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diff --git a/38584.txt b/38584.txt new file mode 100644 index 0000000..2a99a54 --- /dev/null +++ b/38584.txt @@ -0,0 +1,6457 @@ +The Project Gutenberg EBook of On the Variation of Species, with Especial +Reference to the Insecta ; Followed by an Inquiry into the Nature of Genera, by Thomas Vernon Wollaston + +This eBook is for the use of anyone anywhere at no cost and with +almost no restrictions whatsoever. You may copy it, give it away or +re-use it under the terms of the Project Gutenberg License included +with this eBook or online at www.gutenberg.org + + +Title: On the Variation of Species, with Especial Reference to the Insecta ; Followed by an Inquiry into the Nature of Genera + +Author: Thomas Vernon Wollaston + +Release Date: January 15, 2012 [EBook #38584] + +Language: English + +Character set encoding: ASCII + +*** START OF THIS PROJECT GUTENBERG EBOOK VARIATION OF SPECIES *** + + + + +Produced by Charlene Taylor, Bryan Ness, Matthew Wheaton +and the Online Distributed Proofreading Team at +http://www.pgdp.net (This file was produced from images +generously made available by The Internet Archive) + + + + + + + + + + ON THE VARIATION OF SPECIES + + WITH ESPECIAL REFERENCE TO THE INSECTA; + + FOLLOWED BY AN INQUIRY INTO THE NATURE OF GENERA. + + BY + + T. VERNON WOLLASTON, M.A., F.L.S. + + + "No compound of this earthly ball + Is like another, all in all." + + TENNYSON. + + LONDON: + + JOHN VAN VOORST, PATERNOSTER ROW. + + 1856. + +"I do not enter so far into the province of the logicians as to take +notice of the difference there is between the _analytic_ and +_synthetic_ methods of coming at truth, or proving it;--whether it is +better to begin the disquisition from the subject, or from the +attribute. If by the use of _proper media_ anything can be showed to +be, or not to be, I care not from what term the demonstration or +argument takes its rise. Either way propositions may beget their like, +and more truth be brought into the world."--_Religion of Nature +Delineated_, p. 45 (A.D. 1722). + + +PRINTED BY TAYLOR AND FRANCIS, RED LION COURT, FLEET STREET. + + +TO CHARLES DARWIN, ESQ., M.A., V.P.R.S., + +Whose researches, in various parts of the world, have added so much to +our knowledge of Zoological geography, this short Treatise is +dedicated. + + + + +PREFACE. + + +To make a dry subject entertaining, is impossible; but to render it, +at any rate, readable, has been my endeavour in the following pages. +How far I have succeeded in the experiment, it is not for me to +decide. + +It having been suggested, by several of my friends, that it might be +desirable to bring together into a small compass some of the evidence +on Insect variation (with reference to external disturbing causes) +which my researches in the Madeira Islands have supplied me with, I +have been encouraged to do so: and I have added numerous conclusions +from other data also, which have from time to time fallen in my +way,--so as to confer on the volume a more practical interest, for +the general naturalist. + +One of my main objects, however, has been to call attention to the +fact, that the Annulosa have not been hitherto sufficiently +considered, in the great questions arising out of the distribution of +animals and plants; hoping that, by so doing, some few of our British +entomologists, who have not looked into this branch of their science, +may be induced to enlist themselves in the cause of Insect geography. + +If such a result be brought about; or if I be fortunate enough to open +for discussion any of the topics which have been touched upon, and so +lead to a more perfect solution of the problems which I have attempted +to explain, I shall consider myself more than repaid. + + 10 Hereford Street, Park Lane, London. + May 10th, 1856. + + + + + CONTENTS. + + + CHAPTER I. + Introductory Remarks + + CHAPTER II. + Fact of Variation + As a matter of experience + As probable from analogy + + CHAPTER III. + Causes of Variation + Sec. 1. Climatal causes generally (whether dependent + upon latitude or upon altitude) + Sec. 2. Temporary heat or cold, of an unusual degree + Sec. 3. Nature of the country, and of the soil + Sec. 4. Isolation; and exposure to a stormy atmosphere + + CHAPTER IV. + Organs and Characters of Variation + + CHAPTER V. + Geological Reflections + + CHAPTER VI. + The Generic Theory + + CHAPTER VII. + Conclusion + + + + +CORRIGENDUM. + + +Page 90, for _Pecteropus Maderensis_ read _Pecteropus rostratus_. + + + + +SPECIFIC VARIATION IN THE INSECTA. + + + + +CHAPTER I. + +INTRODUCTORY REMARKS. + + +A very small amount of information gained by the student in the field +of Nature is sufficient to kindle the desire to increase it. The more +we know, the more we are anxious to know; though the less we seem to +know. It is one of the distinctive privileges of the naturalist that +he has to labour in a mine which is inexhaustible: the deeper he digs +beneath the surface, the richer is the vein for excavation, and the +more interesting are the facts which he brings successively to light. +Dive he ever so deep, Truth, "at the bottom of the well," is assuredly +present, under some form or other, to reward him still; nor will she +even for once elude his grasp, provided he be content to receive her +as she is, instead of endeavouring to mould her to his preconceived +ideas of what she ought to be. In these times of patient research, +when the microscope is disclosing, day by day, fresh wonders to our +view, and new lines of speculation are springing out, as it were +spontaneously, from the regions of thought, it is remarkable that many +of the commoner questions relating to the members of the external +world around us have remained comparatively unsolved; nor indeed have +some of them ever been discussed at all, except in a desultory manner +and with insufficient data to reason from. Foremost amongst these, +numerous problems affecting the distinction between "varieties" and +"species" (as usually accepted) of the animal kingdom stand +pre-eminent,--especially in the Annulose Orders, in which those +distinctions are less easy, _a priori_, to pronounce upon. + +The descriptive naturalist, whose primary object it is to register +what he sees (apart from the obscurer phaenomena which come within the +province of the more philosophical inquirer), can have scarcely failed +to remark the variation to which certain insects are at times liable +from the external agencies to which they have been exposed: and yet, +in spite of this, it is but too true that even physiologists have +frequently shunned the investigation of the _circumstances_ on which +such variations do manifestly in a great measure depend, as though +they were in no degree accountable for the changes in question, and +did not indeed so much as exist except in theory. In the following +pages I purpose, _inter alia_, to throw out a few general hints; +first, on the fact of aberration, as a mere matter of experience; +and, secondly, on some of the _causes_ to which the physiologist +would, in many instances, endeavour to refer it. + +The _former_ of these considerations (namely, the _fact_ of specific +instability as ordinarily noticed) nobody will be inclined to dispute: +and yet it is abundantly evident that it cannot be taken into account, +at any rate satisfactorily, without involving the _latter_ also,--it +being scarcely possible to attach the proper value to an effect +without first investigating its cause. The importance of assigning its +legitimate weight (and that only) to a variety, is perhaps the most +difficult task which the natural historian has to accomplish; since on +it depends the acknowledgment of the specific identity of one object +with another,--whilst, to draw the line of separation between +varieties and species is indeed a Gordian knot which generations have +proved inadequate to untie. Now it is not the object of this +publication to attempt to throw positively new light upon a subject +which has ever been one of the main stumbling-blocks in the lower +sciences, and which is perhaps destined to be so to the end; still +less would I wish to imply that the causes of variation _are_ +altogether overlooked in these days of accurate inquiry,--when +thousands are accumulating data, in all parts of Europe, destined to +be wielded by the master's hand whensoever the harvest-time shall have +arrived: but I do, nevertheless, believe that there exists a growing +tendency, especially in some portions of the Continent, to regard +every difference (if at all permanent) as a specific one; and hence I +gather the information that a reviewal of our first principles is +occasionally necessary, if we would not restrict (however gradual and +imperceptibly) that legitimate freedom which Nature has had chalked +out for her to sport in, or strive to impose laws of limitation in one +department which we do not admit to be coercive in another. + +Perhaps, however, before entering on the subject-matter of this +treatise, my definition of the terms "species" and "variety,"--so far +at least as such is practicable,--will be expected of me. I may state, +therefore, that I consider the _former_ to involve that ideal +_relationship amongst all its members_ which the descent from a common +parent can alone convey: whilst the _latter_ should be restricted, +unless I am mistaken, to those various aberrations from their peculiar +type which are sufficiently constant and isolated in their general +character to _appear_, at first sight, to be distinct from it. + +The _first_ of these enunciations, it will be perceived, takes for +granted the acceptance of a dogma which I am fully aware is open to +much controversy and doubt,--namely, that of "specific centres of +creation." Without, therefore, examining the evidences of that theory +which would be out of place in these pages (and which has been so ably +done already by the late Professor Edward Forbes), I would merely +suggest that the admission of it is almost necessary, in order to +convey to our minds any definite notion of the word "species" at all: +and that, hence, whilst I would not wish to reject the hypothesis as +involving an absurdity (which I believe to be the exact opposite of +the truth), I would, in the present state of our knowledge, desire +rather to regard it as a _postulate, assumed to illustrate the +doctrine of species_, than as a problem capable of satisfactory +demonstration. + +The _second_ of the above definitions may likewise require briefly +commenting upon; for I have frequently heard it asserted that +everything is to be regarded as a "variety" which has wandered in the +smallest degree from its normal state. Now this I contend is +essentially an error; for a "variety," to be technically such, must +have in it the _prima-facie_ elements of stability,--and to an extent +moreover that, without the intermediate links (which, although rarer +than the variety itself, _must nevertheless exist_) to connect it with +its parent stock, its condition is such that it might be registered as +specifically distinct therefrom. Thus, to take an example for +illustration, there are many darkly coloured insects which, as every +entomologist knows, vary, by slow and regular gradations, into a +pallid hue, sometimes into almost white. It also most frequently +happens, in such instances, that the _extreme_ aberration is of more +common occurrence than the intermediate ones. Here then is a case in +point: there is but a _single_ variety involved, namely a pale +one,--the gradually progressive shades which imperceptibly affiliate +it with its type not being regarded in themselves as "varieties" at +all. If this indeed were not so, then would our position be far from +pleasant, since we should be compelled to record, as a variety, +_every_ separate degree of colour which could possibly be found +between the outer limits,--seeing that (increasing, as they did, in an +even ratio) no _one_ could be tabulated in preference to another. + +This however is an example in which the rate of alteration (so far as +colour is concerned) is _equal_; and one therefore in which the +extreme end of the series can be alone singled out as _the_ aberration +to be specially noticed. It sometimes occurs that, between the two +extremes, there are several nuclei, or centres of radiation, to which +the name of varieties may be legitimately applied,--inasmuch as they +may possess a series of characters which do not, all, in combination, +progress evenly; and which consequently stand out as it were, to as +certain extent isolated, from the remainder. + +As a corollary arising out of these remarks, it would seem to follow +that even small differences _should be regarded as specific ones_ so +long as the intermediate links have not been detected which may enable +us to refer them to their nearest types. In a general sense, I believe +that it would be proper to do so: nevertheless there are instances, +the results, for example, of isolation, in which _abrupt_ +modifications may be _a priori_ looked for; and in which our judgment +must be regulated by our knowledge of the local circumstances which +may be reasonably presumed to have had some influence in producing +them. The consideration of these, however, and other kindred +questions, must be deferred to a subsequent chapter of this work. + + + + +CHAPTER II. + +FACT OF VARIATION. + + +It is scarcely possible to survey the members of the external world +around us without being struck with the instability with which +everything is impressed. The very shadows, as they pass, leave a moral +lesson behind them on the mountain-slope, which the student of Nature +would do well to contemplate. Whatever be our preconceived ideas of +the "immutability of the universe," from first to last the same truth +is re-echoed to our mind,--that here all is change. Organic and +inorganic matter are alike subjected to renovation and decay; and, +dependent on that general law, _variability_ within specific limits +would seem to be an almost necessary consequence. In the animal and +vegetable kingdoms, this principle of fluctuation is peculiarly +apparent; and not more surely do the winds of heaven ruffle the +forests over which they rage, than does the ebb and flow which is +perpetually going on amongst created things mar their boasted +constancy. + +The _fact_ of aberration, to which we would briefly allude in this +chapter, requires but little comment; it is patent _a priori_. As a +matter of experience, every observer who has spent a week in the field +of Nature knows it to exist. However difficult it may be, in some +instances, to distinguish aright between species and varieties, as +rigidly defined, there is an instinct within us which often recognizes +the _latter_, even at first sight, as unmistakeably such: and in these +cases, a well-educated eye, although of course occasionally deceived, +will not often be found to err. + +In the vegetable world this proneness to variation is self-evident; +and botanists innumerable, who have investigated the _causes_ on which +the modifications of certain plants have been presumed to depend, have +not been behindhand in acknowledging it. Soil, climate, altitude, and +a combination of other circumstances and conditions, have been +successively taken into account, and to each an amount of disturbing +influence (more or less, as the case may be) has been conceded. "The +more powerful agents," writes Professor Henfrey, "enforce their +general laws, but every little local action asserts its qualifying +voice; and we see that all these irregularities and uncertainties (as +we in our ignorance call them, and complain of) are necessary and +important parts of a great whole,--are but isolated features of a +comprehensive plan, in accordance with which all work in concert to +bring about that _change_ absolutely indispensable to the existence of +animal and vegetable life upon the earth's surface, and that _variety +of conditions_ by which is ensured a fitting abode for each kind of +its multifarious and diversified inhabitants." + +Whilst exploring the barren moor, or bleak upland heights, the +botanist would as assuredly look for a change in the outward +configuration of certain species, which colonize equally the rich +meadows and teeming ravines, as a geographical difference is _a +priori_ anticipated between the hard, sturdy mountaineer and the more +enervated denizen of the plain. A daisy, gathered on the cultivated +lawn, has usually attained a greater degree of perfection and +luxuriance than its companion from the sterile heath; and the bramble +which chokes up the ditches of the sheltered hedgerow, wears a very +different aspect from its stunted brother of the hills. + +Nor is this dependency on external circumstances less apparent in the +animal kingdom also,--the domesticated races of which every +agriculturist is aware are capable of modification, artificially, to +an almost unlimited extent; and which exhibit, when even in a state of +nature, nearly as great a variety, from purely natural causes, as they +have been proved to do when subjected to the laws and routine of +agrarian science. Take the sheep, for example, of Dartmoor or Wales, +and compare them with those from the wolds of Lincolnshire and the +downs of Kent; or contrast the Hereford oxen with those of the midland +counties, or of the Caledonian breed, still extant in Cadzow Forest, +and it will require but little argument to convince us how important +is the operation of local circumstances in regulating the outward +contour of these higher creatures. If therefore this general obedience +to influences from without be self-evident in the vegetable world, and +equally traceable amongst the Mammalia, why, we may ask, are the +lower members of the animal creation to be denied analogous effects +from the same causes? + +We are often told that the Annulosa present so many anomalies in their +organization, that we cannot apply the argument of analogy, when +reasoning on their structure and attributes; and that we must +consequently be content to leave it an open question, as to whether or +not they possess anything in common with the Vertebrata, or can be +presumed to be acted upon, by external agencies, in at all a similar +manner. Now, whilst there is clearly some truth in this assertion +(especially as regards the _senses_ of insects, which must ever remain +a subject of obscurity), I contend that to accept it in all its +fullness would be in the highest degree unphilosophical; whilst, to +endorse it to the extent which even its partial advocates do insist +upon, would at once involve us in a host of difficulties (affecting +other departments of natural science), the very existence of which +they have themselves tacitly repudiated. + +"Creation," says one of our most intelligent writers of modern times, +"_is full of analogies_, pointing to one general originator, and +linking all sentient things into one great family of related +fellow-creatures:"--and there is an amount of sagacity in the remark +which it would be wise for us to digest. Throughout the whole of +animated nature, it is impossible not to perceive that certain +circumstances do, in the main, produce certain results. They may often +fail to produce them, and the results themselves may frequently be +modified (or, apparently, even reversed), from counter influences of +divers kinds. This touches not, however, the existence of the law; and +the effect is not the less specifically dependent on its own peculiar +cause, because those "counter influences" prevail,--and because +_different_ effects may chance, therefore, to be occasionally brought +about by causes which may possibly _seem_ to be identical. We should, +rather, bear in mind that the agents which operate in moulding the +outward contour of organic beings are various, and capable _inter se_ +of permutations innumerable; so that it is only on a broad scale that +parallel results can be looked for in creatures severally exposed to +the action of elements, which are _liable_ to be differently +compounded from what may _prima facie_ appear to be the case: and +that, consequently, where opposite phaenomena are displayed under +circumstances seemingly coincident, our first object should be (_not_ +to regard the phaenomena as indicative, that no constant result can be +anticipated from causes which are similar, but), to inquire whether +the circumstances in question _are_ really coincident or not,--seeing +that some counteracting stimulus may have been, here or there, +unexpectedly at work, which shall enable us, so soon as it is +detected, to account for the discrepancy. + +It is by this process alone that we can hope to make real use of +analogy, without abusing it: for whilst there is danger, on the one +hand, of needlessly rejecting the argument which it suggests to us, +through opposite effects being observed (amongst the members of the +organic world) from conditions which _we assume to be_ co-ordinate, +but which in fact are not so; we may, on the other, run a similar risk +(and thus fail to discern a _corresponding modus operandi_ in the +maturation of like results), from a mere _a priori_ belief that the +lower animals cannot be acted upon, by external influences, in a +manner at all equivalent to that which is self-evident in the higher +ones. + +"To make a perfect observer in any department of science," writes Sir +John Herschel, "an extensive acquaintance is requisite, not only with +the particular science to which his observations relate, but with +every branch of knowledge which may enable him to appreciate and +neutralize _the effect of extraneous disturbing causes_. Thus +furnished, he will be prepared to seize on any of those minute +indications which often connect phaenomena which seem quite remote from +each other. He will have his eyes as it were opened, that they may be +struck at once with any occurrence which, according to received +theories, ought _not_ to happen; for these are the facts which serve +as clews to new discoveries[1]." + +There can be no doubt that amongst a large proportion of our +naturalists, _differences_, as such, are too exclusively studied. +Essential as their investigation is (for we could not progress a step +without some presumptive notion as to the specific identity, or not, +of the objects about which we have to treat), we should not forget +that there are other questions, likewise, which ought to occupy our +attention in, at any rate, an almost equal degree,--as being of +eminent significance in guiding us to a correct interpretation of the +phaenomena with which we have to deal. Such are, more especially, +similitudes and analogies, in their widest sense,--which are too often +neglected, even by those who admit the necessity of recognizing them +where they may be shown to exist. Lord Bacon, in referring to a +similar tendency amongst a certain section of the naturalists of his +day, remarks (though perhaps his love of analogies may have led him to +somewhat overrate their importance): "Up to this time the industry of +men has been great, and very curious in marking the variety of things, +and explaining the accurate differences of animals, herbs, and +fossils,--the _chief part of which_ are the mere sport of Nature, +rather than serious and of use toward the sciences. Such things tend +to our enjoyment, and sometimes to even practical use; but little or +nothing towards an insight into Nature. And so our labour is to be +turned to inquiry into, and notice of, similitudes and analogies, both +in the whole and in the parts of things: for these are they which +unite Nature, and begin to establish sciences[2]." + +I believe that, if analogies were more carefully studied in the lower +departments of the animal kingdom, we should be less inclined to deny +some sort of uniformity to the action of elements and conditions +which, by a law of Nature, must at times operate equally upon the +various and dissimilar members of the organic creation. Amongst the +Insecta, where the individuals exist in such multitudes that accuracy +in generalizations concerning them, becomes, as it were, peculiarly +within our reach, this doctrine cannot be too rigidly insisted upon; +and it is not difficult to foresee that, should the principle of +external disturbing influences ever be admitted by entomologists to +the extent which it has been accepted by the students of the +Vertebrata, our so-called "species" will have to submit to a process +of elimination and inquiry, which at present would be well nigh +incredible. The time for such a step is yet far off: perhaps indeed, +considering the innovations of nomenclature which it would +necessitate, it will never arrive at all; yet the fact remains the +same, that, _if_ analogy with creatures of a more perfect development +be not altogether disallowed us, during our researches into the insect +tribes, or _if_ similar causes may be presumed to have somewhat +similar effects in opposite sections of the animate world, an +enlargement of our prescribed limits, for