On the Variation of Species, with Especial Reference to the Insecta Part 3

[14] Introduction to the Modern Cla.s.sification of Insects (London, 1840), ii. p. 473.

[15] Id. ii. p. 158.

[16] Introduction to the Modern Cla.s.sification of Insects, ii. p. 431.

[17] Journal of Researches, p. 238.

[18] That I may not be misunderstood by those of my readers who conceive Madeira to be a kind of "arva beata," with the sky for ever blue, and (as a consequence) an unclouded sun; I would repeat, that I am not speaking of the vicinity of Funchal only (from which the invalids, who resort thither for their health, almost exclusively draw their deductions), but of _Madeira_,--and, more-over, of Madeira _as it was_, and not of Madeira as it is. More or less of cultivation during a period exceeding four centuries, in conjunction with the overwhelming fire which completely devastated the entire south of the island, immediately after its first settlers had taken possession of it, and which is stated (in the accounts which are transmitted to us) to have smouldered on for nearly seven years, have so altered the features of the country, that it is only in the untouched regions of the north (on which the woodman's axe is nevertheless encroaching, season after season, with lamentable rapidity) that we can catch even a glimpse of its pristine condition. The dense forests which then everywhere abounded must have caused an amount of moisture and exhalation of which even the northern districts as they now are (though saturated, even yet, with dampness; and at a certain elevation almost constantly enveloped with clouds) can give us but a faint idea.

So tremendous indeed must have been the aqueous acc.u.mulations which then hung around the island, that even the splendour of a southern sun cannot have penetrated the atmosphere as it does at present; and, hence, the historical fact that Madeira proper (although separated by a channel of only thirty miles in breadth, and _now_ usually visible in bold relief against the sky, during a portion, at least, of every day, from a far greater distance) was not discovered for _an entire year_ after the colonization of Porto Santo, on account of the thickness of the canopy which shrouded it from view, is at once rendered intelligible. It is narrated, that, in the year 1419, Prince Henry of Portugal organized an expedition to attempt the doubling of Cape Bojador; but the commanders, having lost their reckoning, were driven ash.o.r.e on an island,--which they named Porto _Santo_, in commemoration of their escape from the perils of the sea. "On their return," says Mr. Harcourt, "Prince Henry sent out Zargo, Vaz, and Pestrello, to plant a new colony in the island. It was not long before a dark spot was observed on the western horizon of Porto Santo. This was regarded by some with superst.i.tious awe; but Zargo concluded it to be clouds attracted by high land; and shaping his course in that direction, in spite of the endeavours of his crew (by menaces and supplications) to prevent him, he discovered, in the year 1420, the island to which, from the trees that covered it, he gave the name of _Madeira_."--_A Sketch of Madeira_, London, 1851, p. 16.

[19] Journal of Researches, pp. 209, 210.

[20] Zoologist, x. 3616.

[21] Considering that I have already detected more than one thousand species in those islands, it may perhaps be questioned whether the same truth _is_ to be gathered from the result of my Madeiran researches. I would wish it therefore to be understood, first, that my statement refers to that group _as contrasted with countries in a similar lat.i.tude_; and, secondly, that its _real_ fauna is alone taken into account,--the host of introductions from more northern regions, a large proportion of which have probably taken place within a very recent period (as may be fairly presumed from the knowledge that fresh arrivals, an almost necessary consequence of the importation of plants, _are_ occurring nearly every season), having been dismissed from our present inquiry.

[22] I perceive, on reference to the original examples, still in my collection, that this was wrongly quoted as the _Haltica rufipes_. It is the _H. exoleta_, Fabr., and it is thus entered in Messrs. Hardy and Bold's 'Catalogue of the Insects of Northumberland and Durham;'

where they make the observation, "variable in colour; specimens from the sea-coast are frequently of a dark mahogany tint." I have myself indeed, since I communicated the above remarks to the 'Zoologist,'

taken its precise counterpart, in abundance, along the Yorks.h.i.+re coast,--from Bridlington to the extremity of Flamborough Head; so that it may perhaps be regarded as a topographical state which is more especially peculiar to the eastern sh.o.r.es of England, north of the Humber.

[23] Zoologist, iv. pp. 1283, 1284.

[24] Geodephaga Britannica (London, 1854), p. 186.

[25] Zoologist, iii. p. 900.

[26] Zoologist, v. p. 1941.

