoptera. In the pools of water I found but few aquatic beetles. I have already contrasted the climate as well as the general appearance of Tierra del Fuego with that of Patagonia; and the difference is strongly exemplified in the entomology. I do not believe they have one species in common; certainly the general character of the insects is widely dissimilar[17]." Now, it is impossible to read this account without being at once struck with two primary considerations: first, that there must exist some great peculiarity (apart from climate) in a region the fauna of which is thus singularly constituted; and, secondly, that latitude (however important it may be in a comprehensive point of view) must exercise in this case a very secondary influence, to allow of localities separated only by the Straits of Magellan to present differences thus extraordinary.

Although so dissimilar in many respects, Madeira and Tierra del Fuego have evidently much in common as regards the conditions which they afford for the increase of organic life. Mr. Darwin describes the latter as "a mountainous region, partly submerged in the sea." So is Madeira. He also adds, that it is "covered to the water's edge with one dense, gloomy forest;" that "to find an acre of level land in any part of the country is most rare;" and that "within the forest, the ground is concealed by a mass of slowly putrefying vegetable matter, which, from being soaked with water, yields to the foot." Such _was_ Madeira, in its normal state[18]; and such it still is throughout a large district towards the northern coast. I cannot indeed refrain from quoting the following, since it portrays the characteristic features of Madeira so vividly, as to be, literally, as suggestive of that island as it doubtless is of Tierra del Fuego. "Finding it nearly hopeless," says Darwin, "to push my way through the wood, I followed the course of a mountain-torrent. At first, from the waterfalls and number of dead trees, I could hardly crawl along; but the bed of the stream soon became a little more open, from the floods having swept the sides. I continued slowly to advance for an hour along the broken and rocky banks, and was amply repaid by the grandeur of the scene. The gloomy depth of the ravine well accorded with the universal signs of violence. On every side were lying irregular masses of rock and torn-up trees; other trees, though still erect, were decayed to the heart and ready to fall. The entangled mass of the thriving and the fallen reminded me of the forests within the tropics; yet there was a difference,--for in these still solitudes, Death, instead of Life, seemed the predominant spirit[19]."

As regards the paucity of species in Tierra del Fuego, there are many instances on record of other countries, and in various latitudes, in which the same anomaly (though perhaps in a less degree) prevails. I have myself observed, in Madeira, large forest tracts, at a considerable elevation above the sea, and which are so densely clothed with wood as to be scarcely penetrable, almost destitute of insect life. Around such altitudes however the clouds perpetually cling, and the rain is well nigh incessant; and it would seem as if the very dampness which causes the vegetation (especially the ferns) to flourish in such rank luxuriance, and the timber to rot with such rapidity that the gigantic trunks are washed, reeking with moisture, down the mountain-slopes, was too extreme for animal existence.

Now, it will be remembered that the Madeiran group is situated at a corresponding distance from the Equator as Morocco, Algeria, the lower limits of Syria, Texas, and Upper Florida are,--all of which literally teem with life; and that Tierra del Fuego lies between the same parallels of south latitude as Durham and Central Russia do in the northern hemisphere. From which it is evident, that the equal removal of countries from the earth's greatest heat does not necessarily imply an equal _exuberance_ in their Faunas,--seeing that in both the regions just appealed to, we not only perceive a vast difference in the _numbers_ of the insects which they respectively contain, from those in other districts which have a similar divergence from the tropics; but we are even able to recognize a certain _resemblance of physical conditions_ (and, therefore, of the creatures which have been either adapted to, or modified by, them) in lands so far asunder, not merely with respect to latitude, but longitude also, as Madeira and Tierra del Fuego.