specific variation, ought in +reality to follow (sooner or later) as an inevitable consequence. + +In whichever light, therefore, insect aberration is viewed by +us,--whether as a matter of experience (which, being self-evident, +will satisfy the practical observer), or as probable from analogy +(which will hardly be denied, at any rate to a certain extent, by even +the most theoretical),--we affirm that _it does, ipso facto, exist_. +"There is no similitude in Nature that owneth not _also to a +difference_;" let this be constantly borne in mind, for it is a truism +almost beyond controversy, and one which, to a reflective mind, will +scarcely admit of a doubt. + +It will be perceived, from the above remarks, that I draw a +distinction between insects which simply vary (that is to say, which +aberr from their normal state), and those which afford (in +the sense as enunciated in the last chapter) one or more actual +"varieties,"--technically so called and it will be further gathered, +that, whilst I regard the former as universally to be met with, the +latter are, on the contrary, of only occasional occurrence. That +positive and well-defined varieties, or races, should be confined to +certain species, is not remarkable; but that every individual insect +should differ, however slightly, from its nearest relation and ally, +may perhaps require some few words of explanation, even to a +naturalist. It is not essential however to our present subject (which +is merely a plea for specific variation generally, as commonly +understood) that any such dogma should be propounded; nevertheless, +since all analogy teaches us to anticipate it, and observation tends +more and more, as our knowledge advances, to corroborate the fact, I +shall be pardoned for venturing a passing thought upon a question even +thus difficult of demonstration. + +Perhaps we are too prone to regard those specific characters, which +are so subtle that they cannot be grasped by our clumsy faculties +except in their broadest and plainest features, as incapable of +fluctuation. Yet a practised eye can detect discrepancies innumerable +in specimens which appear absolutely alike to one that is uneducated; +whilst a third person, better qualified still, will trace out other +and more delicate distinctions, with even greater precision. And thus +it is that we rise, step by step, even amongst the humbler +representatives of the animal kingdom, to the comprehension of that +great truth which is so conspicuous in the nobler ones, and which we +have already summoned to our aid, that "there is no similitude in +Nature which owneth not also to a difference." Let us not forget that +the sphere of our senses is limited; and that, although tuition will +do much to enlarge their capacity for perception, we are at the best +but a dim-sighted race: hence, we should be careful to avoid +conclusions which are not warranted by analogy, and which our +understanding, as it becomes gradually brighter, no less assuredly +condemns. True it is, that we may not be able, as in the higher +animals, to appreciate the differences between individuals without a +rigid inspection, and that sometimes we may fail to do so even when +the objects are critically examined; yet the fact that new +peculiarities do unquestionably open out upon us, as we become more +and more trained for the recognition of them, ought to warn us that +others _may_ exist likewise, despite our _prima-facie_ conclusions; +whilst analogy with what we know to be the case in other departments +of the organic world should suggest, unless indeed there is +presumptive evidence to the contrary, that they in all probability +_do_. + +The Alpine range, when seen from afar, appears a monotonous mass of a +dull uniform hue; and nothing, of all the wondrous details which it +includes, can be distinguished, except perchance the outline of its +jagged peaks projected in faint relief against the distant sky. One by +one, however, as we approach it, inequalities present themselves; the +surface which lately seemed so uniform and grey that it could be +compared only to a cloud, is found to be cleft by ravines; and +valleys, in all their magnificence and breadth, expand slowly to our +view. Yet, marvellous as is the change, this is not all: wood and +water, without which the landscape would be barren, are in turn +revealed; whilst the play of light and shade upon the mountain-slopes +proclaims at length that the picture is well nigh complete. Still more +to be disclosed does in reality remain; and we must advance nearer yet +if we would either fully realise the whole, or enter into the +surprising minutiae of each of its component parts. And so it is with +the objects which we have been just discussing. When contemplated in a +mass, and by an uneducated eye, hosts of them may appear to be +identical; but as our vision becomes clearer and more acute, +differences, formerly inappreciable, are gradually made +manifest,--until at last we can detect modifications innumerable, +throughout the entire length of the living panorama; and are enabled +to endorse the belief (repugnant _a priori_ though it be), that +_individual variations_, even to the extent which I have ventured to +suggest, are not incompatible with _specific similitudes_. + +FOOTNOTES: + +[1] Preliminary Discourse on the Study of Natural Philosophy (London, +1830), p. 132. + +[2] "Magna enim hucusque atque adeo curiosa fuit hominum industria, in +notanda rerum varietate, atque explicandis accuratis animalium, +herbarum, et fossilium differentiis; quarum pleraeque magis sunt lusus +naturae, quam seriae alicujus utilitatis versus scientias. Faciunt certe +hujusmodi res ad delectationem, atque etiam quandoque ad praxin; verum +ad introspiciendam naturam parum aut nihil. Itaque convertenda plane +est opera ad inquirendas et notandas rerum similitudines et analoga, +tam in integralibus, quam partibus: illae enim sunt, quae naturam +uniunt, et constituere scientias incipiunt."--_Novum Organum_, lib. +ii. 27. + + + + +CHAPTER III. + +CAUSES OF VARIATION. + + +"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:-- + +"The extraordinary changes which many insects are subject to when +occurring near the sea, is a fact worthy of notice, and one which I do +not remember to have seen recorded. The strictly maritime species must +be left out of the question; for although many of them are exceedingly +variable both in size and colour, still we have no means of +ascertaining whether that variation is referable to the locality in +which they are placed,--for, never being found inland, nobody can have +an opportunity of asserting that the same changes would not take +place, were they to occur in positions far removed from the influence +of the sea. When we find, however, the same insects in profusion both +inland and on the coast, and observe also numerous and marked +deviations from the typical forms peculiar to the latter situation; +then, _a priori_, we have strong presumptive evidence that the +changes in question are the result of local circumstances, and not +referable to chance. The alteration in size I have almost always +observed to be from large to small, and scarcely ever the reverse; +whereas in colour the change takes place very nearly as much from +light to dark as it does from dark to light: nevertheless the majority +of instances I possess come under the latter department. It has been +remarked that all the specimens of _Mesites Tardii_, which I captured +in Devonshire, were much smaller than the original series taken by Mr. +Tardy at Powerscourt Waterfall, in the county of Wicklow; and so +decided was the difference, that many of my friends, at first sight, +concluded the two to be distinct species. This, however, I consider +entirely owing to their locality, for my specimens were found only on +the coast, and Mr. Tardy's at a considerable distance inland. And, +inasmuch as neither of these instances rested on mere individual +examples, but on long and conspicuous series, the certainty of the +change from large to small was the more apparent. Mr. Holme of Oxford +mentions having taken _Olisthopus rotundatus_ in the Scilly Islands, +in great profusion, none of the specimens of which exceeded two lines +and a half in length. At Whitsand Bay in Cornwall I have captured +_Gymnaetron Campanulae_, none of which exceeded three-quarters of a +line,--the usual length being from a line to a line and +three-quarters. _Anthonomus ater_, the average length of which is two +lines, I have taken a series of in Lundy Island, none of which +exceeded one. In the same locality, also, the common _Ceutorhynchus +contractus_ scarcely ever reaches its natural size; and is, moreover, +so variable in colour, that I was long before I could persuade myself +that the species was not distinct. Instead of the bluish-black elytra +which I had always considered invariable, they all possess a yellowish +or brassy tinge; and the legs, instead of being black, are in most +instances entirely of a light yellow,--and in all, more or less +inclined to that colour. I have received from Mr. Hardy, of Gateshead, +specimens of _Haltica rufipes_[22], captured by him on the coast, in +which the entire insect is of a uniform brownish-red hue. Of the rare +_Mantura Chrysanthemi_ I have taken beautiful varieties at Mount +Edgcumbe and in Lundy Island,--many of which inclined to a rich +metallic-yellow, instead of the brassy-brown of the ordinary +specimens: also, in the latter locality, particularly dark specimens +of _Telephorus testaceus_. In like manner, I might enumerate other +species equally remarkable; but I trust that those already mentioned +are sufficient to verify my observations, of the extreme liability to +change which, more or less, most insects possess when placed within +the immediate influence of the sea. How to account for it, I know not. +I mention it as a mere fact, and leave it for others to assign a +reason for its existence[23]." + +Apparently dependent, in a large measure, on the same circumstance +(namely proximity to the coast), the _Bembidium saxatile_, Gyll., so +common at the edges of the mountain streams in the north of England, +in Scotland, and throughout a portion of Ireland, presents itself +along our southern shores in the form of a permanent variety; being, +as the Rev. J. F. Dawson remarks, "more depressed, never narrower in +front (the sides therefore more parallel), whilst the colour is always +much paler and the spots larger,--that before the apex being round and +very conspicuous, and the anterior one occasionally expanding over the +surface very considerably[24]." I have taken it in profusion on the +coasts of the Isle of Wight, Dorsetshire, and Devon. And so with the +_Cistela sulphurea_, Linn., which in certain maritime localities (as I +have particularly noticed on the sand-hills at Deal) is liable to +become so dark in colouring, that, without the intermediate shades to +judge from (which however may usually be obtained _in situ_), it might +stand a fair chance, occasionally, of being mistaken for a separate +species. A _Psylliodes_ in Lundy Island, allied to (if not identical +with) the _chrysocephala_, Linn., found in abundance on a _Brassica_ +along the ascent from the eastern landing-place, varies "in every +consecutive shade between the limits of light yellow and dark +metallic-green[25]," the former of which states (the normal one on +that rock) might have been fairly set down as specifically distinct +from the latter, did not observation on the spot decide the question +for us without doubt. + +Another curious example of the effect of local influences (amongst +which proximity to the shore plays, in all probability, an important +part) on the external aspect of insects exists in the _Aphodius +plagiatus_, Linn.,--which in this country is generally deep black. "It +is a circumstance worth noticing," I remarked in the 'Zoologist,' in +1846, "that the form which is looked upon by the continental +naturalists _as the variety_, is in England evidently the typical +one,--for out of about sixty specimens which I captured [at Tenby in +South Wales], only _two_ possess the conspicuous red dashes on the +elytra which are considered abroad as the almost invariable +accompaniment." I have observed the same peculiarity in the flat and +damp spots between the sand-hills at Deal, where I have never detected +a single individual which is not perfectly dark; and I believe that +the greater number of the specimens which were originally taken at +Wisbeach, in Cambridgeshire, offered the same geographical +characteristics; whilst those which were found near the more inland +towns of Peterborough and Norwich present a larger proportion of the +ordinary European state. The _blood-red dashes_, however, with which +the elytra of numerous insects are adorned, I have constantly remarked +possess a singular tendency to become evanescent. It is indeed almost +diagnostic of the genus _Gymnaetron_, either that its representatives +should be thus ornamented typically, or else that those which are +normally black should, _when they vary_, keep in view, as it were, +_this principle_ for their wanderers to subscribe to. Thus, I have no +doubt that the _G. Veronicae_, Germ., is but a variety of the _G. +niger_,--an opinion which I expressed in the 'Zoologist' nine years +ago. Whilst commenting on the Coleoptera of Dorsetshire, I then +stated, that "for my own part I must confess I should have doubted the +_G. Veronicae_ being really distinct from the _G. niger_, for red +dashes on the elytra seem naturally peculiar, more or less, to the +whole genus; and I should therefore have suspected that, had +occasional aberrations from a black type existed (which is not +unlikely), those aberrations would probably assume a form which is so +common in the other species of the generic group[26]." + +The _Bembidium bistriatum_, Dufts., is usually much paler when found +in saline districts (under which circumstances it was described as a +distinct species by Mr. Stephens) than when occurring in more inland +positions. The _Blemus areolatus_, Creutz., I have frequently remarked +is similarly affected in brackish places: and I think it far from +improbable that the _Stenolophus Skrimshiranus_, Steph., is but a +local modification (though not altogether, perhaps, through marine +influences) of the _S. Teutonus_, Schr. The _Dromius fasciatus_, +Gyll., not being _exclusively_ littoral, may be quoted as another case +in point,--the specimens which are collected near the coast being for +the most part singularly pale. In speaking of the _Anthicus +bimaculatus_, Illig., M. de la Ferte observes: "Il y a sculement lieu +de remarquer que les individus du bord de l'ocean sont generalement +plus pales que ceux des contrees orientales de l'Europe, et que ceux +des cotes de France et de Belgique sent entierement depourvus de tache +discoidale[27]." And bearing, in much the same manner, on the subject +of variations, the _Anthicus humilis_, Germ., "est une des especes le +plus generalement repandues en Europe; mais il lui faut le voisinage +de l'eau salee. Aussi on le rencontre non-seulement sur les rivages de +toutes les mers, meme de la Baltique, mais encore aux bords des lacs +sales, tels que celui de Mannsfeld, en Saxe. _Ceux de cette derniere +localite sont generalement noirs_; ceux que j'ai pris a Perpignan sont +d'un rouge tres-clair, ce qui me porte a croire que cette espece est +dans le meme cas que quelques autres _Anthicus_, dont les varietes les +plus foncees appartiennent au nord de l'Europe, et les plus pales au +midi[28]." + +Whilst touching on this immediate question of variability _as +dependent to a great extent_, in numerous cases, _on proximity to the +sea_, we may just notice the marked tendency which even the insects +_peculiar to_ saline spots would seem in a large measure to possess, +of converging, more or less obviously, to a lurid-testaceous, or pale +brassy hue, in their colouring. True it is that we cannot (as above +suggested) deduce any evidence of direct physical modifications from +amongst species which are _strictly maritime_,--seeing that we have no +means of judging in such instances whether similar phaenomena would or +would not be produced in central districts also: nevertheless we may +perhaps detect in this general law some slight indication of the +effects which an atmosphere and soil constantly impregnated with salt +would be likely to bring about in the external aspect of those members +of the insect tribes whose range is sufficiently extensive to expose +them to its operation. The bare mention of such names as _Nebria +complanata_ and _livida_, _Calathus mollis_, _Pogonus luridipennis_, +_Trechus lapidosus_, _Aepus marinus_ and _Robinii_, _Cillenum +laterale_, _Bembidium scutellare_, _ephippium_ and _pallidipenne_, +_Ochthebius marinus_, _Psylliodes marcida_, _Phaleria cadaverina_, +_Helops testaceus_, and _Anthicus instabilis_, so eminently +characteristic as they are of briny situations, will at once appeal to +our native entomologists; whilst the acknowledgement of the same +principle is no less conspicuous in a host of other species which are +not included in the British fauna. + +Hence, when we see the tendencies of coloration (not to mention other +particulars, often readily apparent) essentially the same, both in +insects which are peculiar to, and in those which have overspread +(from without) certain regions or localities, it is impossible not to +associate some inherent controlling power with the regions themselves; +and we are driven to the conclusion, that _either_ well-defined +_races_ have been gradually shaped out, by means of the physical +influences to which they have been exposed, or else that the _species +themselves_ (as witnessed by the intermediate geographical links, +which, although sometimes rare, are in all instances to be found) do +assuredly merge into each other. + +In addition to those which we have been just discussing, there are +other influences (equally independent of mere heat and cold) by which +insect modifications may be brought about,--modifications moreover of +that precise character which must be referred, in general terms, to +the nature of the country and of the soil in which they severally +obtain: a very few examples, however, in illustration of their action, +must suffice for our present purpose. The _Tarus lineatus_, Schoenh., +is slightly shorter in Madeira, as also somewhat darker on its head +and prothoracic disk (and with its elytral striae less deeply +impressed), than it is in Algeria and Spain. The Madeiran specimens of +the _Aphodius nitidulus_, Fabr., are usually a little paler, and more +distinctly punctulated, than their northern analogues; as are also, in +the latter respect, those of the _Clypeaster pusillus_, Gyll. The +_Scydmaenus Helferi_, Schaum, is permanently smaller in the Madeiran +group than it is in Sicily; and I believe that the _Achenium +Hartungii_, Heer, of those islands, is but a local state of the _A. +depressum_, Grav., of Central Europe. The _Bembidium tabellatum_ and +_Schmidtii_, Woll., may be in reality but geographical modifications +of the _B. tibiale_ and _callosum_ of higher latitudes; and the +_Malthodes Kiesenwetteri_, Woll., of the common European _M. +brevicollis_. Calcareous deposits would appear, ever and anon, to have +considerable efficacy in regulating the outward aspect of such species +as are able to adapt themselves to different geological districts; and +when in juxtaposition with the shore, their effects are often very +conspicuous. The _Dromius arenicola_, Woll., is the Portosantan +representative of the _D. obscuroguttatus_, Dufts.; and distinct as it +is in colouring from that insect (as evinced both in Madeira proper +and throughout Europe), I believe it to be in reality but a local +condition of it, occasioned by a residence through a long series of +ages on a calcareous soil. For the same reason perhaps (though +assisted, in all probability, by the qualifying power of isolation), +the _Hadrus illotus_, Woll., may be specifically identical with the +Madeiran _H. cinerascens_. In like manner, the _Bembidium Atlanticum_, +Woll., which in Madeira proper is frequently so dark that its elytral +patches are sub-obsolete, and which is but seldom brightly arrayed in +that island, assumes in Porto Santo (which is not only more calcareous +than the central mass; but is strongly impregnated, as its streams and +rills everywhere testify, with muriate of soda) a permanently paler +hue,--being at times almost testaceous. Some districts seem to be +more prolific in varieties, generally, than others. The neighbourhood +of Ipswich, in our own country, has been cited by Mr. Curtis[29] as +possessing this peculiarity; and I have remarked a similar tendency in +certain parts of Ireland. The common _Haliplus obliquus_, indeed, of +the Blackwater river, in the county of Cork, is usually so dark and +suffused in colouring, that it might be almost taken for a distinct +species,--its fasciae, especially the hinder ones, being occasionally +evanescent. + +One more example must satisfy us under this section,--namely, the +_Harpalus vividus_, Dej., of the Madeiran group. So curiously is that +insect affected by the nature of the areas through which it +successively ascends, and that too irrespectively of heat and cold (as +may be gathered from the fact that its phases on the shore and upland +heights are well nigh coincident), that it may be appropriately +singled out as a concluding instance of the effects of those obscure +local influences to which we have been drawing attention. "Ranging +from the beach to the extreme summits of the loftiest mountains, +accommodating itself at one time to a low barren rock of 20 yards +circumference, at another to the deep-wooded ravines of intermediate +altitudes, around which the clouds perpetually cling, and where +vegetation and decay are ever rampant, or harbouring beneath the rough +basaltic blocks of the weather-beaten peaks (6000 feet above the sea); +we should naturally expect, _a priori_, to discover some slight +modifications of outward structure, according as the respective +localities differed in condition. And such we find to be everywhere +the case. I am satisfied, moreover, that it is only by a careful +observation on the spot that an insect like the present one can be +properly understood; for, to anybody acquainted with it practically in +all its phases, it is but too evident how many 'species' (so called) +might be established on undoubted varieties, where there exists a +desire for creating them, and where our sole knowledge is gathered +from a few stray specimens collected by another person, and +unaccompanied by local information to render the aberrations +intelligible. For it must be tracked from the shore to an elevation of +more than 6000 feet before we are enabled to discern the causes by +which its development is controlled, or even to connect by slow and +easy gradations its opposite extremes of form. And it is an +interesting fact, that the distance between its variations does not +increase in proportion to the distance between its altitudes. On the +contrary, it would seem to pass through its minimum of size and +maximum of sculpture at about the elevation of from 3000 to 4000 feet; +both above and below which,--that is to say, as it recedes from the +upper and lower limits of the sylvan districts,--it becomes gradually +modified, and almost in a similar manner. Thus, to a person who had +visited Madeira and had picked up specimens on the coast, and to +another who had perchance penetrated into the interior, as passing +visitors from the vessels are accustomed to do, and had brought away +examples from the wooded mountain-slopes, the two insects would appear +altogether distinct. For, commencing on the level of the beach, the +usual type is broad, flat, more or less opake, with the prothorax +almost impunctate, and the elytra soldered together. As we ascend +higher, the breadth invariably diminishes, the brightness, and depth +of sculpture, seem (up to a certain altitude) to increase, and the +elytra are seldom, or but very imperfectly united; until, on entering +the lower limits of the forest region, at an elevation perhaps, _ore +rotundo_, of 3000 feet, we find that it has gradually put on a very +different aspect,--being small, narrow, bright, convex, comparatively +ovate and deeply striated; the legs and antennae have become +exceedingly pale; the prothorax has altered considerably in shape, +being much narrowed behind and punctured; and the elytra are nearly +always free. In this state it continues for about 1500 feet; when +again emerging into the broad daylight of the open hills, it +recommences to mould itself as it did below; until, having reached the +summits of the loftiest peaks, more than 6000 feet above the sea, it +has almost (though not entirely) assumed the features which +characterized it on the shores beneath[30]." + + +Sec. IV. _Isolation; and exposure to a stormy atmosphere._ + +Having in the