[27] Monographie des _Anthicus_ (Paris, 1848), p. 149.

[28] _Id._ pp. 127, 128.

[29] Proceedings of the Entomological Society of London (Part 3. New Series), p. 4.

[30] Insecta Maderensia, pp. 55, 56.

[31] Trans. of the Ent. Soc. of London, ii. pp. 59, 62.

[32] Considering that the true _Vanessa Atalanta_, of more northern lat.i.tudes, _does_ occasionally occur around Funchal, it may be reasonably contended that the fact of its coexistence (on the same spot) with the _V. Callirhoe_ is strong presumptive proof that the latter is a true species, and no climatal or insular modification of the former. And so, judging from a distance, and without local evidence to explain this phaenomenon, I should have concluded myself: nevertheless, recollecting how easy of transport the larvae and pupae of Lepidoptera necessarily are (of which we have the plainest a.s.surance in the almost certain introduction of the _Pontia Bra.s.sicae_, _Sphinx Convolvuli_, _Acherontia Atropos_, &c. into those islands), especially in a region which for more than a century has been receiving a constant supply of vegetables and ornamental plants from western Europe; I am induced to believe that the appearance of the _Atalanta_ is a comparatively recent one, whilst that of the _Callirhoe_ (which, unlike the typical _Red Admiral_, has naturalized itself in nearly all portions of the group) must be referred to the remote period when migrations over a long-lost continuous land were in regular operation.

The _slowness_ of the change, in external aspect, which the isolation of insects from geological causes would seem to bring about (and which follows, as a corollary, if the above conclusion be true), I propose to discuss in a subsequent chapter of this work.

[33] Insecta Maderensia, p. 260.

[34] Insecta Maderensia, pp. 268, 269.

[35] Religion of Nature Delineated, p. 99.

[36] Although the result of a primary (or creative) adjustment to special circ.u.mstances, rather than of a secondary adaptation, brought about by a self-modifying capability; we may just call attention to the fact, that most of the blind insects, whether a.s.sociates within the nests of ants, or natives of subterranean caverns, have either their palpi _or_ antennae anomalously developed,--as though, partially (although how, and in what degree, we cannot possibly ascertain), to make amends for the inconvenience which a total want of sight must, necessarily entail.

[37] This is certainly rendered _probable_, however, from the fact that a large proportion of these apterous species are members of _genera_ which are usually winged,--such as _Tarus_, _Loricera_, _Calathus_, _Olisthopus_, _Argutor_, _Trechus_, _Hydrobius_, _Ephistemus_, _Syncalypta_, _Phloe ophagus_, _Tychius_, _Longitarsus_, _Chrysomela_, _Scymnus_, _Corylophus_, _Helops_, and _Othius_,--whilst the knowledge that, out of twenty-nine genera which I believe to be endemic in those islands, six only are winged (the remaining twenty-three being apterous), will not tend to diminish the probability that there is something peculiar in the action of Madeiran influences generally on the alary system of the insect tribes.

[38] I do not think it necessary to apologize for the apparent disposal of this _quaestio vexata_; because, from the wildness of the upland ridges to which the _D. obscuroguttatus_ is in Madeira exclusively confined, I deem it an absolute impossibility that it could ever have been _introduced_, through any chance agencies whatsoever. And hence, unless we reject the doctrine of specific centres _in toto_, I contend that it must have migrated, together with other insects similarly circ.u.mstanced, by ordinary means, and without natural impediments, from its own area of diffusion.

[39] I am informed by Dr. Hooker, that the only two insects (belonging respectively to the orders Coleoptera and Lepidoptera) which he detected in Kerguelen's Land were wingless.

[40] Insecta Maderensia, p. 6.

[41] Insecta Maderensia, p. 36.

[42] _Id._ p. 310.

[43] Insecta Maderensia, p. 452.

[44] Insecta Maderensia, p. 11.

[45] Religion of Nature Delineated, p. 84.

CHAPTER IV.

ORGANS AND CHARACTERS OF VARIATION.

Having in the preceding chapter briefly alluded to some of the princ.i.p.al causes by which the outward aspect of the insect tribes would seem to be in a large measure (though within definite specific limits) regulated, it may perhaps be desirable to gather into a small compa.s.s, from those remarks, what the chief organs and characters are which appear to be more peculiarly beneath the control of the various influences which we have been just discussing. To imagine that when an insect has become much altered in its general contour, all the parts of which it is composed are equally affected, is contrary to experience; since observation warns us that there are but few actual _members_ which are capable of change,--whilst even the external features, or secondary diagnostics, are only interfered with according to a fixed law, the workings of which are necessarily modified, in proportion as the const.i.tutions of the several animals are differently organized and acted upon.