Other instances might be cited, in support of the immediate principle for which we are now contending,--namely, that many areas have (from local circumstances) a natural superiority over others for the increase of the animal tribes, even _apart from the direct action of heat and cold_:--but space will only permit me to glance at a very few of them. We may detect evidences of this fact, in Ireland; which, in spite of the narrowness of the straits which separate it from our own country, and of its independent commerce with all parts of the civilized world, has an insect fauna curiously limited. From what cause this may arise,--whether from some obscure physical influences peculiar to the soil, or (as Professor E. Forbes has suggested) from the sudden impediment which the establishment of St. George's Channel presented to the westward progress of the various species from the Germanic plains,--it is difficult to speculate: yet the _fact_ of its poverty remains, and we must explain it as best we are able. There can be no question, that, from more frequent communication with England, its entomological fauna has of late years been considerably increased; and it is equally easy to detect, through an examination of its less inhabited provinces, that at a period geologically recent its insect population must have been singularly scanty. I know of few regions (not even excepting the uplands of Madeira) which are more deficient in insect life than the mountains of Kerry. Although abounding, throughout extensive districts, with wood and water, and presenting every apparent requisite for its full development; the naturalist will often be disappointed by finding that a hard day's work has not ensured him the same amount of success as he would have reaped in less than half an hour in many an English meadow. Do we ask, why this is so?--it is impossible to reply, except on the supposition that there are real physical agents, independently of heat and cold, which are unfavourable in Ireland to the existence of these lower creatures. We may perhaps be told, by the advocates of Professor Forbes's theory, that it is the result of isolation,--the quondam land of passage having been broken up before the proper complement of species had reached this large portion of their western destination. But even this, although I believe it to contain much presumptive truth, will not altogether suffice to account for the phaenomena which we see; for Ireland is not only remarkable for the paucity of its _species_, but also for the paucity of its _individuals_,--and the latter fact cannot be explained by any stretch of the migration-hypothesis. We are compelled therefore to conclude, that Ireland, like the other countries to which we have already alluded, presents conditions (altogether irrespective of _latitude_) which must be regarded as adverse to the general prosperity of the insect races.

And so it is with _localities_ (no less than with larger countries),--many of which are eminently unproductive, when compared with others situated at but a short distance from them. Thus, the south-western corner of England is by far the most unprofitable portion of our island, unless indeed I am much mistaken, for insect ascendency. I have made some remarks on this subject in the 'Zoologist,'--from which I extract the following: "Unlike the easy collecting to which we are accustomed in the more favoured East, miles of unprofitable country have often to be gone over, be it swampy moorland or iron-bound coast, where scarcely an insect is to be seen; or, at any rate, where the few which exist are so ordinary, and so sparingly dispersed, as to be scarcely worth the labour of obtaining them,--more especially since the identical species are many of them to be met with in the utmost profusion in more central, or eastern districts. Whether it be the moisture of the climate, or the violence of the south-west winds, which (continually sweeping, as they do, over the high central mass of Devonshire and the bleak, barren downs of Cornwall) present as great an obstacle to the development of animal, as they clearly do of vegetable life, I will not venture to suggest; yet certain it is, from observation, that insects not only become fewer in number in proportion as they are exposed to these external agencies of wind and water; but likewise, in many instances, diminish so considerably in stature as to be scarcely reconcileable with their normal types[20]."

There can be no doubt that islands are, for the most part, more unproductive (even in proportion) than continents; and that, the smaller the area, the less favourable will it be for the development of insect life. Mr. Darwin has noticed this fact in the Galapagos (which he remarks are only equalled by Tierra del Fuego, in barrenness), on Keeling Island (in the Indian Ocean), where he succeeded in detecting but thirteen species, in St. Helena, and at Ascension; and I have added fresh evidence to the same in the various portions of the Madeiran Group[21]. It is however to geological causes that we must mainly look for the explanation of this phaenomenon; and, therefore, since I propose to examine that branch of our subject in a future chapter of this treatise, we will not discuss it now. It will also be better perhaps to defer for the present the general question of self-_diffusion_, which, at the opening of this section, we proposed to consider, along with that of insect _productiveness_ (as dependent on other local influences, besides climatal ones),--it being scarcely possible to render the problem of dispersion in any degree intelligible without calling in geology to our aid.

Having then disposed of this preliminary appendage to our inquiry, by expressing our belief (which I am satisfied that observation will tend more and more to corroborate) _that certain countries and spots are by constitution more favourable than others for the increase_ (apart from the after dissemination) _of the insect tribes_,--and that too through local influences amongst which mere heat and cold are but secondary in importance; let us proceed to consider, how far the _nature of the several districts_ may assist us in accounting for some of those numerous aberrations from the typical state which various insects are accustomed to display, and on which it has too often happened that "species" (so called) have been attempted to be established. I may premise however, that, whilst (as already urged) I would regard climate _per se_ as subsidiary to many other agents, I would not wish to ignore its action altogether even under the present section, since in combination with peculiar circumstances and conditions it may have (and probably has) considerable controlling power: nevertheless I would desire it to be looked upon here as, at any rate, an inferior element, and as working in conjunction with physical influences of greater significance than itself. If therefore under the preceding heads it has been treated (so far at least as the exceptions would permit) as a great geographical principle, possessing a certain modifying quality on a large scale, let us now merely recognize it to the extent in which we are actually compelled to do, when dealing with areas of smaller magnitude,--namely as a _topo_graphical one.

From amongst the many results which I have been long accustomed to associate (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 assume 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:--