preceding pages touched upon the subject of insect +variability, as the occasional result, to a greater or less extent, of +climatal and other influences; let us now proceed to consider the +importance of a certain physical condition, which will be found, I +believe, on inquiry, to be accompanied by a more decided modifying +power than any which we have yet discussed. + +Every one who has examined the natural history of islands, both in +theory and practice, must be aware of the many difficulties which have +constantly to be encountered, before the several phaenomena can be +satisfactorily explained. Laying aside those forms which are +manifestly endemic (the numerical proportion of which usually accords +with the _distance_ from the nearest mainland), again and again are we +baffled by the near resemblance of the various creatures to +continental types,--whilst the minute _differences_ which they +display, from them, are at the same time so permanently fixed, that we +are almost precluded, under the ordinary acceptation of a "species," +from regarding the two as undoubted descendants of a common stock: and +thus it is that insular faunas have frequently been magnified, in the +novelties which they are supposed to contain, far beyond what is +right. A person however who looks to the causes of things, and is +prepared to recognize _effects_ where there are fair grounds for +anticipating them, will not be slow to perceive, that, in the small +deviations which we are so often accustomed under such circumstances +to behold, _the results of isolation itself_ (as an active controlling +principle) may be traced out; whilst geology, ever ready to lend a +helping hand when appealed to, will seldom fail to supply those +intermediate links of probability which the believer in specific +centres of creation must needs subscribe to, before he can draw any +deductions on a broad scale, or be competent to analyse even the +general bearings of a question thus necessarily comprehensive. + +Having thought it desirable to defer to a subsequent chapter of this +treatise the few geological reflections which our subject may give +rise to, it will not be my aim to allude to them in the present +section more than is absolutely requisite. I propose rather to +consider some of the ordinary effects of isolation, as mere matters of +experience; and to allow geology to tell its own tale when we come to +examine the problem of _self-dispersion, as occasionally interrupted +by subsidence_. + +If we except a few of the _Heteromera_ and apterous _Curculionidae_, +which appear to be influenced in a different manner, the power of +isolation over insect form is perhaps more especially to be detected +in a deterioration of stature. Whether this principally emanates from +the constant irritation of a stormy atmosphere, such as small islands +are of course exposed to, and which would seem to have stunted the +development (during a long series of ages) of the animal and vegetable +worlds, or from a diminution of area consequent on the breaking up of +a continuous land, it is difficult to pronounce: nevertheless, it is +most consistent with both reason and analogy to suppose that each of +those causes has operated to induce a similar result; and that we must +therefore view them as working in concert, if we would appreciate +their action aright. + +It is a law to which a large proportion of the organic creation would +appear to be subject, that the exuberance of life (not so much, +however, as regards the number of individuals which the various +species may present, as in the grandeur of their size) has reference +to the magnitude of the spot over which it is permitted to range. The +unnatural breeding-in of a single race, which must of necessity happen +unless the intercourse with other varieties of its kind be possible, +has always been attended with effects more or less pernicious; and in +the Annulose tribes I believe that the reduction of space which +geological convulsions have at various epochs brought about, has been +commonly succeeded (_inter alia_) by a reduction of stature in those +species which have been cut off from their fellows. I do not assert +that there are no exceptions to this rule; for counter-influences may +at times prevail (as we shall shortly see), to neutralize the above +tendency. I hold it, however, as an absolute truism, in physics, that +a law without an exception is an anomaly. If, therefore, we were once +to admit the latter to negative the former, no such thing as a law +could exist. Hence it follows, as a corollary (unless, indeed, we are +prepared to endorse that conclusion), _that_ _where there is a law +there must be an exception to it_; and that, consequently, exceptional +cases, if not exceedingly numerous, should never pervert our belief +from an otherwise presumptive truth. + +This dwindling-down of size has seldom failed to attract my attention, +more or less, in almost every island which I have hitherto had an +opportunity of exploring: space, however, will not permit me to dwell +upon many instances. I have already adverted to the diminished stature +of _Anthonomus ater_, Mshm, and _Ceutorhynchus contractus_, Mshm, in +Lundy Island,--the first of which scarcely ever reaches, on that rock, +more than half its natural bulk. The late Mr. Holme, of Corpus Christi +College, Oxford, in like manner, captured the common _Calathus +melanocephalus_, Linn., and _Olisthopus rotundatus_, Payk., in +Scilly,--the former of which seldom exceeded two lines, and the latter +two and a half, in length: and he also recorded, that the _Bolitochara +assimilis_, Kby, is invariably smaller in those islands than it is in +the neighbourhood of Penzance[31]. The _Vanessa Callirhoe_, Fabr. (a +geographical analogue of the Red Admiral Butterfly[32], so common in +our own country), is permanently smaller in Porto Santo than it is on +the larger, more luxuriant and varied, and therefore more protected, +island of Madeira proper. And, as regards the _Ptini_ of that group, +so completely are some of them "affected by isolation, and by exposure +to a perpetually stormy atmosphere, that they do not attain half the +bulk on many of the adjacent rocks that they do in the more sheltered +districts of the central mass; and so marvellously is this verified in +a particular instance, that I have but little doubt that five or six +_species_ (so called) might have been recorded out of one, had only a +few stray specimens been brought home for identification, without any +regard having been paid to the respective circumstances under which +they were found[33]." That "one," Protean, representative is the +_Ptinus albopictus_, Woll.; and it is so eminently a case in point, +that it may be admissible to quote, _in extenso_, a few of the +observations which I have already published concerning it:-- + +"The _P. albopictus_ is the commonest of the Madeiran _Ptini_, and by +far the most variable, having a separate radiating-form for almost +every island of the group,--whilst, at the same time, the whole are so +intimately connected together (and merge into each other) by +innumerable intermediate links, that it is impossible to regard them, +in spite of the opposite contour of the _extremes_, in any other light +than as different aspects of a single species, according as +circumstances may favour, retard, or otherwise regulate its +development. Instability in fact (in its broadest sense) may be +considered to be one of its most prominent characteristics, since it +appears to be more sensitive to isolation and altitude than any of the +other members of the genus with which we have here to do,--as may be +proved to a demonstration by a careful study of its habits on the +spot, where the influences of position and exposure are, in nearly all +instances, more than sufficient to account for the successive phases +assumed. Thus, commencing with _var._ alpha, which reaches its +maximum in the sheltered ravines of the central mass, the bulk is +usually large, and the tints comparatively intense. _Var._ beta. +is likewise brightly variegated, but it is smaller. Now, if our +premises be correct, that locality and the action of the external +elements have much to do with the changes in question, we might have +expected, _a priori_, that this state, from its peculiarity to the +Dezerta Grande, would not only have reduced in dimensions (which it +is), but in colour also (which it is not). Here, therefore, +observation, _in situ_, becomes extremely important; since such does +at once convince us that its almost exclusive attachment to the +interior of the stalks of the _Silybum Marianum_, Grtn. (the _Holy +Thistle_ of the ancients), with which the more protected portions of +that island everywhere abound, affords it ample conditions, even on so +bleak a rock, for its completion. Nevertheless, its _stature_ (as +already stated) is slightly diminished in spite of this: and when we +come to examine the individuals which infest the lichen of more open +situations (aberrant however on the Dezerta Grande, and answering to +the _var._ gamma. of the diagnosis), we immediately perceive that +_both_ of our required results are indicated,--the reduction not being +limited to size, but extended also to hue. In Porto Santo this +modification is the normal one,--where the insect likewise displays +the same lichenophagous tendency, and where the districts in which it +exists are equally barren. But, if its maximum be attained in Madeira +proper, and a certain number of minor deviations range throughout +Porto Santo and the Dezerta Grande, it still remains for us to show +where its _minimum_ is to be obtained:--which, true to the _modus +operandi_ by which we have conjectured its divers degrees of abortion +to have been brought about, would seem to be centred on the Northern +Dezerta, or Ilheo Chao. When we bear in mind the minute dimensions of +that flattened rock, which does not include so much as a single +valley, or depression, within its bounds, and is consequently seldom +free from the violence of the winds (which sweep across it +incessantly, from whatever quarter they may arise); it could hardly be +supposed that an insect which is so obviously subservient to +atmospheric control should not have become materially affected, in its +outward guise, through long seclusion on such a spot:--and accordingly +we are not astonished to find the race which has been thus cut +off for ages on this extraordinary little island, itself _as_ +extraordinary. It is indeed very remarkable to trace out how clearly +the agencies we are discussing have here operated on the species under +consideration,--for both sexes (though especially the male) descend on +the Ilheo Chao to somewhat less than half a line in length, being +literally of scarcely greater magnitude than some of the larger +representatives of the _Ptiliadae_!"[34] + +I stated above, that, although this diminution of stature is a very +general accompaniment of isolation, amongst insects which have been +_long_ cut off from the rest of their kind, there is no rule without +an exception to it; and that, therefore, we must not always anticipate +the result which has been described. We should remember that _immense_ +periods of time are apparently necessary before any perceptible change +can come over creatures from the stoppage of their migratory progress, +and the unnatural in-breeding of their several tribes; so that in +islands geologically recent (which often implies, however, their +existence through epochs which would sound vast indeed to ears +unscientific) we must not invariably expect to discover evidences of +this law. On the contrary, we must first of all take into account the +age of their formation, before we can judge _a priori_ as to the +probability of its operation through a sufficient interval of time to +have become conspicuous in its effects. I say "through a sufficient +interval of time," because the process of deterioration may be +silently going on, even now, in many an island, _which has not yet +shown any matured traces of its action_, except perhaps in the case of +a few species which appear to be more particularly susceptible to +contingencies from without. We should then call to mind, that an +enormous proportion of nearly every insular fauna is composed of +accidental colonists during the last few centuries, in which +civilization and commerce have been unintentionally at work in the +cause of animal diffusion; and that, therefore, if modifications in +outward contour have not necessarily resulted during a positive +_geological_ interval, it would be absurd to look for them in the mere +settlers (as it were) of yesterday. + +Thus, it will be perceived, how necessary it is to take every element +and contingency into account before we venture to pronounce +dogmatically on either the existence or non-existence of any physical +law; and how cautious we should be of denying the legitimate operation +of external influences in one region, because they would seem, _prima +facie_, to be contradicted in another. It is surely more philosophical +to endeavour to reconcile the two, by tracing out (as may frequently +be done) some opposing principle in the latter, which shall enable us +to understand the discrepancy, and to believe that the same action may +be going on in both cases, but that in one of them it is either +overruled by a greater controlling power than itself, or else has not +had sufficient time to bring its fruits to maturity. If a proposition +be true, we should recollect that it is _always_ so (under all the +circumstances and conditions to which it is applicable); for, +otherwise, it would be both true and false,--which is absurd: hence, +_if_ my premises be true, that the general tendency of isolation is to +diminish the stature of those insects which have become isolated; it +follows that that tendency must remain, so long as there are no other +special disturbing influences to absorb or neutralize it. "When any +observation," says a writer of the last century, "hath hitherto +constantly held true, or hath _most commonly_ proved to be so, it has +by this acquired an established credit: the cause may be presumed to +retain its former force; and the effect may be taken as probable, _if +in the example before us there doth not appear something +particular,--some reason for exception_[35]." Hence it is, that, even +amongst the _opposite_ phaenomena which one island may occasionally +present from those of another, I have often been able to recognize the +working of a selfsame law; and clearly to detect, that it is not from +_its failure_, in either instance, that contending results are brought +about, but simply that some counteracting agent has been exerting its +energy in the one case, so as to nullify what would have otherwise +come to pass. + +The main object however of the present section being to show that a +considerable amount of power is due to isolation itself, in regulating +(after a long series of ages) the outward aspect of the insect tribes, +it is not strictly necessary that we should so rigidly insist on +deterioration of size as one of its primary consequences,--since +(whether it be so or not) we are merely concerned here to demonstrate, +that its influence, _in some shape or other_, is absolute and real. + +After the above remarks, we shall not be surprised that the phaenomena +displayed in certain islands, as regards size, are sometimes (though I +believe it to be an exception to the ordinary rule) the exact opposite +of what we have been describing. Let us not however be alarmed at this +fact, on the bare statement of it,--as though the proposition which we +have been lately advancing were at once disproved; since we shall +find, on inquiry, that the case is not so desperate as might be +imagined; and that in many islands where even this principle is to be +detected, we may recognize traces of the other also. But how, it will +be asked, can this be? for, since the influences are the same, +creatures similarly exposed to them must be similarly affected. Now, +although, on a broad scale, such a notion contains much presumptive +truth; on a narrower one it does not always apply; for species are +differently constituted _ab ovo_, and will sometimes give a different +result from the operation of causes which are identical. Moreover, +there is a curious tendency which I have remarked in most islands, +that the wings (especially the metathoracic ones) of their insect +inhabitants are liable to be retarded in their development,--often +indeed to such an extent as to become actually evanescent: and I +believe it to be a law of Nature, that when any particular organ is +either stunted or taken away, the creature receives a compensation for +its loss either by the undue enlargement of some other one[36], or +else in a general increase of its bulk. If such be the case, the +presence of two apparently conflicting effects in a single island is +rendered somewhat more intelligible; nevertheless, on the above +hypothesis, the specimens which increase in dimensions should +undoubtedly have their organs of flight more or less enfeebled, whilst +those which diminish should be regularly winged. And hence we arrive +at the question, is this so? My own experience would certainly tend to +prove that it is; and I suspect that future observations will confirm +the fact. Meanwhile, I must content myself with simply advancing the +subject for consideration, and with recording such few examples, in +support of the theory, as space will permit, and which occur to me +almost spontaneously. + +The Madeiras would seem to inherit, as it were, a more than usual +control over the alary system of their insect population; for, out of +about 550 species of Coleoptera which I have hitherto met with in that +group, nearly 200 are either altogether apterous, or else have their +organs of flight so imperfectly developed, that they may be +practically regarded as such; so that, if our preceding conclusions +(from the compensation-hypothesis) be correct, we should _a priori_ +anticipate an increase of bulk in those islands, rather than a +decrease of it. Unfortunately the greater number of these 200 +representatives are now, through the submergence of the once +surrounding continent, _endemic_, so that we have no means of judging +whether the obsoleteness of their wings is to be referred to the long +action of Madeiran influences[37], or whether they were thus created +severally in the beginning; and, for the same reason (that is to say, +having no others of their kind to compare them with), we cannot +pronounce, even if we might assume this partial organic decay to be +the consequence of their isolation on these rocks, whether their +general stature has been subsequently augmented or not. Still, there +are some few, out of the 200 just alluded to, which are of common +European distribution; and, as these would appear to have obeyed the +principles to which we have been calling attention, it is not +unreasonable to suppose, that many of the others (could we but behold +them as they formerly were,--emigrants over a vast continuous land) +would be found to have done so also. + +I alluded, in a previous section, to the _Dromius obscuroguttatus_, +Dufts., as presenting permanent characteristics in Madeira,--the +combined result of latitude and isolation; and I also stated that it +was not always possible, whilst dealing with physical agents which are +necessarily obscure, to refer the respective phaenomena (whatsoever +they may be), which would seem to have departed from their types, to a +single disturbing cause. Hence, whilst I there acknowledged latitude +as in part answerable for the changes which that insect has undergone, +I may here suggest that it is, in all probability, to _isolation_ that +we must mainly look, if we would understand those changes aright. But +what _are_ the distinctive features, it may be asked, which the _D. +obscuroguttatus_ has adopted, since its first arrival from more +northern latitudes over an unbroken[38] continent? It has not altered +much, after all: it is, however, the _nature_ of the alterations, and +their constancy, which give them their real importance. In a few words +then, the insect is rather larger and more robust than its European +analogue, and (to omit other minor differences) _its wings are +evanescent_. But this, on our above hypothesis, is precisely what we +should have expected: for, since it is self-evident that the species +cannot have been naturalized accidentally on these mountains, and +since geology informs us that a _vast_ interval has elapsed since the +Madeiran islands were portions of a continuous whole, we have at once +a sufficient _time_ assured us for the modifications to be completed, +and to appear at length permanently adjusted in accordance with the +conditions and influences which locally prevail. + +There are other examples which might be quoted in support of my +theory,--that isolation, when involving a sufficient period of time, +has a direct tendency either to diminish the stature of the insect +tribes, or else to neutralize their power of flight; but that, in the +latter case, the creatures, when thus despoiled of a function, do, on +the contrary (instead of deteriorating in size), often receive a +compensation for their loss by an actual _increase_ in their bulk. The +common _Bradycellus fulvus_, Mshm, is another instance in point. From +its occurrence in the almost inaccessible districts of the Madeiran +group, far removed from cultivation, I am inclined to refer its entry +into this southern region to that remote period when a connective land +offered a natural passage to wanderers from the north. Hence our first +stipulation, that of _sufficient time_, is satisfied; and what is the +result? The insect is a trifle more robust than its ordinary European +representatives, and it is _invariably apterous_. The _Calathus +fuscus_, Fabr., is also, as is clear from its special attachment to +the mountain tops, strictly indigenous in Madeira (that is to say, it +must have arrived there during the migratory epoch); and the +consequence is, that, although usually winged in our own country, it +is permanently subapterous in that island. I think it far from +unlikely that the _Dromius negrita_, Woll., may be the ultimate phasis +(from isolation) of the common _D. glabratus_, Dufts.,--from which it +may be distinguished by its somewhat larger bulk, more robust head and +prothorax, and by the obsoleteness of its wings. True it is, that the +latter species flourishes alongside it in Madeira; but, like the +_Vanessa Atalanta_ (when considered with respect to the _V. +Callirhoe_), may it not be of more recent importation from the +European continent, and as yet in a transition state?