As regards positive structure, indeed, we can have but few observations to communicate,--seeing that the limbs and appendages themselves are usually of so constant a nature, that disturbing agencies have little or no power to divert them from their typical states. Still, there are occasional facts on record, which would tend to prove that even these are not altogether exempt from the deranging force of certain contingencies from without: the number of the antennal joints, for instance, in the tribes where those organs are multiarticulate, is said to vary; but how far this may be dependent on physical influences, I am not in a position to decide. The connateness of the elytra, again, is a character which we may at any rate define as _sub_-structural; and this I have myself noticed, at times, to fluctuate, according to the circ.u.mstances and conditions of the respective localities in which the particular species obtain. Such is eminently the case with the universal _Harpalus_ (the _H. vividus_, Dej.) of the Madeiran Group. Speaking of this peculiarity, in my volume on the Coleoptera of those islands, I made the following remarks: "But perhaps its most singular character, and in which it differs from every other _Harpalus_ with which I am acquainted, consists in the tendency of its elytra to become united or soldered together. I say 'the tendency,' because it is not always the case that they are joined (which, since the law exists at all, is perhaps the more remarkable), although in most instances, especially in localities much exposed and but slightly elevated above the sea-sh.o.r.e, they are.

I have examples, however, from the upper as well as the lower regions, in which both states are represented; and others again in which the elytra are only partially connected, being free at the apex though firmly attached towards the scutellum. In every instance, however, even where they are united throughout their entire length, a little force will succeed in separating them, showing their structure, as I have indicated in the diagnosis, to be _sub_-connate rather than connate. But that it does require force to effect the disjunction, when they are really in the condition described, is proved to a demonstration to any one who has seen the _remains_ of the insect beneath the slabs of stone on many of the small adjacent islands where it most abounds, or drifting about over the surface of the rocks,--under which circ.u.mstances I have observed them in immense numbers, apparently the acc.u.mulation of two or three generations, which the violence of the elements had not been able to sever. It is rare in the sylvan districts to find them joined; nevertheless such is sometimes the case,--thus proving that the peculiarity is not actually essential, but merely one which it is the tendency of the species to a.s.sume, and which is more developed in some specimens, and under certain conditions, than in others.[46]"

But by far the greatest amount of variability to which insect structure is liable, is presented by the _wings_,--especially the metathoracic ones. The wings, indeed, unless I am much mistaken, are essentially (as compared with other primary details) organs of variation, capable of being more or less developed, according as the several countries in which the creatures are placed may necessitate their action. I will not recapitulate the evidence which I have already adduced, proving that islands have an especial capability of their own, either for increasing or neutralizing, as it may happen, the powers of flight (in which _latter_ case, however, a compensation is usually made for the loss); but I will point to the data which are there brought together, in support of the hypothesis for which I am now pleading,--believing that they will be found sufficient, on inquiry, to establish the doctrine of alary mutability, so far at least as it is connected with isolation as an element of control. If, however (irrespectively of its cause), the thing itself be recognized, the _principle_ is at once established; and we may reason upon it as a matter of fact. So that, if we can ensure this concession or acknowledgment, the occasional _p.r.o.neness_ to variation of these thoracic appendages is, as a law, admitted. The only questions which would then appear immediately to suggest themselves, are: Under what circ.u.mstances do they princ.i.p.ally fluctuate? and why should it happen that organs which are apparently so necessary as a medium of subsistence, should be subject to inconstancy?

Both of these have, in reality, been already replied to in the preceding chapter. Nevertheless, we may briefly repeat, that, so far as the first is concerned, it is in islands that we detect the maximum of instability to which the wings of the Insecta are liable, and that it is in seasons of extraordinary heat that their development is everywhere inclined (if at all) to be especially stimulated: whilst, as regards the second, it will be sufficient to state, that in _continents_, when any decided alteration takes place in the organs of flight, it for the most part comes to pa.s.s that an _increased_ (rather than diminished) action is the result; whereas in _islands_, provided that the species are not absolutely dependent on aerial progression for their food (in which case, in order to prepare for the contingency of being blown out to sea, the capacity of the wings is commonly augmented), the _reverse_ is nearer the truth. So that the _second_ problem,--the _reason why_ appendages thus apparently essential should be subject to inconstancy,--is at once rendered intelligible from the consideration, that it is only under circ.u.mstances in which the indiscriminate employment of those organs would be apt to bring the creatures into trouble that (when not an actual _sine qua non_ to their existence) they are liable to be taken away; whilst, even in that case, it generally happens that some partial equivalent for the privation incurred is granted, as a recompense.