--an idea which +the _smallness_ of its wing, as compared with those of its British +analogues, would seem rather to corroborate. + +But, if this slight increase of stature would appear generally to +accompany that gradual extinction of the powers of flight which +isolation is apt to induce, it follows, on the other hand (as +indeed I have lately intimated), that where wings are so essential +to the continuance of a species that they cannot, without its +positive destruction, be taken from it, the _primary_ effect of +isolation,--namely a diminution of bulk,--will for the most part +happen instead. As this fact, however, has been already commented +upon, we will not discuss it afresh. + +Why it is, in the Insecta, that _islands_[39] should predispose to an +apterous state more than continents, it is not easy to speculate. Mr. +Darwin has indeed suggested, and with much apparent reason, that, were +wings fully developed, the indiscriminate use of them might lead to +unhappy results, by tempting the creatures to venture too far from +their native rocks; and that, therefore, this partial decay is, under +such circumstances, a wise provision in their favour: whilst it has +been urged, on the other hand, that since insular species are at all +times liable during heavy gales to be blown out to sea, they should +in reality be gifted with _stronger_ powers of flight (rather than +weaker ones), to fortify them against such disasters; and that, +consequently, the above phaenomena are not explicable on Mr. Darwin's +hypothesis. For my own part, I am inclined to accept that theory, in +all its fullness; and, furthermore, I do not believe that the latter +consideration (though it unquestionably contains much presumptive +truth) does at all interfere with the admission of it,--seeing that +either requirement may be fulfilled, according to the nature of the +several species which are destined to be acted upon. Thus, if _flight_ +is absolutely indispensable, as in the greater number of the +Lepidoptera, and beetles of a flower-infesting tendency, we shall find +that the wings remain unaltered (if indeed they be not actually +increased in capacity, of which I am by no means certain), and that +the effect of isolation is more particularly evident in a diminution +of stature. But if, on the contrary, the creatures are less dependent +on aerial progression for their sustenance, as in the predacious +tribes generally, especially those of nocturnal habits, the reduced +area in which they are confined, in conjunction, it may be, with the +danger to which they would constantly expose themselves by the +promiscuous employment of organs which their modes of life do not +positively need, would seem to render the presence of wings +unnecessary; and they are accordingly, by degrees, removed:--in which +case, however, a compensation for the loss is not unfrequently granted +by an increase (more or less perceptible) in bulk. + +In the Madeiras, this diminution and enlargement of stature, +accompanied for the most part respectively by the retention and +annihilation of the powers of flight, is singularly traceable on the +selfsame rocks, particularly the smaller ones of the group. Thus, on +the Flat Deserta, or Ilheo Chao, the _Scarites abbreviatus_, Koll., +_Laparocerus morio_, Schoen., and the _Helops Vulcanus_, Woll., attain +a gigantic size; yet it is on that very island that the _Ptinus +albopictus_, Woll., finds its minimum of development,--scarcely +exceeding in dimensions some of the larger members of the +_Trichopterygia_. The Deserta Grande has some special modifying +capability of its own,--the _Eurygnathus Latreillei_, Lap., +_Notiophilus geminatus_, Dej., _Zargus pellucidus_, Woll., _Calathus +complanatus_, Koll., _Olisthopus Maderensis_, Woll., _Caulotrupis +conicollis_, Woll., _Laparocerus morio_, Schoen., _Omias Waterhousei_, +Woll., _Helops Vulcanus_, Woll., and the _Ellipsodes glabratus_, Fab., +being also larger on that rock than is typical: all of them, however, +with the exception of _Notiophilus geminatus_, are there, as +elsewhere, apterous. + +Other qualifying results, from isolation, are equally apparent. Take +_colour_, for instance; and we shall perceive that in the _Dromius +sigma_, Rossi, it is sensibly affected. The normal state of that +insect "does not occur at all in Madeira proper, but only in Porto +Santo. True it is that the modifications in the several islands +present but slight differences _inter se_; nevertheless, being +constant, I would lay particular stress upon them, since they go very +materially to prove that the effects of isolation on external insect +form are even more important, if possible, than those of latitude. +That this is the case in the present instance, appears clear from +facts so minute as these. For, out of the many specimens which have +come under my observation from various countries of Europe, if there +is one point more constant than another in this otherwise variable +species, it is, I believe, to all circumstances, its immaculate +prothorax. Now, whilst this (we may almost say essential) character +obtains in Porto Santo, in Madeira it does not hold good: the +prothorax there is invariably infuscate in the centre; and on a small +adjacent rock (the Ilheo de Fora) it is entirely dark. Nor let anyone +suppose that details apparently so trivial are beneath our notice, or +the mere result of chance, since it is by the observation of such-like +points, and by marking their development according to the +circumstances of the several localities in which they obtain, that we +are alone able to appreciate their importance, and so to form, in a +wider and geographical sense, a correct estimate of their value[40]." +The _Olisthopus Maderensis_, Woll., is much paler, larger, and more +opake, on the Dezerta Grande than it is in Madeira proper. So great +indeed is the change which it has undergone through a long isolation +on that rock, "that, had the case been a solitary one, I should not +have hesitated in regarding the specimens obtained from thence as +specifically distinct; nevertheless, with the knowledge both of the +modifying effects of isolation, and also of the _kind_ of modification +essentially peculiar to that island, I am perfectly satisfied that it +is a mere local state, although a very remarkable one, and has no +claim whatsoever to be otherwise considered[41]." The _Pecteropus +Maderensis_, Woll., is of a greenish-brassy tinge in Porto Santo, and +much acuminated in front; whereas on the Dezerta Grande it is almost +invariably _coppery_, and less narrowed anteriorly. The _Caulotrupis +lucifugus_, Woll., although ranging through no very opposite phases, +either of outline or sculpture, "appears to possess a slight +modification for every island of the Madeiran Group: and hence small +shades of difference, which might otherwise be regarded as trifling, +become directly important, and cannot be ignored in a local +fauna,--even though a general collector may deem it unnecessary to +recognize them. In real fact, however, such distinctions, when viewed +geographically, are of the greatest interest, as serving to illustrate +what we have so often had occasion to comment upon, namely the +influence of isolation and other circumstances on external insect +form[42]." The _Psylliodes vehemens_, Woll., is permanently paler in +Porto Santo than it is in Madeira proper, being almost entirely +testaceous. "That the species is identical, however, with the Madeiran +one I have not the slightest doubt,--the sculpture and colour, as I +conceive, having merely undergone a change since the remote period +of its isolation on a comparatively calcareous soil[43]." The +_Scarites abbreviatus_, Koll., occupies the loftiest peaks of nearly +all the Madeiran islands, and was probably once abundant over the +entire ancient continent, whatsoever its limits may have been, of +which the present group forms but an isolated part. "There are traces +of it in the Canaries, from whence occasional specimens have been +brought, and which, from the want of local data and of sufficient +numbers to reason upon, have in their turn been severally regarded as +distinct. The fact however is, that the species in question is an +extremely variable one, assuming differences of size according to the +altitude at which it lives, and differences of sculpture according to +the circumstances of the spot on which it is isolated. That such is +actually the case, a careful observation of the many minute changes +which the insect has undergone in the various islands and altitudes of +the Madeiran group will, I think, prove to a demonstration. For it is +impossible to suppose that every rock contains its own _species_, that +is to say, has had a separate creation expressly for itself,--a +conclusion at which we must assuredly arrive, if small and even +constant differences are _of necessity_ specific. Rejecting therefore +this hypothesis as utterly untenable, and as contrary to all +experience, we are driven to acknowledge that isolation _does_, in +nearly every instance, in the course of time, affect, more or less +sensibly, external insect form;--which being admitted, we have at once +an intelligible principle whereby to account for modifications +innumerable, each of which, when viewed simply as a difference, +independently of the circumstances producing it, might have been +regarded as sufficient to erect a 'species' upon, had the desire for +multiplying them overbalanced the love of truth[44]." + + * * * * * + +Such are a few of the circumstances, influences, and conditions, by +which the outward aspect of the insect tribes is liable, within +definite limits, to be more or less regulated: and it is impossible to +view them with an unbiassed mind and not arrive at the conclusion, +that physical agents generally have a very decided control over the +external contour of these lower creatures. In selecting the examples +which we have lately discussed, I have avoided as much as possible +those startling instances of variation which distant quarters of the +globe will readily supply, because there are vast numbers of our +naturalists who will not acknowledge the validity of any evidence +which would tend to amalgamate, in a broad sense, the species of the +Old and New Worlds. I have therefore contented myself with such data +as must fall within our common experience, feeling satisfied that if +the principle be allowed in the one case, it cannot long be objected +to in the other. There are few entomologists who would not recognize, +in the abstract, a legitimate capacity for adaptation in every insect +with which they have to do; yet I believe there are not many, who, if +modifications were to be shown them as the fixed result of +disturbances from without, would be prepared at once practically to +accept them as such. The collectors of the present day are so prone to +regard every _permanent_ difference as a specific one, that a large +proportion of them do not sufficiently realize, that well-marked +races, or states, are no longer matters of hypothesis, but of fact; +and that, therefore, a sensible amount of aberration should not only +be _conceded_ to the action of certain physical combinations and +elements, but even anticipated and looked for. Such however ought not +to be; and earnestly therefore would I advocate a greater latitude for +geographical influences than has been hitherto admitted by many of us. +Especially would I urge the necessity for a more careful study of +_insular_ phaenomena, for I am convinced that a due allowance is +seldom, if ever, made for the qualifying power of isolation, _per +se_,--the most significant perhaps of all the conditions which we have +attempted in the preceding pages to examine. + +"Felix qui potuit rerum cognoscere causas" is a motto which the +student of Nature should keep constantly in view; for it is +undoubtedly a more honourable task to discover the _reasons_ for what +we see, than the mere appearances themselves. He who has dived deeply +into the everyday circumstances around him will be reluctant to +ascribe so much as a single item of all that comes within his ken, to +chance; for to him the whole system of created things is, from first +to last, replete with design. _Natura nil agit sine causa_ is as true +now as it ever was, and it will be so to the end. Let us not therefore +be discouraged at the apparent smallness of the data from which many +of our conclusions have to be drawn, for nothing is in reality trivial +which is the effect of a wisely appointed law; and, even were such the +case, it would not be thereby proved that the investigation of the law +itself (however liable it may be to exceptions) is unimportant. Nor +ought we, on the other hand, to be discouraged if we cannot always +reconcile conflicting phaenomena, and detect in each a primary +controlling cause. We should rather bear in mind, that the elements +with which we have to deal are obscure, and subject to permutations +from which various results must of necessity arise; and that it is +only, therefore, on a broad scale that we can look for uniformity of +action, even from conditions which may appear to be identical. "Nature +is not irregular, or without method, because there are some _seeming_ +deviations from the common rule. These are generally the effects of +that influence which free agents, and various circumstances, have upon +natural productions[45]." + +FOOTNOTES: + +[3] Religion of Nature Delineated, p. 103. + +[4] Journal of Researches (London, 1852), p. 381. + +[5] The great preponderance of the phytophagous over the predacious +tribes, in the hotter regions of the earth, is a remarkable fact, and +strongly suggestive of the relation which the insect and vegetable +worlds (both of which attain their maximum in those zones) bear to +each other. "The carnivorous beetles, or _Carabidae_," says Mr. Darwin, +"appear in extremely few numbers within the tropics. The +carrion-feeders and _Brachelytra_ are very uncommon; on the other +hand, the _Rhynchophora_ and _Chrysomelidae_, all of which depend on +the vegetable world for subsistence, are present in astonishing +numbers. The orders _Orthoptera_ and _Hemiptera_ are peculiarly +numerous; as is, likewise, the stinging division of the _Hymenoptera_, +the bees, perhaps, being excepted."--Journal of Researches, p. 34. + +[6] Mr. Westwood states that he possesses an individual of the +_Papilio Machaon_ from the Himalayan Mountains, captured by Professor +Royle, "which scarcely exhibits the slightest differences when +compared with English specimens."--_The Butterflies of Great Britain_, +p. 4. + +[7] Zoologist, xiii. p. 4655. + +[8] The Butterflies of Great Britain (London, 1855), p. 17. + +[9] _Id._ p. 94. + +[10] Insecta Maderensia (London, 1854), pp. 7, 8, 9. + +[11] Insecta Maderensia, p. 516. + +[12] Insecta Maderensia, p. 17. + +[13] I possess specimens of this insect captured on the summit of +Mount Olympus by my friend E. Armitage, Esq., who is also of opinion +that it may be but a mountain state of the _C. sylvatica_, Linn. + +[14] Introduction to the Modern Classification of Insects (London, +1840), ii. p. 473. + +[15] Id. ii. p. 158. + +[16] Introduction to the Modern Classification 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 accumulations 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 ashore 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 superstitious 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 latitude_; 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 Yorkshire +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 shores 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 +latitudes, _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 assurance +in the almost certain introduction of the _Pontia Brassicae_, _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 circumstances, 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 associates 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 circumstanced, 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 +principal 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 +compass, 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 constitutions 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 circumstances 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-shore, 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 circumstances I have observed them in immense +numbers, apparently the accumulation 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 +assume, 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 _proneness_ 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 +circumstances do they principally 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 pass 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 circumstances 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 +Classification 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 principally 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 _Coranus +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 sunshine. 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 space to cross over, to the African continent, constitute +(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 pass to and fro between Europe and Africa in +that particular latitude, 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 prone 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 +circumstances 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 +Chao) 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 anticipate, 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_ (constituting in its +paler garb the _P. nigricollis_, Mshm) may be quoted, as cases in +point. Thus, also, in Madeira, the _Mycetoporus pronus_, 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] + +As regards the instability displayed by _colour_, in the insect +tribes, when subjected to the action of certain conditions and +influences from without, so much has been said in the fourth section +of the preceding chapter, that it is unnecessary to repeat it here. +True it is that it was then my sole province to discuss the _causes_ +which would appear to regulate, in a large measure, the external +aspect of the Annulosa; yet the _existence_ of inconstancy, in the +several organs and characters involved (with which alone we are now +concerned), was, by the nature of the case, implied: so that if the +_disturbing element_ was demonstrated, the mere fact that the thing +(whatsoever it may have been) _was interfered with_, was surely proved +_a fortiori_. I there pointed out the great proneness to a change in +hue which divers circumstances are apt to induce; and I particularly +instanced proximity to the sea-shore, and other saline spots, as well +as an attachment to calcareous districts, as amongst the most powerful +of the deranging contingences. In case, however, that any further +evidence should be looked for, on this immediate subject, I will quote +the following,--relating to the _Bembidium Atlanticum_ of the Madeira +islands, which was but just touched upon in that chapter,--as a +concluding example of the general effect of physical agents on the +colour of these lower creatures. "Throughout all the Madeiran +Coleoptera there is perhaps no insect which displays such an +extraordinary range of colouring as the present one does; and although +it is true that the section of _Bembidium_ to which it belongs is +essentially a variable one, yet I am not acquainted with any +_Peryphus_ in which the paler patches of the elytra are so remarkably +unstable, or which appear to be so completely under the control of +external circumstances, as are those of the _B. Atlanticum_: and +indeed unless viewed in the mass, we should scarcely be inclined to +recognize the same species in the many aspects which it puts on +between its extremes. The examination, however, of a very large number +of examples, and a careful consideration of the several localities and +altitudes in which they were taken, has convinced me that there is +unquestionably but a single type of form amongst my entire series, +since the whole are so intimately connected, by successive gradations +both of outline and colour, that it is perfectly impossible to isolate +even a single specimen, or to draw a line of specific demarcation +between any two consecutive members of the chain. It will be +perceived, by a reference to the diagnosis, that the insect in +question passes imperceptibly from nearly a pure green, through a +well-defined spotted state, into one which has the elytra almost +testaceous,--the paler portions being at last so largely developed as +to become confluent, and almost to cover the entire surface. In +Madeira proper the darker varieties would seem to be typical; whereas +in Porto Santo the brightly coloured ones preponderate, and in fact +are all but universal. Both extremes do nevertheless occur in both +islands, the tendency being merely, in either case, to assume the +particular modification characteristic of the spot[58]". + +And so it is with the outline and sculpture (no less than with bulk +and hue): they also are equally liable to disturbance from physical +causes, as indeed has been already insisted upon. Like most of +the minutiae of variation, however, to which we have called +attention, it is more particularly on islands that this is to be +observed,--isolation, during an interval sufficiently long, appearing +to possess some especial control over the external contour and surface +of the insect races. Thus, in the Madeiras, for instance, the +_Caulotropis lucifugus_ has its prothorax more distinctly punctured, +and its elytra more perceptibly striated, in the principal island, +than on any of the smaller members of the group; in Porto Santo, +indeed, it is almost free from sculpture of any kind; whilst its ally, +the _C. conicollis_, apart from being somewhat larger, is, on the +contrary, both more punctured and striated on the Dezerta Grande than +it is in Madeira proper. The _Omias Waterhousei_, again (in addition +to its slightly increased bulk and less shining envelope, in that +locality), is more lightly impressed on the Dezerta than it is in +Madeira and, not to mention other differences, the _Ellipsodes +glabratus_ is densely beset with most minute granules on that same +rock--whereas on the mountain slopes of the central mass, it is highly +polished and glabrous. The _Helops confertus_, we have intimated at a +previous page, is less coarsely sculptured in the lofty regions of +Madeira, than in the lower ones: and the _H. futilis_ has its elytral +tubercles apparent in Madeira proper, but evanescent on the Dezerta +Grande. The _Eurygnathus Latreillei_ assumes a permanent variety on +the Dezerta, the insect having become modified through a long +isolation on those weather-beaten heights,--here it not only attains a +more gigantic stature than in Porto Santo, but is invariably also more +parallel and opake, has the sides of its prothorax more recurved, with +the punctures towards the lateral angles almost obsolete, and the +striae of its elytra somewhat more evidently punctate[59]. + +Such examples, however, might be multiplied _ad infinitum_; and I will +not therefore devote further space to the bringing together of facts +which it is hardly possible will be disputed,--especially as it has +been my wish, in the present chapter, merely to _enumerate_ what the +organs and characters principally are which are more peculiarly +sensitive to change, throughout the Annulose tribes. This I may +venture to hope, though briefly, I have in part done; and I will +consequently pass on to other considerations, which, even if somewhat +alien to the immediate question of insect instability, should scarcely +be altogether omitted in a treatise like this. + +FOOTNOTES: + +[46] Insecta Maderensia, pp. 56, 57. + +[47] Introduction to the Modern Classification of Insects, ii. p. 466. + +[48] _Id._ ii. p. 469. + +[49] _Id._ ii. p. 454. + +[50] Introduction to the Modern Classification of Insects, ii. p. 480. + +[51] Essai, p. 103. + +[52] Introduction to the Modern Classification of Insects, ii. p. 473. + +[53] Trans. of the Ent. Soc. of London, ii. p. 60. + +[54] _Id._ ii. p. 59. + +[55] Insecta Maderensia, pp. 260, 261. + +[56] Vide _supra_, p. 5. + +[57] Although, in our ignorance of their real nature, we cannot cite +them as actually analogous to these separate phases in certain members +of the Insecta, yet we are forcibly reminded by the latter of the +distinct states which many of the Terrestrial Mollusca present +(frequently in equal proportions) in the same localities. Thus, most +of the _Pupae_ have at least two abruptly-marked forms,--a larger and +smaller one. Many of the _Helices_ also exhibit this tendency in an +eminent degree: I have indeed been shown specimens by Sir Charles +Lyell of the _Helix hirsuta_, Say, from North America, one state of +which is considerably more than double the dimensions of the other; +and I believe it is a well-known fact that intermediate links _have_ +not yet been observed to connect the extremes. May not therefore the +gigantic _H. Lowei_ and _Bowdichiana_, which are now extinct in the +Madeira Islands, have been but forms of the _H. Portosanctana_ and +_punctulata_, respectively,--co-existent with them, though more +sensitive to the great diminutions of altitude and area which were +consequent on the breaking-up of a once continuous land? If such be +the case, however, it is certain that they were far commoner at an +early period than their smaller colleagues (which, now, in their +proper districts, absolutely teem),--seeing that the _latter_ are +extremely rare in the fossil deposits, whilst they themselves +literally abound. + +[58] Insecta Maderensia, p. 78. + +[59] Insecta Maderensia, pp. 21, 22. + + + + +CHAPTER V. + +GEOLOGICAL REFLECTIONS. + + +We frequently hear it asserted, that, since the members of the Insecta +are so numerous and minute, when compared with those of other +departments of the organic world, the entomologist, whose province it +is to collect and classify them, can have but little time, if he +attempt the real advancement of his particular science, for +generalizations on a broad scale. Now, whilst there is necessarily +some reason in this remark (for the investigation of species is a work +of such labour and drudgery that it is apt to monopolize all the +leisure hours which the greater number of us are able to command), we +should recollect, on the other hand, that the soundest theorists have +ever been the most patient and accurate observers; and have, many of +them, spent whole years of their lives as humble students in Nature's +domain. We need not be afraid that an occupation amongst what is +microscopically small is liable to cramp the mind, and render it unfit +for wider processes of induction, since the very opposite of this +would seem to come nearer to the truth. The understanding which has +been well tutored by a system of close and steady observation, which +has been trained to seize upon differences amongst the objects of our +common experience, to balance the importance of generic and specific +characters, as tested in the acquisitions of our daily walks; and +which has been gradually brightened and matured by the habitual +exercise of its judgment on the most trifling phaenomena around us, has +usually gained strength enough to form conclusions from such data, +which will not only stand the test of analysis, but will be free from +those eccentricities of genius which too often mar the speculations of +less practical naturalists. The mind, moreover, having been chained +and fettered for a season to the mere detail of facts, breaks forth, +under such circumstances, with all the vigour with which the +contemplation of truth has gifted it, and takes its flight as it were +to a clearer sky; and, though a reaction may at times set in, hurrying +it away into regions beyond its sphere, it will assuredly return at +length, fraught with the soberness which its vocation has inspired, +and commence to build up its hypotheses, step by step, in harmony with +the material which it has amassed. + +Yet though entomologists may be in reality as well qualified as any +other natural historians for drawing general conclusions from the +result of their researches, it is impossible to conceal the fact, +that, as a body, they have not ordinarily done so. Whether this has +happened through an accidental disinclination on their part to occupy +themselves in such matters, or (which is more probable) from their +whole time having been engrossed by the dry routine of their science, +I do not pretend to determine: be the solution, however, what it may, +the inference is practically the same,--that the Annulosa have not +hitherto been sufficiently regarded, in the great questions of +zoological geography. But especially have they been ignored during +that most significant of considerations which has been so ably brought +forward of late years by some of our keenest observers,--namely, the +distribution of animals, as affected by geological changes, on the +earth's surface. + +It would be well if the collector of insects would devote at least a +tithe of his energies to the speculative branch of his subject. +Certain it is that much would probably be advanced, at first, on +slender premises; and would, as a consequence, fall to the ground, +leaving no record behind it. Yet such must inevitably be the case, at +the outset, in every region of inquiry; and we are prepared to expect +it. It does not however follow that _good_ would not be developed +also; whilst we are confident of the fact, that unless the trial be +made, it cannot possibly arise. No question has ever yet been mooted +without beneficial results: it has either been shown to be absurd, and +has received its death-blow on the spot, or else truth has been +elicited (indirectly perhaps), which has at once shed a new ray of +light on some of its obscurest bearings. And so, assuredly, it would +be in the present instance. We cannot doubt that there is much to be +discovered in the past history of insect dissemination, which would +tend, when rightly interpreted, to explain many of the occult +phaenomena of the present day; and we may be equally satisfied that +this cannot by any possibility be attempted without the assistance of +geology. Let us therefore glance hastily at a few of those more +undeniable convulsions which we are aware have, at various epochs, +taken place; and endeavour to catch a glimpse of how, in the common +course of things, that portion of the insect world would be affected +which was exposed to their influence. + +First and foremost, perhaps, in importance, of all the changes which +it is self-evident have happened, may be mentioned _subsidence_. +Including, as it does, both the general lowering of some countries, +and the actual isolation of others, there are, I believe, no physical +crises to which we could point, through the instrumentality of which +the very _existence_ of the insect races (not to allude to their +diffusion) has been, by the nature of the case, more seriously +interfered with. We know that there are certain species of an alpine +and boreal character, which cannot live except in a climate of low +temperature,--guaranteed to them either by _elevation_ in one land, or +by a higher latitude in another: and let us picture the consequences +of the gradual sinking of a mountain chain, even to a small extent, +the _summits_ of which only just afforded the conditions of atmosphere +necessary for the continuance of creatures like these. Now this is an +example by no means far-fetched, and such as _must_ have occurred in +instances innumerable. But, what would be the many results of a +diminution in the level of our imaginary range? It needs no argument +to prove, that _one_ at least would be manifest in the total +extinction of those forms which could not adapt themselves to the +increased heat. Others, which were able with difficulty to endure the +alteration, would in all probability, even though they had now +emigrated to the loftiest peaks, flourish less vigorously than before; +and it is not unlikely, moreover, that they would become _somewhat +modified from their normal states_,--states which, be it recollected +(for this is an instructive lesson), would still exist in more +northern zones. + +During my researches in mountain tracts, I have usually remarked, that +the highest points of land either teem with life, or else are +perfectly barren. My own experience would certainly tend to prove, +that, in a general sense, one or the other of these extremes does +almost constantly obtain. And, although I would not wish to dogmatize +on phaenomena which may in reality be explicable on other hypotheses, +it would perhaps be worth while to inquire whether the geological +movements of subsidence and elevation will not afford some clew to the +right interpretation of them. Be this, however, as it may, I can +answer, that in many countries, where there are strong indications of +the former, the alpine summits harbour an insect population to a +singular extent; whilst in others, where the latter is as distinctly +traceable, the upland ridges are comparatively untenanted. Now we have +already shown, that where the gradual lowering of a region has taken +place, there will be, of necessity, an undue accumulation of life on +its loftiest pinnacles,--for, even allowing a certain number of +species (which _even formerly_ were only just able to find a +sufficient altitude for their development) to have perished, we shall +have concentrated at that single elevation the residue of all those +which have survived _from the ancient elevations above it_. But, if, +on the other hand, an area, already peopled, be in parts greatly +upheaved, there will be _either_ a universal dying-out, from the cold, +of a large proportion of its inhabitants, or else an instinctive +striving amongst them to desert the higher grounds on which they have +been lifted up, and to descend to their normal altitudes: in both +cases, however, the present summits will display the same +feature,--namely, utter desolation. + +Such are a few of the effects which elevation and subsidence, even on +a small scale, would seem (when tested by theory and practice) to +produce. It yet remains for us to suggest, that the latter, when +carried to its maximum, so as to cause the actual separation by the +sea of one district from another, is a contingency of immense +significance in regulating the distribution of the Annulose tribes. +Their outward contour and aspect we have shown in a previous chapter +to be very largely beneath the control of isolation, provided a +sufficient _time_ can be granted for the change: but their ultimate +absence from any particular place, through the impediment which it +offers to their migratory progress, we have not yet touched upon. Let +us conceive, therefore, an extensive continent; and, since the +insects which at present inhabit our earth must, if the doctrine of +specific centres be true, have been originally created in certain +definite spots, let us suppose a limited proportion of them to have +been first produced upon this tract. Self-dissemination, we will +assume, has been going on for centuries: those species which were +gifted with quick diffusive powers have become pretty evenly dispersed +over its surface; whilst those of naturally slow or sedentary habits +have peopled, comparatively, but small areas around the respective +localities of their birth. Such may have been the case, at some fixed +period, amongst the aboriginal beings of any country which we choose +to select as an illustration. But there is another element to be +considered. If this region be not insular, it will have received +colonists from foci of radiation situated beyond its bounds; and +these, therefore, according to their several capabilities for +progression, will have, likewise, in parts, overspread, or tenanted, +it. Now it is impossible to cite a more simple example than this. But +let us endeavour to realize what would be the necessary consequence of +the breaking up of such a district as that which we have imagined. If +a _general_ sinking should take place, causing its higher points to be +alone visible above the ocean, or merely a _partial_ one, so as to +admit of the sea encompassing portions of it which would remain +unaffected in their altitude; the result practically would be the +same,--namely, the constitution of a group of islands out of a once +continuous land. Then, as regards the animal population of this +tract, the main phaenomena are almost self-evident. Should any of its +isolated fragments chance to contain a portion of one of _those +limited areas_ which a species of slow progressive powers had +succeeded in colonizing, it would of course harbour (provided that the +other portion has disappeared) what would now be defined as _endemic_. +Numbers of these small areas, or, in other words, of the species which +had overspread them, would in all probability be lost for ever; whilst +the occurrence of any of the surviving ones in more than a single +island would manifestly depend on the proximity of the islands _inter +se_. Those forms which had diffused themselves over the whole original +continent would now be found in all the detachments of the cluster; +whilst others, which had wandered over the greater portion of it only, +might be traceable perhaps in every island _except a few_. + +Such are the primary facts which suggest themselves, whilst discussing +the question of isolation as regulating the _distribution_ of the +Annulose tribes. Its _after effects_, on their external configuration +and development, we have examined in a preceding chapter of this +treatise; and we have also lately intimated what might be a few of the +presumptive consequences of a subsidence (in a general sense), _apart +from_ the still more important principle of isolation. Before, +however, we dismiss these brief and elementary reflexions on the +upward and downward movements which geology testifies to have +occurred, at various epochs, on the earth's surface, I shall perhaps +be pardoned if I digress so far from my immediate subject as to trace +out some of the actual results of isolation in the diffusion of the +Insecta (especially recognizable in the stoppage of a former migratory +progress) in a few of the northern Atlantic groups. I should premise, +however, that it is from the Coleoptera alone that I shall attempt to +draw my inferences; nevertheless, since that order is more extensive +than any of the others, and has moreover been closely investigated in +most of those islands, it may possibly afford us data of sufficient +comprehensiveness and accuracy for practical purposes. + +To commence, then, with the Madeiras and Canaries; the first facts +which isolation discloses to us, concerning the statistics of a region +which was once continuous throughout that portion of the Atlantic, are +the _slowness_ and the _direction_ of the ancient migratory movements. +The former of these is rendered evident from the vast number of +endemic species which are at present contained, not merely in the two +groups combined, but in the several islands of which each of them is +composed. True it is, that these peculiar forms are, most of them, +apterous, and of naturally sluggish self-disseminating powers; yet, +still the circumstance remains, that these various creatures had not +overrun areas of any extent before the land of passage was +destroyed,--for otherwise they must have occurred, now, on islands and +rocks but slightly removed from each other, _which they do not_. The +latter of the above conclusions, namely, the _direction_ of the +migratory current, will become apparent in the sequel. We may premise +however, that, so far as the aborigines of this province are +concerned, their course will be found, upon the whole, to have been a +_northerly_ one. + +As regards the slowness, and the direction, of the _quondam_ migration +(questions which can scarcely be treated apart from each other), some +light may be thrown on the subject from considerations like the +following. The Canaries are the head-quarters of the genus _Hegeter_; +Teneriffe may indeed be called the land of Hegeters. No less than +thirteen or fourteen species have been recorded as indigenous to those +islands; and there can be no reasonable doubt whatsoever that that +ancient region (when continuous and entire) was the primaeval centre, +or range, of that Heteromerous group. The Hegeters are an apterous +race, and of a sedentary temperament; hence, when the area (whether by +general or partial subsidence, it signifies not) was broken up, it is +not surprising that those local fragments of it should have become the +nucleus of reception, as it were, for the members of that genus. +Nevertheless, a few of these many representatives (of more discursive +capabilities perhaps than the rest) had found their way, before the +period of dissolution, to a considerable distance from their original +haunts. Thus, one of them (the _H. latebricola_, Woll.) had arrived at +what now constitutes the rocks of the Salvages; another (the _H. +elongatus_, Oliv.), at least, if not two, had colonized the Madeiras, +and is said (though I believe incorrectly) to have even reached the +present coast of Portugal. This latter species is clearly of a more +adaptive nature than its allies, inasmuch as it has, also, naturalized +itself (though this may be a more recent, and accidental, +circumstance) on the opposite shores of Africa. One thing, however, is +at any rate manifest,--that the Hegeters attain their maximum in the +Canaries, and that a few members only have been sent off, in a +northerly, or north-easterly, direction, from thence. + +In like manner, the genus _Tarphius_ is distinctively Madeiran. I have +detected nearly twenty well-defined species of it in that group; yet, +out of so large a number, two only have occurred beyond the central +island. Now the _Tarphii_ are, also, wingless; and creatures of very +sluggish propensities,--scarcely ever stirring from the masses of +loose rotting timber which they so assimilate in hue, and to the under +sides of which they affix themselves, day and night. Although +difficult to investigate in their precise economy, it is extremely +probable (may I not say, certain?) that some important and peculiar +office is assigned to them in the remote upland districts to which +they exclusively belong: and there cannot be any question, to a person +who has studied them carefully on the spot, but that the region which +they now inhabit is the actual area of their primaeval appearance on +this earth. Many kindred species may of course have been lost, during +those gigantic subsidences which caused the Madeiras to be shaped out, +and to tell their tale above the waves as ruins of an ancient land; +yet our existing cluster of forms could not have wandered far at that +early period, from the Serras and ridges of their birth,--perhaps not +_so_ far indeed (considering the limited bounds within which they are +now confined, and that time should in reality have increased their +range rather than diminished it) as they have succeeded in doing at +the present day. Hence we may reasonably conclude, that Madeira proper +is an example of what we have alluded to in a preceding page,--namely, +of the accidental retention, during a vast downward movement, of a +nucleus of small specific areas of colonization, the colonizers of +which _had not extended elsewhere_. But I stated, that two of the +above-mentioned _Tarphii_ have occurred beyond the central mass. It is +in Porto Santo that they make their appearance; nevertheless, since +one of them is apparently peculiar to that island, it is only the _T. +Lowei_, Woll. (an insect of a different, and more active, nature than +the rest) which has violated that _local exclusiveness_ which would +seem to be almost a generic character, as it were, of its allies. That +species, however, both in its manners and aspect, recedes materially +from the remainder. Although, like them, nocturnal in its habits, it +is able to run with considerable velocity; and, instead of attaching +itself to the blocks of putrefying wood, which both fall and decay _in +situ_ on those elevated tracts, it hides within the bunches of +_Evernia scopulorum_ and _prunastri_ which clothe the trunks of living +trees, and fill up the crevices of the weather-beaten peaks. Hence, +when contrasted with its comrades, we can easily understand how the +varied processes of accidental transportation would operate to +increase the range of a creature which differs so essentially, in many +respects, from them. It is indeed, not unfrequently, brought down, at +the present day, by _human_ agencies from the mountain-slopes; for, +since the cutting of faggots is one of the few sources of livelihood +to a large proportion of the poor of Funchal, numerous insects of +subcortical and lichen-infesting tendencies are subject to be +naturalized (provided they can adapt themselves to the change) in +altitudes lower than their normal ones: so that there are many +chances, even _a priori_, in favour of the _T. Lowei_ having +overspread, whether by natural or artificial means, a wider area than +its congeners. I believe that there is no such thing as a _Tarphius_ +in the Canarian Group: nevertheless, singularly enough, a +representative, which is more akin to the _T. Lowei_ than to any other +hitherto discovered (and which was imagined until lately to have been +the sole exponent of the genus), namely, the _T. gibbulus_, Germ., +occurs in Sicily. From which data we arrive at this significant fact: +that, whilst Madeira proper is, without doubt, the original centre of +the _Tarphii_, two species (one of which is, likewise, Madeiran) are +found in Porto Santo, to the north-east of it; whilst a third makes +its appearance in an island of the Mediterranean. + +The genus _Acalles_ presents a nucleus of species in the Canaries, +moulded on a very large pattern. A closely allied member, the _A. +Neptunus_, Woll. (which may perhaps be in reality but an insular +modification of the _A. argillosus_, Schoen., from Teneriffe), has been +detected on the rocks of the Salvages, to the north of them; whilst on +the Dezerta Grande, one of the most southern stations of the Madeiran +Group, we have a third, which displays far more in common with the +Canarian type than it does with that which obtains in Madeira +proper;--which last is gradually, in its turn, merged into the +ordinary European form. The genus _Pecteropus_, Woll., is another +instance in point. I possess three or four species from the Grand +Canary, Fuertaventura, and Teneriffe; and I believe it will be found, +on inquiry, to attain its maximum in that cluster. Unlike the others, +however, which we have just cited, it is powerfully winged; and we +should consequently expect to trace the evidences of its northward +progression with comparative perspicuity. Can we therefore do so? Yes: +in Madeira proper it has two representatives, and in Porto Santo (to +the north of it) one. And so with _Xenostrongylus_, Woll. (which is +likewise winged), we have two species, at least, in the Canaries; one +in the Madeiras; and a third, unless I am mistaken, in Sicily. The +genus _Ditylus_ is shadowed forth in the Canary Islands by two or +three singular representatives of a pallid, testaceous hue; and, +although the group is entirely absent in Madeira, a species (the _D. +fulvus_, Woll.) is found on the 'Great Piton' of the Salvages, so +nearly resembling, except in its smaller size, one of those from the +Canaries that I think it far from improbable that it is a fixed +insular state of that insect. _Deucalion_, also, may be quoted in +support of this twofold hypothesis, of the direction, and the +slowness, of the former migratory movements. It is an apterous genus, +and of eminently sluggish habits; and what is the consequence?--we +have a very remarkable species (the _D. oceanicum_, Woll.) on one of +the rocks of the Salvages, whilst another (the _D. Desertarum_, Woll.) +has been isolated on the two southernmost islands of the Madeiran +Group; and of so sedentary a nature is this last, that, although +physically unimpeded, it has not, even to this day, overrun the +diminutive areas on which, when the surrounding region was submerged, +it was originally saved from destruction. So strongly indeed was this +fact impressed upon me, when I first detected it, that I shall perhaps +be excused for recapitulating _in extenso_ the few reflexions which +then suggested themselves to my mind. "There is no genus, perhaps, +throughout all the Madeiran Coleoptera, more truly indigenous than +_Deucalion_. Confined apparently, so far as these islands are +concerned, to the remote and almost inaccessible ridges of the two +southern Dezertas, it would seem to bid defiance to the most +enthusiastic adventurer who would scale those dangerous heights. Its +excessive rarity, moreover, even when the localities are attained, +must ever impart to it a peculiar value in the eyes of a naturalist; +whilst its anomalous structure and sedentary[60] mode of life give it +an additional interest in connexion with that ancient continent, of +which these ocean ruins, on which for so many ages it has been cut +off, are the undoubted witnesses. Approximating in affinity to +_Parmena_ and _Dorcadion_, yet presenting a modification essentially +its own, it becomes doubly important in a geographical point of view; +and it was therefore with the greater pleasure that I lately +received a second representative, from the distant rocks of the +Salvages,--midway between Madeira and the Canaries. Differing widely +in specific minutiae, yet agreeing to an identity in everything +generic, they offer conjointly the strongest presumptive evidence to +the _quondam_ existence of many subsidiary links (long since lost, and +radiating in all probability from some intermediate type) during the +period when the whole of these islands were portions, and perhaps very +elevated ones, of a vast continuous land. * * * * * The _Deucalion +Desertarum_ is of the utmost rarity, the only two[61] specimens which +I have seen having been captured (the first by myself, in 1849; and +the second by the Rev. R. T. Lowe, in 1850) on the respective summits +of the Middle and Southern Dezertas. So local indeed does it seem to +be, that it, apparently, has not extended itself even over the Dezerta +Grande (where there are no external obstacles to bar its progress); +but retains the very position which in all probability constituted its +original centre of dissemination at the remote period of time when +this ancient continent received its allotted forms. Judging from the +slowness with which creatures of such habits must necessarily, under +any circumstances, be diffused, it is at least unlikely that the +present one could have circulated far, when the now submerged portions +of that region began to give way; and hence it is not impossible that +the Southern Dezerta, with the adjacent part (then united to it) of +the Central one, may have embraced the _whole area_ of its actual +primaeval range,--the remains of which (though they be now separated by +a channel) it still continues to occupy, and from which, even when +physically unimpeded, it has never roamed[62]." + +Although it is not my province in this volume to draw inferences from +data which are not strictly entomological, I shall perhaps be pardoned +for adding a few words on the testimony which the Land Mollusca of the +Madeiras would seem to afford, in support of the general slowness of +the animal migrations over that primaeval continent. The researches of +the Rev. R. T. Lowe, and of myself, on every rock and island of the +group, have, it appears, so nearly exhausted the whole number of +species which lately remained to be found, that the conchological +statistics are perhaps, at the present time, more accurate than those +of any other department of the fauna: and, independently of the +modifications which have been manifestly brought about, in some few +instances, by isolation, since the periods of subsidence, it is truly +singular to remark how every detached portion of the entire cluster +harbours real species, which are now peculiarly its own. Thus (to +select an illustration from amongst