Mr. Westwood, in his admirable _Introduction to the Modern Cla.s.sification of Insects_, has recorded many instances of alary variation; which, however, as he does not appear to have noticed the peculiarity of island faunas, are princ.i.p.ally in corroboration of what I have just insisted upon as the usual tendency in continents,--namely, an _enlargement_ of the erratic powers. Speaking of the _Aphelocheirus aestivalis_ (a member of the Hemiptera), he observes: "My British specimens have but short, rudimental, oval hemelytra, like those of the bed-bug; but I possess one of Bosc's original examples, described by Fabricius, not quite so large as the others, in which _the wings are fully developed_. I do not, however, on that account, regard the former either as pupae or distinct species, but as undeveloped specimens in the imago state[47]." And whilst discussing the _Hydrometridae_, he expresses himself thus: "It appears to me, that, from causes of which we are ignorant, numerous individuals of many of the species of these tribes are subjected to an inferior kind of development in the imago state, which does not allow the acquirement of wings,--which, however, in certain cases, _acquire their full size_. Hence, I consider that the apterous specimens of _Hydrometra stagnorum_, those with very short elytra, and those with the full-sized wings and wing-covers, are all in the imago state, although some are more perfect than others[48]." And, again, in his reflections on the Hemiptera, Mr. Westwood says (and most entomologists are aware of the fact): "The species of _Gerris_, _Hydrometra_, and _Velia_ are mostly found perfectly apterous, though _occasionally with full-sized wings_. _Chorosoma miriforme_, _Prostemma guttula_, _Pachymerus brevipennis_, &c., are generally found with very short wing-covers, but sometimes with full-sized wings[49]." In like manner, the _Cimex apterus_, Linn. (one of the _Lygaeidae_) "exhibits, in an eminent degree, the ordinary occurrence of an imperfect perfect-state; whilst individuals are occasionally found _with fully developed organs of flight_[50]". _Lyaeus brevipennis_, Lat., also ordinarily occurs with abbreviated hemelytra; but it has been found with them perfect by Westwood, as well as with metathoracic wings.

None of the above examples however would appear to do more than refer to the alary instability of the Insecta, as a matter of fact; but this is all for which we are now contending,--the preceding chapter having been in part devoted to some of the presumptive _causes_ of it.

Whether the specimens of _Oncocephalus griseus_, to which Spinola called attention, were insular ones, I cannot say; but he seems to have noted an example in which an _opposite_ phaenomenon to those which Mr. Westwood has cited, was displayed, and moreover to have speculated on the conditions producing it, when he suggests: "L'influence du climat septentrional parait avoir arrete le developpement des organes du vol[51]." And, again, when commenting upon the other tendency in a representative of the _Reduviadae_, he says ('Essai,' p. 96): "Je pense que la presence des ailes et leur developpement dependent du climat."

Whilst treating of two British species of the same family, Mr.

Westwood observes: "The _Prostemma guttula_, Fab., and _Cora.n.u.s subapterus_, Curt., are interesting on account of their being generally found in an undeveloped imago state,--the latter being either entirely apterous or with the fore-wings rudimental, although occasionally to be met with having the fore-wings completely developed[52]." The common _Phosphuga atrata_ of our own country has the organs of flight very rudimentary, and much too small for use: yet the late Mr. Holme of Oxford has mentioned[53], that he has several times taken it on the wing, during the hot suns.h.i.+ne. And, concerning the _Olisthopus rotundatus_, he states[54] that every specimen which he captured in the Scilly Islands was subapterous.