the most anomalous of the endemic +forms), we have in Madeira proper, Porto Santo, and on the Southern +Dezerta, respectively, true representatives, in the _Helix tiarella_, +_coronata_, and _coronula_,--which in all probability still occupy the +positions (or nearly so) of their original _debut_ upon this earth. +Considering the sluggish, or sedentary, nature of the Terrestrial +Mollusks, it is extremely likely (nay, almost certain) that many +intermediate links, radiating from the same type, were lost for ever, +when the gigantic movements which rent this ancient region were in +course of operation: so that, if such were in reality the case, we +need not be surprised that one at least of this small geographical +nucleus should have been preserved on three of the existing islands of +the group. That these are actual species (saved alive from their +fellows, after the wholesale destructions in this Atlantic province +had been completed), and no results of insular development, is +demonstrated by the fact that two of them (for the third has +apparently become extinct) have not altered one iota since the _fossil +period_, which, in the opinion of Sir Charles Lyell, is anterior to +the dissolution of the intermediate land;--whereas, had they been mere +modifications of each other, induced by the local conditions and +influences to which they have been, through a long series of ages, +severally exposed, the difference between their recent contour and +that of their fossil homologues would have been doubtless at once +conspicuous. I gather, therefore, that like the _Tarphii_, to which we +have lately drawn attention, they are veritable surviving members of +an esoteric assemblage which found its birth-place on this +post-miocene (?) tract. + +In a similar manner, the _H. undata_ in Madeira proper, the _H. +Vulcania_ on the Dezertas, and the _H. Porto-sanctana_ in Porto Santo, +are representative species,--each occupying the same position, and +being equally abundant, on their respective islands: and, although it +may be a problem whether the second of these is not an insular +modification of the first (or _vice versa_); yet, with the analogy of +the three already mentioned before us, I am inclined _a priori_ to +view it as distinct. These, also, occur in a subfossil state; and no +alteration appears to have been brought about, by either circumstances +or time. And so it is with numerous others (as the _H. latens_ in +Madeira, and the _H. obtecta_ in Porto Santo; the _H. squalida_ in +Madeira, and the _H. depauperata_ in Porto Santo; the _H. Delphinula_ +in Madeira, and the _H. tectiformis_ in Porto Santo), which are no +less representative _inter se_. From which we are driven to +conclude;--first, that this _quondam_ continent was densely stocked at +the beginning with foci of radiation created expressly for itself[63]; +and, secondly, that the areas which these various creatures had +overspread, before the land of passage was broken up, was extremely +limited,--or, which amounts to the same thing, that _their migratory +progress was unusually slow_. + +Touching the two-fold question, of the _local engagement_ of this +Atlantic district with specific centres of diffusion, and the _extreme +slowness of their diffusive progress_, much instruction may be derived +from a contemplation of the conchological statistics. Porto Santo, for +instance, is a very small island (not more than seven miles in +length), yet the number of endemic species which it includes is so +perfectly astounding that it may be appropriately termed a _generic +area of radiation_. Nor does this primaeval excess of its aboriginal +beings strike us more forcibly than does the utter quiescence (if I +may so express it) which has been going on amongst them since the +remote era of their birth. Although a few have apparently died out[64] +since that epoch, consequent perhaps on the change of level and +diminished range which took place during the process of subsidence; we +are amazed to find that certain species which are now limited to +particular spots (even whilst unopposed by physical barriers) have +been absolutely peculiar to them from the first,--or, in other words, +that, whilst the fossil deposits extend throughout the lower regions +of the island, far and wide, it is only in those respective portions +of the beds which join on to the present "habitats" that the fossil +homologues of several of the species are to be met with. The _H. +Wollastoni_ is eminently a case in point. That most interesting of the +Madeiran mollusks was first detected by myself on the southern ascent +of the Pico de Conseilho, of Porto Santo, April 22, 1849; and the +subsequent explorations of the Rev. R. T. Lowe, in conjunction with my +own, have, I think, satisfactorily proved that it occurs nowhere else +except upon that single slope. Throughout the large expanse of +calcareous incrustations which are spread over the island elsewhere, +and on the adjoining Ilheo de Baixo, all of which teem with shells, I +think I may assert, without fear of contradiction, that the _H. +Wollastoni_ does not so much as exist. Yet at the Zimbral d'Areia, +which the Pico de Conseilho directly overhangs,--a rich tract for +these fossil remains,--as well as in the muddy composition of a cliff +near at hand, it literally abounds. + +In like manner, we might recall many others which are peculiar, +_recent and fossil_, to the self-same precincts. Such, for example, +are the _H. calculus_ and _commixta_, which swarm on the summit of the +Ilheo de Baixo, in both states. The _H. attrita_, again, is the Pico +d'Anna Ferreira modification of the _H. polymorpha_; and it is only in +the beds towards the base of that mountain that its fossil homologue +is found. But what do these facts indicate? Surely they tell us +plainly of what we have already so often insisted upon,--namely, the +redundancy of this once continuous land with specific foci of its own, +and the sluggish or sedentary nature of those primaeval radiating +forms. + +We must not however omit to notice, that some few of these endemic +_Helices_ appear to have been gifted (as we should _a priori_ +anticipate) with more rapid capabilities for diffusion than the rest. +Thus, the _H. erubescens_ and _paupercula_ seem not only to have +colonized the entire province of which the Madeiras are detached +fragments, but to have even found their way to that distant portion of +it which now constitutes the Azores. The _H. polymorpha_ has also +penetrated the Madeiran region throughout; and being, like the _H. +erubescens_, peculiarly sensitive to the action of external +influences, we perceive, in consequence, that almost every island and +rock has now its own especial phasis of it. So greatly indeed is that +species beneath the control of local circumstances, that the very +districts of an island as insignificant as Porto Santo have each their +separate races to boast of. On the Pico d'Anna Ferreira it assumes a +form to which the name of _H. attrita_ has been applied; when on the +Ilheo de Baixo, it is the _H. papilio_; at the Zimbra d'Areia, on the +Pico de Conseilho, and in the Ribeira da Coxinha, it is the _H. +pulvinata_; and, in many other situations widely removed _inter se_, +it puts on the shape (variable, both in size and hue) to which the +title of _H. discina_ has been given. But, if we leave Porto Santo, +and follow this Protean _Helix_ into the other divisions of the group; +we meet with it on the Dezertas as the _H. senilis_ (those moreover +from the central island having a much more open umbilicus than is the +case in the northern and southern ones), whilst in Madeira proper it +constitutes the _H. lincta_ (with an additional pale variety for the +calcareous district of Canical),--and the _H. saccharata_, from the +Sao Lourenco promontory. + +In the same way we might pursue the _H. erubescens_, and show that in +the sylvan regions, and on the low barren Ponta Sao Lourenco of +Madeira, on the Pico de Facho of Porto Santo, on the Ilheo Chao, on +the Central Dezerta, and on the Bugio (where it attains a gigantic +size), it has its distinct and permanent phases,--the evident results +of isolation, and other topographical influences, since the +subsidence of the intervening tracts. And in like manner, the +_Clausilia deltostoma_ is universal throughout the Madeiran +Archipelago,--displaying, however, in Porto Santo a fixed and strongly +ribbed state, peculiar to that island. Thus, if the examples which we +previously cited tend to establish the extreme slowness of the +migratory movements of the terrestrial mollusca across this former +continent, the present ones (which refer to a few exceptional species +of quicker self-diffusive powers) will show, no less than the +_insects_ to which I have lately called attention, that where +sufficient areas had been overspread (before the periods of +subsidence) for the creatures to have reached what now constitute the +various islands of the cluster, we at once detect traces of this fact, +through their more or less altered aspects,--the result of isolation, +and diminished range, during the enormous interval which has elapsed +since the successive convulsions which caused the partial destruction +of this Atlantic province were brought to a close. + +To return, however, to the insects, after this long conchological +digression,--I need not multiply evidence, in corroboration of my +theory. Enough has been said to render intelligible the idea which I +wished to convey, concerning the _general direction_ of the migratory +current over that ancient tract, and the _extreme slowness of its +progress_,--the former of which I consider probable from the +north-easterly course in which creatures _generically identical_ were, +if we may so express it, "given-off;" whilst the circumstance of their +being for the most part _specifically dissimilar_ (or, in other words, +of the islands harbouring, many of them, species which are endemic) +would seem as it were to establish the latter. + +We must not however forget, that it is only to the _aborigines_ of +this _quondam_ land that the above speculations apply. Assuming the +region not to have been insular, that is to say, to have been +connected, on its outer limits, with a European, or Mediterranean, +continent; it would necessarily follow, that a certain number of +colonists must have found their way over its area, and moreover _in an +opposite direction_ to the living stream (if we may so call it) which +had been long flowing in a north-easterly course across its surface. +Whatever be the length of the periods, however, during which these +counter migrations were going on, I think it sufficient to state that +I would refer them to epochs altogether different,--so that, +accompanied as they may have been by special geological phaenomena, +which, if known, would in all probability become at once explanatory, +we should be the less inclined to regard as absurd what might appear +at first sight difficult to understand. In the case of the British +Isles indeed, no less than five of these distinct migratory eras have +been assumed, and specified[65], by Professor Edward Forbes; therefore +(whatever value be attached to his able and interesting theory) I do +not consider it necessary to apologize for requiring _at least two_ in +behalf of this ancient Atlantic province. Not to insist upon those of +his faunas and floras which are of a less evident, or more +questionable, character, he has at any rate proved, I think, almost to +a demonstration, the _westward progress_ of the great mass of our +British animals and plants, over a then unbroken land (the upheaved +bed of the glacial sea), from the central Germanic plains; whilst the +accurate calculations of the late Mr. Thompson of Belfast, concerning +the reptile statistics of Ireland, England, and Belgium, respectively, +have succeeded in showing, with much presumptive reason, how the +formation of St. George's Channel, _before_ that of the German Ocean, +interrupted the march of these wanderers to the far West, and debarred +an immense proportion of them from an entry into Ireland,--which would +otherwise have colonized that country equally with our own. + +As regards Professor Forbes's views of the creation of a vast +continent (reaching far into the Atlantic[66]) at the close of the +miocene epoch, through the upheaved bed of a shallow miocene sea,--a +region moreover of such an extent as to have connected the various +island groups between the Fucus bank and the shores of the Old World, +not only with each other, but with a Mediterranean province, Asturias, +and even the south-west of Ireland,--I must be content to pass them +by, hazarding only a few crude and desultory remarks. So large a +question, indeed, cannot be safely handled without a corresponding +amount of data, in all departments of natural science, to reason +from,--which I do not possess: still, if a speculation from +entomological premises, _per se_, be not altogether worthless, I would +point to the conclusions (lately adverted to) which my Madeiran +researches have forced upon me, concerning the _direction_ of the +former insect migrations,--inferences which are, from first to last, +of necessity erroneous, if the requisite medium for transit (into +South-European latitudes, at all events) be a mere conjecture or +romance. Such a notion, however, I would not for a moment +entertain,--for there is too much direct evidence in support of +distinct epochs of diffusion, to allow of any hypothesis, when +endeavouring to account for the phaenomena which we now behold, to +supersede the assumption of a once continuous tract. No matter if we +be compelled to suppose, whilst attempting to interpret what we see, +that the disseminating current has flowed in exactly opposite courses, +at different and remote periods, over the surface of that ancient +land,--seeing that the _fact_ (if such in reality it be) remains +untouched, that _the land itself is_ at any rate _there_. I am not, +however, prepared to assert that the opinion at which I had +independently arrived, from the insect statistics, does positively +require a northerly prolongation of that area beyond the line of the +central Mediterranean districts; yet, after making every possible +allowance for accidental introductions since the subsidences have +taken place, there is still left a large residuum which I am convinced +can never be explained (unless the doctrine of specific centres be a +myth) except through the means of ordinary and regular migration over +an unbroken continent. Nevertheless, though I would not presume, from +insufficient material, to insist upon an extension of this Atlantic +region into higher latitudes than those which I have just referred to, +I must express my individual belief that, the more the subject is +examined, with reference to the distribution of the Annulosa, the +less will Professor Forbes's idea suffer from the inquiry. In the +'Insecta Maderensia,' I have already thrown out a few scattered hints +which bear on this immediate consideration; and, since no subsequent +reason has induced me either to withdraw or modify them (but rather +the reverse), I will select the following,--extracted from my preface +to that work. + +"Taking a cursory view of the Coleoptera here described, the fauna may +perhaps be pronounced as having a greater affinity with that of Sicily +than of any other country which has been hitherto properly +investigated. Apart from the large number of our genera (and even +species) which are diffused over more or less of the entire +Mediterranean basin, this is especially evinced in some of the most +characteristic forms,--such as _Apotomus_, _Xenostrongylus_, +_Tarphius_, _Cholovocera_, _Holoparamecus_, _Berginus_, _Litargus_, +_Thorictus_, and _Boromorphus_. There is, moreover, strange though it +may appear to be, some slight (though decided) collective assimilation +with what we observe in the south-western extremity of our own country +and of Ireland,--nearly all the species which are common to Madeira +and the British Isles being found in those particular regions; whilst +one point of coincidence at any rate, and of a very remarkable nature, +has been fully discussed under _Mesites_. Whether or not this partial +parallelism may be employed to further Professor E. Forbes's theory of +the _quondam_ approximation, by means of a continuous land, of the +Kerry and Gallician hills, and of a huge miocene continent extending +beyond the Azores, and including all these Atlantic clusters within +its embrace, I will not venture to suggest: nevertheless, it is +impossible to deny that, so far as the Madeiras betoken, everything +would go to favour this grand and comprehensive idea. Partaking in the +main of a Mediterranean fauna, the _northern tendency_ of which is in +the evident direction of the south-western portions of England and +Ireland, and with a profusion of endemic modifications of its own +(bearing witness to the engorgement of this ancient tract with centres +of radiation created expressly for itself), whilst geology proclaims +the fact that _subsidences_ on a stupendous scale have taken place, by +which means the ocean's groups were constituted; we seem to trace out +on every side records of the past, and to catch the glimpses, as it +were, of a _veritable_ Atlantis from beneath the waves of time[67]." + +The _Mesites Maderensis_, Woll., to which I alluded in the above +quotation, is undoubtedly a strong case in point. Although +specifically dissimilar from the _M. Tardii_, its Irish counterpart, +it nevertheless approaches it so closely, that it might be literally +mistaken, _prima facie_, for that insect; and we know that it is one +of the plans on which Nature commonly proceeds, that species which are +not merely representative of (or analogous to) each other, but which +are actual homologues, or allies, should usually emanate at first from +foci not far removed _inter se_; or, at all events, if distant, +connected by an intervening land:--in other words, that _generic +areas_, no less than specific centres, of radiation, form a +substantial item of the comprehensive scheme on which the system of +created things was originally planned. We detect traces of this +primary law in each division, or class, of the organic world; nor is +its reality _as a law_ interfered with, through the occasional +exceptions which are liable, as in every other instance, to present +themselves. Such deviations are often easily to be accounted for, +whether by natural or artificial means; and do not affect the +subject, as a whole. Sometimes indeed they become at once intelligible +from the historical records connected with them, proving that human +agencies have been at work acting as transporting media, within a +period comparatively recent; whilst at others, the fact of the +creature having been endowed with self-diffusive powers to an +extravagant degree may succeed equally in rendering the phaenomena +explicable. But, even where neither of these solutions would seem to +suffice, we should still recollect that it is only in the mass that +such questions can be pronounced upon; and that, consequently, where +we are able to discover a rule which is _for the most part_ adhered +to, it is more philosophical to conclude that the departures from it +are the result of special disturbing causes (whatsoever they may have +been), than to permit them to undermine our faith in what would be +otherwise universally true. Thus, the botanist tells us of Ixias, +Stapelias, Mesembrianthemums, Pelargoniums, and Euphorbias, as +concentrated in Southern Africa; of Magnolias in Central America; of +Calceolarias on the Andes; of Myrtles, Banksias, Mimosas, and +_Eucalypti_, in Australia; and of the Bread-fruit Trees in the South +Sea Islands: the ornithologist points, _inter alia_, to the Toucans +and Humming-Birds from South America and the West Indies; whilst the +student of the higher animals informs us of the Kangaroos (indeed of +the whole of the subclass _Marsupialia_, except the genus _Didelphys_) +as peculiar to Australia and a few islands to the north of it; of +_Lemur_ _proper_ to Madagascar; of the Sloths, Armadillos, Tree +Porcupines, and of Alligators, and of the _Platyrrhini_ (amongst the +Monkeys), to South America; and of the Ourangs to the islands of the +Indian Archipelago. + +And so it is with the Insecta; many of the larger groups of which (as +_Amycterus_ and _Paropsis_, in Australia; _Pachyrhynchus_ and +_Apocyrtus_, in the Philippine Islands; _Hipporhinus_, _Monochelus_, +_Dichelus_, and _Moluris_, in Southern Africa; _Macronota_, in Java; +and _Naupactus_, _Hypsonotus_, _Centrinus_, _Platyomus_, and +_Cyrtonota_, in South America) are confined to countries of +proportionate magnitude, whilst the smaller ones are more commonly (as +it were) shaped out for special provinces or regions, according as +local circumstances may require primary adaptations to harmonize with +them. Thus, whilst we frequently find an extensive genus diffused over +the greater portion of the known world, we perceive that even its +_structural_ characteristics are not uniform throughout, but afford +fixed geographical modifications (_not_, in this case, however, the +effect of development),--which have often, in their turn, obtained the +name of 'genera,' and have been described as such. Whether genera, +however, or not, they are undeniably small topographical assemblages, +satellites around their central types; and they may therefore be +safely regarded as genera, if we choose to view them in that light. Of +such a nature I have already pointed out[68] is _Saprinus_, as +compared with _Hister_; _Atlantis_ with _Laparocerus_; and _Oxyomus_ +with _Aphodius_; and, I might also add, _Mesites_ with _Cossonus_. I +believe indeed that _Mesites_ will be found to attain its maximum on +the Pyrenees (I already possess two or three species, in abundance, +from that region); and, if such should be the case, we shall be able +to appreciate the significance of two representatives so closely +allied as the _M. Tardii_ and _Maderensis_,--one of which has been +given off in the direction of Ireland, and the other of the Madeiran +Archipelago. + +But I will not digress further on the subject of this Atlantic +province; since, however much I may individually regard it as a +reality of the past (which the Coleopterous statistics have compelled +me to do), it must of necessity remain, as heretofore, a matter of +much controversy and doubt. I should indeed apologize for having +trespassed on the reader's attention, in wandering this far from the +immediate results of _subsidences_,--which I proposed, at the outset +of this chapter, to examine, with reference to the impeded diffusion +of the Annulose races. Nevertheless, concluding that a practical +illustration of the effects of one of those great downward movements +to which geology so repeatedly bears witness would not be irrelevant +to the _assumed consequences_ which I had previously ventured to +define, I have acted on that judgment; and, having finished my task, +will now proceed to notice, briefly, a few other considerations which +should not be omitted, when inquiring into insect distribution as +influenced by geological phaenomena. + +Next in importance, perhaps, to the elevations and sinkings (traces of +one or the other of which are more or less manifest in almost every +region of the world), _natural barriers_ may be cited,--as presenting, +not unfrequently, insurmountable obstacles to the self-dissemination +of the insect tribes. By natural barriers, however, I would be +understood to imply natural _primary_ barriers,--or, in other words, +such as have continued as barriers ever since the present animals and +plants came into existence upon the earth. For, the _ocean_ (by way of +illustration) is a natural barrier; and yet it is not necessarily a +primary one, as may be readily gathered from the above remarks, in +which the results of _subsidences_ are discussed,--subsidences which +have had the effect of letting it in over portions of an _already +tenanted_, and unbroken, continent. Mountain-chains, also, are +barriers; but it may happen that they have not been so from the +beginning,--as in instances, for example, where they have been +gradually upraised during periods geologically recent. But both sea +and alpine ranges are barriers, when (as usually happens) they have +remained as such since the creation of the several species which now +inhabit our globe. Mr. Darwin has acknowledged this distinction, +whilst commenting upon the marked divergence of the faunas on the +eastern and western slopes of the Cordillera. "This fact," says he, +"is in perfect accordance with the geological history of the Andes; +for these mountains have existed as a great barrier since the present +races of animals have appeared; and therefore, unless we suppose the +same species to have been created in two different places, we ought +not to expect any closer similarity between the organic beings on the +opposite sides of the Andes, than on the opposite shores of the ocean. +In both cases, we must leave out of the question those kinds which +have been able to cross the barrier,--whether of solid rock or +salt-water[69]." + +Conceding, therefore, this distinction between barriers of a primaeval +and more recent character, it is not difficult to understand why the +opposite sides of an alpine chain, as well as countries separated by +the sea, should display different phaenomena from each other. On the +contrary indeed, if we could feel satisfied that no means of +accidental transportation had operated to take them there, and that +the animals themselves were incapable of enduring great diversities of +temperature, and other contingencies; we should be startled to +discover creatures specifically identical in such regions,--so long at +least as the doctrine of unique centres of radiation formed part of +our zoological creed. We must not, however, be too hasty in +questioning (if I may be pardoned for the completion of a metaphor of +which I thoroughly disapprove) this article of our faith, through the +occurrence of similar beings in areas between which there exist +barriers, both primary and well-defined; for the methods of diffusion +are so complicated and numerous, that, even where human agency (that +most important of elements) is not concerned, what at first sight may +frequently appear to be impossible becomes clear enough when more +critically inquired into. Some species, we know, are gifted with +greater powers for horizontal and vertical progression than their +comrades, and can (though they are doubtless exceptions to the general +rule) pass through extremes of atmosphere sufficient to render even +lofty mountain summits no obstacles to _them_. Others, as the +_Calosoma Syncophanta_ of Europe, have been stated to traverse the +ocean unhurt[70]; and I believe that many do at times accidentally +arrive, in a half-drowned state, especially after boisterous weather, +across channels of considerable breadth. Mr. Kirby, on examining the +marine _rejectamenta_, during one of these apparent occurrences, along +the Suffolk coast, writes as follows: "Whether the insects I observed +upon the beach, wetted by the waves, had flown from our own shores, +and, falling into the water, had been brought back by the tide; or +whether they had succeeded in the attempt to pass from the continent +to us, by flying as far as they could, and then falling had been +brought by the waves, cannot certainly be ascertained; but Kalm's +observation inclines me to the latter opinion[71]." And Sir Charles +Lyell remarks:--"Exotic beetles are sometimes thrown on our shore, +which revive after being drenched in salt water[72]." Nor should we +forget that chance agencies of every description, which we are too apt +to overlook, are daily at work (and have been so since, at any rate, +the last creative epoch) to transport these variously organized beings +beyond their original spheres. Sometimes they are carried on, or +within, the bodies of larger animals, which is especially the case +with the parasitic tribes; at others on floating trunks of trees, and +casual substances of divers kinds, which are able to resist for a +definite period the destructive action of an element saturated with +salt. Unwilling victims, again, are ever and anon hurried to +comparatively distant lands by the very winds that blow; and not only +to distant lands, but over altitudes in which the severity of the cold +would quickly annihilate them, were they (as perhaps usually happens) +to be deposited there on their headlong and compulsory course. "As +almost all insects are winged[73]," says Sir Charles Lyell, "they can +readily spread themselves wherever their progress is not opposed by +uncongenial climates, or by seas, mountains, and other physical +impediments; and _these_ barriers they can sometimes surmount by +abandoning themselves to violent gales, which may in a few hours carry +them to very considerable distances. On the Andes some sphinxes and +flies have been observed by Humboldt, at the height of 19,180 feet +above the sea, and which appeared to him to have been involuntarily +carried into those regions by ascending currents of air[74]." With +respect to the accidental conveyance of numerous species across the +sea, it is not to the winds alone that we must look for an +explanation. Large and rapid rivers are liable to inundate their banks +and bring down insects in prodigious masses,--which are disgorged into +the ocean, and carried to a distance from the coast, in proportion to +the violence of the ejecting stream. When the body of water is +considerable, the sea becomes diluted to an unusual extent; and +creatures which must have otherwise perished, from the action of the +salt, are able to survive for a time, and may be deposited, by means +of rapid currents into which they are borne, on neighbouring islands +and continents. Even the _Hydradephaga_ are thus occasionally +transported; for Darwin mentions having captured a _Colymbetes_ off +Cape S^{ta} Maria (to the north of the Rio de la Plata), when +forty-five miles from the shore. And, in his 'Journal of Researches,' +he records the following remarkable facts, which bear upon this +immediate question. "On another occasion, when seventeen miles off +Cape Corrientes, I had a net overboard to catch pelagic animals. Upon +drawing it up, to my surprise I found a considerable number of beetles +in it, and, although in the open sea, they did not appear much injured +by the salt water. I lost some of the specimens; but those which I +preserved belonged to the genera _Colymbetes_, _Hydroporus_, +_Hydrobius_, _Notaphus_, _Cynucus_, _Adimonia_, and _Scarabaeus_. At +first I thought that these insects had been blown from the shore; but +upon reflecting that, out of the eight species, four were aquatic (and +two partly so) in their habits, it appeared to me most probable that +they were floated into the sea by a small stream which drains a lake +near Cape Corrientes. On any supposition, it is an interesting +circumstance to find live insects swimming in the open ocean seventeen +miles from the nearest point of land[75]." + +Accidental means of dissemination, such as those to which I have just +alluded, and others to which we might appeal, will generally account, +and with much presumptive truth, for the many exceptional cases which +present themselves, during our investigation into the effects of +natural barriers, as visible in the distribution of the Annulose +races, on the earth's surface. I say "exceptional cases," because any +one who has laboured practically in mountain tracts cannot have failed +to recognize the marked difference which is often displayed by the +insect population on opposite sides of some alpine chain; whilst he +whose lot has been cast amidst island groups, will have become even +more conscious than the former of the permanency of those impediments +which have been placed (in this instance by the broad arms of the +mighty ocean) as checks upon a too rapid system of diffusion. + +But if the sea and mountain ranges, when of a sufficient age _in +situ_, are amongst the most effectual of Nature's barriers against the +self-dispersion of the animate tribes; it follows that, if the two +could be (as it were) _united_, we should have found the greatest +obstacle which physical conditions can ordinarily present against the +wandering capabilities of the latter. The question therefore +arises,--Is it possible for them to _be_ so joined? Undoubtedly it is: +and hence we arrive at the conclusion, that a _mountain island_ should +afford us the _minimum of size, as regards the areas its species have +overspread_, which any country is able to furnish. + +Madeira is a mountain island,--its highest peaks rising, although +resting on so small a base, to an altitude of more than 6000 feet. Yet +it is only partially a case in point; for, although it was a mountain +mass, and perhaps a very elevated one, when its endemic beings made +their first appearance upon its surface, we have already intimated +that it has become isolated _since_ that epoch: so that, whilst _one_ +of the natural barriers against dispersion which it involves (namely, +mountain ridges) may be considered as primary; the _other_ (to wit, +the sea, as it now obtains) has played, as an agent of obstruction, +but a secondary part. Still, there is good reason to believe that the +ancient tract of which it is a portion was broken up at a +comparatively early date after the creation of those peculiar organic +forms which found their birthplace within its bounds; and that, +consequently, the latter could not have wandered far (if we except +those species on which unusual powers of diffusion were bestowed) when +the land of passage began to give way. Hence, even the sea, in this +particular instance, partakes almost of the character (no less than +the mountain heights) of an original impediment; and Madeira therefore +may be safely quoted as an example in which two barriers, of a primary +nature, are united; and where, consequently, we may anticipate those +ultra phaenomena of _areal limitation_ upon which we have been just +commenting. + +But let us now inquire, whether the hypothesis at which we have +arrived will stand the test of experience; for unless it will do so, +we might have been spared the labour of propounding it. Madeira is a +country composed of narrow mountain ridges, which radiate from central +crests, and form the lateral boundaries of deep and precipitous +ravines. Modifications of this structural type are of course traceable +everywhere; the upland tracts are often undulating and broad, and the +buttresses which slope towards the sea are sometimes expansive and +irregular: yet upon the whole the above description is correct, and we +may accept it in a generic sense. Now we may premise that, even to +this day, it is an island of floods; therefore, how much more must it +have been so when its primaeval forests, in all their splendour, caused +an amount of exhalation and moisture of which at present we can have +but a remote conception! Hence, it is hardly to be imagined, that +(however limited may have been the naturally acquired areas of those +of its inmates which are most sluggish and sedentary) a fusion would +not have taken place, in the course of ages, so as to render its +modern fauna, in a large measure, homogeneous throughout. Yet, in +spite of this esoteric tendency, it is surprising how little +amalgamation has been effected amongst the tenants of its several +districts. Scarcely a gorge or woodland serra exists within its bounds +which does not harbour some species essentially its own; and in many +instances the ranges of these creatures are so local or confined, that +they might be easily overlooked even in their respective +neighbourhoods. It is certain, however, that the floods (which happen +periodically) have done considerable work in naturalizing many of the +subalpine forms, which could adapt themselves to the climatal change, +in altitudes below their normal ones: and, in the north of the island, +where the temperature is cooler than on the opposite side, and where +the lofty defiles terminate, even at their lowest outlets, in abrupt +precipices along the coast, so that the _rejectamenta_ during the +annual rains are brought into direct contact with the shore, this +gradual process of deportation is particularly evident,--a +circumstance to which I have already alluded elsewhere[76]. + +But, after making due allowance for these powerful means of +dissemination (which, in the common order of things, must necessarily +obtain in _mountain islands_, as it were, _par excellence_), the fact +still remains, that in the Madeiran Group the acquired areas, even up +to the present date, of a vast proportion of the insect inhabitants, +are wonderfully circumscribed. The real state of the case, however, +would appear to be simply this: that the floods, although they may +have tended to diffuse the members of a comparatively uniform alpine +fauna in the various clefts or gorges beneath, can have had no power +to combine the aborigines of the several gorges themselves; and, since +a large proportion of the endemic species of those islands are (as I +have previously stated) apterous, the perpendicular edges of the +ravines, which in many instances rise to an elevation of 2000 feet, +have acted (and ever _will_ act) as impassable barriers to vast +numbers of the insect tribes. + +With this single example (by way of illustration), which the Madeiras +have supplied, I will take my leave of the question of _natural +barriers, as tending to regulate the topographical diffusion of the +Annulosa_,--feeling that I have already devoted too much time and +space to this portion of the subject (if such indeed it be) which I +had proposed in the present treatise to discuss. Other barriers might +have been adverted to,--such as large rivers, extensive deserts, and +thickly set forests (especially of pine-trees, which frequently offer +a very decided impediment to insect progress),--but they are of +secondary importance, when compared with marine and alpine ones; and +their consequences may be, to a certain extent, deduced from the +considerations which I have just entered into. My main object has been +to draw attention to the fact, that the great obstacles which Nature +has placed against the too rapid dispersion of animal life should be +more strictly taken into account (as a matter of positive reality) +than it is, during our investigations into entomological geography. To +be aware that these barriers exist, and yet to feel surprised, +especially in a country where the species are principally wingless, +that we do not discover indications of a general uniformity in its +fauna, involves an absurdity,--unless the doctrine of specific centres +of creation be a mere coinage of the brain. But, if we believe in that +theory (which, until it can be shown to be impossible, I hold that we +are _a priori_ bound to do), we must at least act consistently with +ourselves, and not anticipate phaenomena where we have neither reason +nor right to look for them. + +We are too apt to draw a line of imaginary demarcation between the +sciences, as though each had its own propositions to establish, and +nothing more: indeed, some of us would appear to assume (though +perhaps tacitly), that what is proved to be true in one department may +be, at least, rendered inconsistent (if not actually negatived) in +another. But surely this requires no argument to refute,--since a +_principle_ which is _true_, is true under every circumstance and +condition; for otherwise, it could be both true and false. We need not +therefore be afraid of comparing truth with truth, under whatever +shape it may arrive, as though it were possible that either of its +phases could ever suffer from the ordeal of a close contact; since, if +they be really true, and free from deception, they must needs go hand +in hand, and _may_ become (however opposite they be in their subjects) +directly explanatory of each other. The astronomer who is not +intimately acquainted with pure mathematical analysis, in its various +aspects and bearings, is in fact no astronomer at all. The geologist +who would interpret the grand phaenomena of the earth's crust apart +from statical and dynamical knowledge, and without the help which the +chemist, mineralogist, anatomist, zoologist, and botanist can afford +him, stands a fair chance of leaving his problems unsolved; whilst the +students of zoology and botany who would endeavour to understand, and +account for, what they see in the animal and vegetable worlds around +them, without calling in geology to their aid, must assuredly be +prepared to fail signally in their attempts. All indeed must work in +concert, if the whole is to be advanced,--and not only in concert, but +as mutually assisting each other. "By the help of truths already +known, more may be discovered; for those inferences which arise from +the application of general truths to the particular things and cases +contained under them, must be just.[77]" + +FOOTNOTES: + +[60] "When we consider indeed the apterous nature of _Deucalion_, its +subconnate elytra, and its attachment (at any rate in the larva state) +to the interior of the stems of particular, local plants, or its +retiring propensities within the crevices of rocks; we are at once +struck with the conviction, that, during the enormous interval of time +which has elapsed since the mighty convulsions which rent asunder +these regions terminated, it has probably never removed many yards +from the weather-beaten ledges which it now inhabits." + +[61] Since the above was published, I have succeeded in detecting one +more example,--namely (in June 1855) on the summit of the Ilheo Bugio, +or Southern Dezerta, within a few yards of the self-same spot where it +was found by the Rev. R. T. Lowe in May 1850. Although I searched +diligently on the Dezerta Grande, during my late campaign in the +Madeira Islands, I was not able (so great is its rarity) to discover +farther traces of it on that rock. + +[62] Insecta Maderensia, p. 435. + +[63] It would seem, when viewed on a broad scale, as if particular +districts throughout the world had been made as it were the special +fields for the exercise of the creative force,--or that, _generic +areas of radiation_ were part of the elementary design. Thus, +Professor E. Forbes records his belief that most, if not indeed _all_, +of the terrestrial animals and plants now inhabiting Britain are +members of specific centres beyond bounds,--they having migrated to it +over a continuous land, before, during, or after the glacial epoch. +Hence, since the greater number of them are supposed to have come from +the central Germanic plains, we may assume that those plains were one +of the primary areas of diffusion for a large mass of created beings. +There is good cause for suspecting that the Pyrenean region may have +been another; and certainly all evidence would tend to prove that this +vast Atlantic province was, also, well stocked with aboriginal forms. + +[64] Assuming the _Helix Lowei_ and _Bowdichiana_ to be gigantic +phases of the _H. Portosanctana_ and _punctulata_, respectively; four +only, namely _H. fluctuosa_ and _lapicida_, _Achatina Eulina_, and +_Cyclostoma lucidum_ (the first three of which are extinct throughout +the entire group), seem to have altogether disappeared. Nevertheless, +the gradual dying-out, as it were, of species, both here and in +Madeira proper, is singularly evident. Thus, in the latter, the +Canical beds show the _H. tiarella_ to have been once most abundant +(it literally teems in those calcareous formations). Yet so rare is it +in a recent state, that, until the summer of 1855, when it was +detected by myself and the Rev. R. T. Lowe in two remote spots along +the perpendicular cliffs of the northern coast, it was supposed to +have been lost for ages. And the same may be said of its counterpart, +the _H. coronata_, in Porto Santo,--which, likewise, swarms in every +fossil-bed of that island; but which was, also, until I met with it, +on the 15th of December 1848, adhering to slabs of stone at a +considerable depth beneath the ground, on the extreme eastern peak +(opposite to the Ilheo de Cima), imagined to have long passed away. +And so, reasoning from analogy, I think it far from improbable that +the third representative of this little geographical assemblage,--the +_H. coronula_ of the Bugio (which has hitherto only occurred in the +mud deposits on the summit of that rock),--may be still alive, though +perhaps in very small numbers, on some of the inaccessible ridges of +those dangerous heights. + +[65] Origin of the Fauna and Flora of the British Isles (in Mem. of +the Geol. Survey of Great Britain, vol. i. p. 336, A.D. 1846). + +[66] "My own belief," says Professor Forbes, "is, that the great belt +of gulf-weed, ranging between the 15th and 45th degrees of north +latitude, and constant in its place, marks the position of the +coast-line of that ancient land." + +[67] Although, for want of a better name, it may be admissible, when +speaking either figuratively or poetically, to allude to this former +region (as I have done in the above quotation) under the title of +"Atlantis;" yet it seems incredible that certain writers (assuming its +_quondam_ existence) should have recently referred to it seriously as +the possible "Atlantis _of the ancients_!" Considering that there is +good reason to believe that all these islands _were islands in a +miocene sea_, and that, if (through a general elevation) they were +subsequently connected, the land of passage was broken up long +anterior to the appearance of man upon the earth, "the ancients" must +have assuredly merited their appellation, if they could have thrown +any light on a problem which belongs to an epoch thus remote. Whether +the "Atlantis" had any being at all except in the imagination of the +Latin poets, or whether (as Lord Bacon has suggested) it was the New +World, will probably never now be known; yet the fact that the _Insulae +Fortunatae_ of Juba are almost universally identified with the present +Canarian Group (as indeed the accurate description of Pliny well nigh +demonstrates), and the _Purpurariae_ with the Madeiras, ought at once, +apart from geological evidence, to point out the absurdity of the +hypothesis, that an Atlantic continent, _in the very position which +those islands occupy_, could have been acknowledged to have any +existence by the literature of either Rome or Greece. + +[68] Insecta Maderensia, p. 214. + +[69] Journal of Researches, pp. 326, 327. + +[70] Many of the _Calosomata_ would appear to possess this power of +crossing, either by flight or by abandoning themselves to the waves +(though more probably by the assistance of both), even marine barriers +with impunity. Numerous instances are on record to this effect; and I +am informed by Mr. Darwin that a _Calosoma_ flew on board the +'Beagle,' off the Bay of San Blas, in South America, whilst they were +ten miles from shore. It seems likely, therefore, that the occasional +occurrence of the _C. Syncophanta_ in our own country, along the +southern and eastern coasts, is due to this generic capability,--and +consequently (as indeed it is usually acknowledged to be), the result +of accident. + +[71] Introduction to Entomology, ii. p. 13. + +[72] Principles of Geology, 9th ed. p. 657. + +[73] Although this is true on a broad scale, a reference to my +observations in a preceding chapter will show, that in some countries, +especially islands, the reverse will frequently be found to obtain. + +[74] Principles of Geology, p. 656. + +[75] Journal of Researches, p. 159. + +[76] Insecta Maderensia, p. 81. + +[77] Religion of Nature Delineated, pp. 73, 74. + + + + +CHAPTER VI. + +THE GENERIC THEORY. + + +How glorious to the observant eye is the great system of the organic +world, how perfect in each separate part, how complete and harmonious +the whole! The unity of the comprehensive plan, amidst the infinite +modifications which it includes, has ever been a theme of admiration +and delight; for the mind, which has once caught a glimpse, even in +physics, of what it is not possible to disprove, instinctively clings +to it, as to a grand material truth. The discovery, at all times, of +what we feel to be actually _certain_ is in itself so fascinating, +that the very data which it gives us are scarcely more prized than the +mere knowledge that we have gained a single additional light to guide +us on our forward way: for, since in the inductive sciences we can but +climb from step to step, at a slow and even pace, we hail with inward +satisfaction whatsoever may tend to lighten our task, and to lead us +more quickly onwards (gradually though we must of necessity advance) +towards its final accomplishment. + +But how, it may be asked, is this general harmony of the organic +creation to be insisted upon, when beings so extravagant and +dissimilar are everywhere to be met with? Is it possible to recognize +anything like a unity of type amongst creatures so differently +constructed, and so widely removed from each other in their habits, +aspects, functions, and attributes? Such questions as these, however, +though they may occasionally perplex the tyro, or amateur, are not +likely to be raised by anyone who has mastered the merest alphabet of +zoology,--and who is aware that the integrity of Nature is something +real and positive, as experience indeed is ever tending more and more +to corroborate, and by no means the day-dream of an enthusiastic, or +fertile, imagination. To trace out the progressive development of +animal life, from its humblest phases; and to mark, as they become +visible in the intermediate grades, the first rudiments of organs and +instincts which are destined to attain their maximum in the higher +ones, embody but a small portion of what it is the naturalist's +mission to investigate. To him belongs the special privilege of +inquiring dogmatically