But facts like those are, after all, nothing more than such as we may trace the counterpart of in higher animals than the Insecta. Mr. Gould informs me, that the Swallows of Malta, which have but a comparatively narrow s.p.a.ce to cross over, to the African continent, const.i.tute (although specifically identical with them) a distinct race from those of England,--all of which, he believes, winter in Morocco. But, what are the differences displayed? From amongst many minor ones, of a climatal or geographical nature, the most conspicuous is _the length of the wings_,--those which have annually a longer journey to perform having, through a course of ages, acquired, as a race, a superior capacity for flight. And, in answer to a late query on this subject, he adds that _all_ the sylvan birds in Malta, such as the Black-caps, Willow-wrens, &c., though unquestionably of the same species as those of Great Britain, exhibit small local characteristics by which they may be immediately distinguished,--such as the length of the wings, size of the bills, and tints of the plumage. So that the migratory birds generally, which pa.s.s to and fro between Europe and Africa in that particular lat.i.tude, would appear to form separate races from those which traverse the ocean to our own country; and to be, most of them, remarkable, _inter alia_, for a slight shortening of their organs of transit.

If, however, the members of the insect tribes are capable of but small variation in actual _structure_, with the exception, in certain instances, of the greater or less development of the wings; we shall find that their external characters are much more p.r.o.ne to instability. There is not an item indeed of all their secondary diagnostics which does not admit of a positive change; and, though it be only within fixed limits that the several modifications can occur, those boundaries are frequently far apart, and include at times numerous phases within their embrace which have been too often looked upon as specific. Thus, whether we regard their bulk, outline, colour, or sculpture, anything like absolute constancy, under all circ.u.mstances and conditions, does not so much as exist; and we are driven to admit, that the physical influences to which these various creatures are exposed have a very decided power over their general configuration and aspect. It would be needless, however, to attempt to discuss the above details of aberration separately; because, where any one of them is especially interfered with, it usually happens that the others are more or less involved with them: but we may offer a few desultory remarks, which will tend to show that disturbing agents are apt to mar them both individually and as a whole,--and not only so, but to affect them in a permanent manner (as indeed has been already intimated), according as similar combinations of them are, from local causes (as it were), _selected_, to be acted upon.

I have stated in the last section of the preceding chapter that insect stature is eminently beneath the control of contingences from without; adducing, amongst other examples, in support of this, the Madeiran _Ptinus albopictus_,--a species which, whilst it averages more than a line in length on the central island of the group, is reduced to _less than half_ that bulk on a small and weather-beaten rock (the Ilheo Cho) at a distance from it. Judging indeed from many hundred specimens of the _Ptini_ which I have submitted to a close comparison, "the most constant of their characters would seem to be outline and sculpture, whilst size and colour are apparently the least to be depended upon:--so that trifling differences may be of specific indication in the former case, where in the latter much larger ones are worthless[55]." I have in fact generally noticed, that size and colour are more peculiarly liable to be affected _together_. This, however, is nothing more than what we should antic.i.p.ate, since the same causes which have stunted the dimensions, during a long series of ages, of any particular creature, will for the most part be found to have also impaired the brilliancy of its tints. Luxuriance of vegetation and sheltered districts are alike conducive, in the Annulosa, to the development both of the body and its adornment; or, in other words, where the vegetable creation attains its maximum (which it certainly does not do in situations which are exposed to the irritating consequences of a perpetually stormy atmosphere), there the animal world will be usually observed to thrive.

There are many insects which appear to have _two distinct states_, both in magnitude and hue, which we are seldom (in some instances, I believe, never) able to unite by intermediate links, or grades; and yet which are universally admitted, although found in actually the self-same spots (a fact which prevents their being looked upon as separate, local modifications of a common type), to be mere varieties of each other. They are, however, exceptions to the general rule; and, although infringing on the strict definition of a "variety," as given at a preceding page[56], we nevertheless feel an _a priori_ conviction that they are by no means specifically dissimilar _inter se_. Such phases, as regards _stature_, are presented by the _Brachinus crepitans_ and _Lamprias chlorocephalus_ of our own country; whilst, as regards _colour_, the _Philhydrus melanocephalus_, _Aphodius plagiatus_, and the _Psylliodes erythrocephala_ (const.i.tuting in its paler garb the _P. nigricollis_, Mshm) may be quoted, as cases in point. Thus, also, in Madeira, the _Mycetoporus p.r.o.nus_, Erich., has a large and small form, living in communion,--which I have never been able to connect, and yet which are unquestionably identical (differing in no respect except in size): and so have the _Stenus Heeri_, Woll., and the _Saprinus nitidulus_, Fab.[57]