into this structural advancement; and of +suggesting methods of classification which shall accord, in their +several component divisions, so far at least as is practicable, with +the constitutional change. We should recollect, however, that this +system, being based upon truth, must, if it would be consonant +throughout, adapt itself to all the various phaenomena (in their +respective positions, in the scale), from the consideration of which +it should be exclusively deduced, or built. To draw broad conclusions +of any kind, or to attempt the establishment of propositions and +principles, from simple dialectics, without a previous training in +the practical bearings of the subject, would be absurd, and almost +certain to beget error. "It cannot be that axioms established by means +of _reasoning_ [alone] should be of any value for the discovery of new +results; because the subtilty of Nature far exceeds the subtilty of +reasoning. But axioms duly and orderly abstracted from _particulars_, +in their turn easily point out and mark off new particulars; and so +render the sciences active[78]." Such were the words of the greatest +philosopher which this country has ever produced; and it would be +well, whilst examining the causes of what we see, and endeavouring to +obtain some faint and distant notion of the vast scheme of Nature as +originally designed, to keep them constantly in view,--lest, by +trusting to theory only, apart from observation and facts; or by +venturing to pervert the latter (instead of being led by them), so as +to tally with our preconceived ideas of what ought to be, we miss our +road, and become lost in the mazy labyrinth of our own fanciful +inventions. + +With this preliminary stricture on the express duty which devolves +upon the naturalist (with whom the phaenomena of the organic world +principally rest, for interpretation) to make facts, rather than +reason and argument, the basis of his various doctrines,--at any rate +of those in which the critical subject of _arrangement_ is concerned; +I shall perhaps be pardoned, after having been drawn, in the preceding +chapters (however involuntarily), into the question of 'species,' as +rigidly defined, if I now offer a few passing remarks on the theory of +_genera_. + +There can be no doubt that amongst a large class of ordinary observers +a clear perception of the generic system, in an abstract sense, does +not by any means prevail. What the nature of a genus really is, would +appear to have been very commonly overlooked, or perhaps +misunderstood, by people of this stamp; and the consequence has been, +that the wildest notions have frequently arisen, even from men of +sound _specific_ attainments, as to the claims (for annihilation or +retention, as 'genera') of certain subsidiary zoological assemblages. +The terms 'genus' and 'species' have been conjointly so long +associated in our minds with the selfsame things (whatsoever they may +be), that they have become almost part and parcel of the objects +themselves; so that the student who does not sufficiently reflect on +their true signification, is apt to regard them as of equal +importance,--or, rather, more often perhaps than otherwise, to make +the latter subservient (or inferior) to the former! This however is, +in reality, the very reverse of what should be the case, as a moment's +consideration will indeed at once convince us: for what are genera, +after all, but _dilatations_ (as it were) along a chain _which is +itself composed of separate_, though differently shaped, _links_? The +links (or the actual, independent bodies which constitute the chain) +are the species; but the knobs, or swellings, which their several +forms may tend, _by degrees_, to establish along its course (through +the slight disparity which each of them presents from that which is +next in succession to it; and therefore through the gradual manner in +which the bulbs, or nodules, may be said, _on the whole_, to be +produced), are the groups into which those species naturally fall. It +matters not a straw whether these assemblages be primary, secondary, +tertiary, &c.,--in other words, whether they be departments, families, +or genera, as usually understood,--the _principle_ is in every +instance the same; the difference being merely relative, and not +absolute. + +Or, if we choose to vary the simile, we may compare the whole system +to a cord, upon which beads, of innumerable sizes, patterns, and +colours, have been densely strung. Now, if there were no such things +as natural divisions in the organic world, these beads (which +represent the separate species) might have been disposed of +anyhow,--their positions, with respect to each other, would under +those circumstances have been of no importance. But such is not the +case: there is an order and method throughout Nature, which shows that +every individual portion of it has been adjusted by the Master's hand, +and that nothing has been left to chance. Those beads (to follow up +the metaphor) of countless magnitudes and hues, have had their proper +places allotted to them,--and moreover with such care and regularity, +that a complete plan, or scheme, of distribution is at once +conspicuous. Although there are not even two, amongst that enormous +multitude, which are _precisely_ alike (for every species, however it +may resemble its next ally, has _some_ distinctive feature of its +own), we immediately perceive that those beads which have most in +common, are, as it were, attracted to each other,--so as, by their +close approximation, or contact, to create excrescences and stripes, +of divers kinds, along the entire length of the cord. If we assume now +that the red beads have been collected together, to the length (for +instance) of a yard, and that within that space a dozen protuberances, +of discordant aspects and dimensions, have (by the union of those +beads which more nearly simulate each other) been brought about; we +shall have a very fair idea of the ordinary grouping of the animate +tribes. The red beads, taken in the mass, may be likened to a perfect +"family;" the differing gibbosities to twelve well-marked "genera," +which that family includes; whilst the "species" (the real _dramatis +personae_, of independent existence, which are nevertheless compelled +to occupy the situations we have described,--thus _causing_ the +divisions to be mapped out) are here typified, as everywhere, by the +several beads themselves. + +I have not thought it necessary to pursue this reasoning into higher +divisions than "families;" but of course it may be extended to any +amount,--so as to shadow forth, equally, the compartments of +_primary_ significance. Nor would I wish to imply, by the above +similes, that I regard a _lineal_ method of arrangement as the correct +one. Every zoologist is aware, that in Nature such does not exist: but +the mode of illustration which I have selected is applicable to all +systems alike, so far as the _principle_ is concerned. + +It will consequently be seen, from what has been said, that the terms +"genus" and "species" not only differ very considerably in +_importance_, but in signification also. Whilst the former is merely +suggestive of a particular _position_ which a creature occupies in a +systematic scale (a position, however, which depends upon the various +structural peculiarities which it possesses _in common with other +beings_,--which thus more or less resemble it); the latter expresses +the actual creature itself: so that while one applies to _several_ +animals (of distinct natures and origins, though bound together by a +certain bond of imitation), the other belongs to _a single race +alone_, which it therefore exclusively indicates. But if such be the +case, it will perhaps be asked,--Why then insist upon a generic name +at all, if the specific one be sufficient to denote all that is +required, namely, the _animal itself_? To which, however, we may +reply, that the binomial nomenclature is demanded for two elementary +reasons,--first, because it is founded upon a natural truth, which (to +say the least) it would be unwise to violate; and, secondly, because +it is _convenient_, both for simplification and analysis. We should +assuredly be surprised were a man to object to his surname, as +unnecessary, because he has a christian (or specific[79]) one which is +the exponent of him _alone_. True it is that his family (or generic) +title applies to the rest of his kin also; but, since there are other +people (of other families) who may have the same _individual_ +appellation as himself, it is clearly desirable, even as a matter of +expediency alone, that patronymic and christian name should be alike +retained. We need not, however, plead expediency, in favour of this +acceptance of what has been so long tested, and shown to be correct; +we appeal to a higher tribunal,--that of experience,--in proof that it +draws its origin from Nature itself, and is implied by the very +existence, or reality, of _natural groups_. The 'Methode Mononomique' +has indeed been attempted[80]; and it has failed,--or at any rate it +has shown itself to be inferior, both ideally and in practice, to the +plan commonly in use: and if I might be pardoned a passing conjecture +on its ultimate success, I should be inclined, since it is contrary to +the canon of the organic world, to regard its case as utterly +hopeless. + +Let us not be unfair, however, towards those who have sought to +establish a nomenclature which they conceived would be less open to +objections than that which we have been hitherto accustomed to +endorse. The notion did, at any rate, arise out of an apparent defect +in the binomial process,--for the inconveniences which they complained +of are real ones; and, having felt them practically, they aspired to +sweep them away by remodelling the whole system afresh. But, had it +not been for an evident misconception of the generic theory, in the +abstract, the trial would in all probability have never been made; and +we should have been spared the downfall of a contrivance which has had +but little to recommend it beyond the ingenuity of its machinery and +detail. If we analyse the motives for this experiment, we shall find +that it originated from a belief, that genera are _either_ purely +imaginary, or else that they must (like species) have a definite and +isolated existence. Now both of these conclusions appear to be equally +gratuitous and untenable; and such as a lack of observation could +alone beget. Genera are _not_ mere phantoms of the brain (as most +naturalists will readily admit); but they are, likewise, by no means +abrupt, or well-marked, on their outer limits (except indeed by +accident,--of which hereafter), but merge into each other by +gradations, more or less slow and perceptible. Such being the case, we +can easily understand why it is that the followers of the 'Methode +Mononomique' (who, paralysed by the fact that genera are seldom +_clearly defined at their extremes_, would seem to repudiate them _in +toto_) have rashly regarded the binomial system as intolerable. +Finding that it was possible for numerous species, whose structural +characteristics were less conspicuously pronounced than those of +their allies, to be enumerated, and with equal plausibility, under two +consecutive groups; they immediately inferred that the groups +themselves could not be upheld on account of these connective links: +and so it was resolved (through a new and artificial scheme) to ignore +them; and to fall back upon the creed, that species alone (and not +genera) are to be recognized in the organic world. This was but the +device, however, at the outset, of a single mind; and the perverts to +it have been but few. It is in direct opposition to the first +principles of nomenclature, and sets at defiance a great natural +truth. + +But what, it may be inquired, is this great primary truth which the +monomial system tends to violate? I repeat what I have already stated, +that it is the _existence of natural assemblages_ which that scheme +would, if it were practicable, discountenance. Order and symmetry, +however (which involve classification, or arrangement), are the law of +Nature, and it is not possible to set them aside. It matters not if +harsh lines of demarcation are undiscernible between the several +consecutive groups,--the _groups themselves_ must still remain +(however equivocal it may be where they exactly commence or +terminate), and cannot be wiped out. To suppose _a priori_ that the +allied divisions of the animate creation are perfectly disconnected +_inter se_, is in fact to break the chain on which the unity of the +organic world depends; whilst to assume that groups cease to be groups +when they can be discovered to merge into each other, would no less +destroy the harmony of that admirable method, or array, which the +naturalist, above all others, delights to contemplate. If things are +no longer to be regarded as dissimilar because they unite on their +outer limits, differences may be given up, as having no special +meaning, and as therefore unworthy of investigation. It requires but a +slight insight into the physical universe to be convinced, that nearly +everything which we see (and, moreover, _without injuring its +individual reality_) is blended into that to which it is the most +akin. Night is distinct from day; yet, so long as the twilight +intervenes, no man can pronounce where the one ends, and the other +begins. Heat is opposed to cold; yet, if by degrees they be +respectively diminished, they will at last amalgamate, in a central +temperature. And thus it is with things material. The sea and the land +are essentially unlike; yet the precise boundary between the two is +never clearly defined,--the ebb and flow are constantly going on, and +the line of separation is variable. The mountain-range is moulded on a +different type to the level country beneath it; yet the turning-point +of them both is, in all instances, on neutral ground. We need not +however adduce further evidence in support of this fact,--that, +throughout the whole of Nature, the _general principle_ of fusion +(either absolute or apparent) is most obvious. From first to last, +traces of it are everywhere to be detected; not only between +_clusters_, or material combinations, of objects (in which case it is +absolute), but even between the objects themselves,--under which +circumstances, however, it is merely apparent; for, since they are +specifically dissimilar, it can only arise from their _near +resemblance_ to each other, and not from their positive coalescence. +But, admitting that this universal blending, throughout the animate +world, does not interfere with the gradual conformation of its several +groups, which _therefore_ should be recognized; we may perhaps be told +by the believers in the 'Methode Mononomique,' that they do not intend +to ignore the _arrangement_ which Nature has so broadly laid down, but +that, on the contrary, they tacitly endorse it,--their device having +reference to the _names_ only. To this however it will be sufficient +to reply, that, if they deem it necessary (of which I am by no means +convinced) to accept the natural genera of the organic creation at +all, why not _acknowledge_ them? and how can they be so well +acknowledged, either in principle or practice, as through the medium +of a binomial nomenclature? Such a system is the only consistent one, +on the hypothesis that they _do_ consider them of primary importance; +it is more in unison with our notions of what ought to be; more +suggestive of what actually _is_; more honest and generous to those +who have laboured (as describers), with such care and diligence, +before us. + +It will be perceived, from the above remarks, that, although +professedly criticizing the 'Methode Mononomique,' into the analysis +of which my subject has unintentionally drawn me, it is the absurdity +of objecting to genera _because they are not rigidly defined +throughout_, that I have been mainly striving to condemn. It is +indeed well nigh incredible that any such strictures could ever have +been advanced; for it must surely have occurred to the most +superficial inquirer, that genera, after all, _cannot_ be +homogeneous,--seeing that they are necessarily composed of detached +species, no two of which are _precisely_ similar, even in the few +structural details which may have been accidentally chosen for generic +diagnostics. How is it possible, therefore, that mere _groups_, even +though they be in accordance with Nature, should be so far isolated +and uniform in their character as to occupy an analogous position to +that of the absolutely independent species (of distinct origins) which +they severally contain? + +Taking the preceding considerations into account, the question will +perhaps arise,--How then is a genus to be defined? To which I may +reply that, were I asked whether genera had any real existence in the +animate world, my answer would be that they undoubtedly have,--though +not in the sense (which is so commonly supposed) of abrupt and +disconnected groups. I conceive them to be gradually formed nuclei, +through the gathering together of creatures which more or less +resemble each other, around a central type: they are the _dilatations_ +(to use our late simile) along a chain which is itself composed of +separate, though differently shaped links,--the links being the actual +species themselves, and the swellings, or nodes, the slowly developed +genera into which they naturally fall. When I say "slowly developed," +my meaning may possibly require some slight comment. It is simply +therefore to guard against the fallacy, which I have so often +disclaimed, that genera are abruptly (or suddenly) terminated on their +outer limits, that the expression has been employed. Though I believe +that a series of _species_, each partially imitating the next in +contact with it, is Nature's truest system; yet we must be all of us +aware that those species do certainly tend, in the main, to map out +assemblages of divers phases and magnitudes, distinguished by peculiar +characteristics which the several members of each squadron have more +or less in common. So that it is only in the middle points that these +various groups, respectively, attain their maximum,--every one of +which (by way of illustration) may be described as a _concentric +bulb_, which becomes denser, as it were, in its successive component +layers, and more typical, as it approaches its core. + +If, then, the theory of genera be such as I have endeavoured to +expound, it results from what has been said, _that every generic type +is to be looked for in, or about, the centre of its peculiar +group_,--or at any rate in that region of it which would seem to be +the most characteristically, or evenly, pronounced. I lay particular +stress upon this conclusion, because (if correct) it will somewhat +modify the notions which are occasionally entertained upon the +subject. A stricture, however, may here be required upon what I have +advanced, lest, through using the metaphors _which I selected for the +elucidation of a principle_, it be supposed that I would wish them to +apply to the smaller details, likewise, of the problem. If a genus has +been portrayed under the similitude of a bulb, or of a nodule (formed +by the approximation of beads which more or less resemble each other +in their primary aspect), it does not follow that either bulb or +nodule are to diminish in a similar ratio towards their respective +circumferences,--or, which is the same thing, that they are to be +symmetrical; whether spherical, ovoid, or otherwise. The general +method of the organic creation is a progressive one; and its +successive types, therefore, will not always be found to radiate +_equally_ from their normal foci: so that it is in the direction of +the _higher_ (rather than the lower) extremities of the assemblages +that those foci are usually to be discerned;--and where the groups are +large, it is not often difficult to pronounce which of their ends are, +as a whole, the more perfectly developed. + +It will, moreover, be further acknowledged (if my premises are +allowed), that, since it is a somewhat central position which the +typical member of a genus usually occupies, _the diagnostic +characters_, although (in combination) carried out to the full, _are +more evenly balanced in a generic type than in any of its associates_; +or, in other words, that a species in which any single organ is +monstrously enlarged, at the expense of the rest, is seldom typical of +the assemblage with which it is placed; but may be _a priori_ regarded +as in all probability a transition form, leading us onwards into some +neighbouring group[81]. + +I will not, however, venture too closely into this question in its +minor bearings;--suffice it to have demonstrated that, whatever be the +rate, law, or direction, of the advancement of the various groups +towards a more perfect model; or in whatsoever position the several +types are to be discerned, with respect to their immediate associates, +genera _cannot_ be isolated and distinct, but must of necessity merge +(each into two or more others) on their outer limits. Hence, if such +be the case, as I contend that it usually is (the exceptions to the +rule being, as I shall hope shortly to prove, the result of accident, +and by no means a part of the original design), it may perhaps be a +problem, how far we are justified in rejecting many large and natural +assemblages, through the fact that they blend, both at their +commencement and termination, imperceptibly, with others,--their +precise boundaries being dimly defined. + +That the recognition of genera is necessary, even as a matter of mere +convenience, is self-evident; for in many extensive departments they +combine with each other so completely at their extremities (although +sufficiently well-marked in the mass), that, unless we are prepared +to accept them as they are, we must needs repudiate them altogether: +under which circumstances, our difficulties, both in determination and +nomenclature, would be increased tenfold. We should also recollect, +that clusters which seem abruptly chalked out whilst our knowledge is +imperfect, are very frequently united with others when fresh +discoveries are made, and the intermediate grades brought to light: so +that their apparent isolation may oftentimes arise from our ignorance +of the absent links, rather than from the fact itself. It would surely +be more desirable, therefore, when viewed even in the light of +expediency alone, to submit to the possibility of a few neutral +species being conceded, _with equal reason_, to different groups, than +to amalgamate the whole, and so lose sight of the general method or +arrangement, into which the various creatures do unquestionably (in a +broad sense) dispose themselves. If, however, there be any truth in +the generic doctrine as above enunciated, the question of +_convenience_ may be omitted from our speculations _in toto_,--seeing +that _all_ genera (except those whose present abruptness is the effect +of accident) fuse into others with which they are in immediate +contact: so that in reality, unless we ignore these natural +assemblages from first to last, we have no choice left us as regards +the equivocal forms; but must consent to recognize them as of doubtful +location, and as possessing an equal right to be placed in one or the +other of two consecutive groups,--according to the judgment of the +particular naturalist who has to deal with them. + +But let us glance at the subject through the medium of an example, and +endeavour to realize what would be the consequence of that wholesale +combination at which we must sooner or latter arrive, if genera are +not to be upheld because they slowly merge into each other as we +recede from their respective types. The immense department _Carabidae_, +of the Coleoptera, is eminently a case in point. In the details of +their oral organs the _whole_ of that family display (as I have +elsewhere[82] remarked) so great a similarity _inter se_, or rather +shade off into each other by such imperceptible gradations, that the +_tendency_ which various clusters of them possess to assume +modifications of form which attain their maximum only in successive +centres of radiation, must oftentimes be regarded as _generic_, if we +would not shut our eyes altogether to the natural collective masses +into which the numerous species (however gradually) are, in the main, +so manifestly distributed. It is possible indeed that, as our +knowledge advances and new discoveries take place, we shall so far +unite many of the consecutive nuclei which are now considered pretty +clearly defined, that we shall be driven at last _either_ to accept +the Linnaean genera only, or else the entire host of subsidiary ones +(albeit perhaps in a secondary sense) which are, one by one, being +expunged. And, since under the former contingency the _determination +of species_ would become practically well nigh hopeless, it is far +from unlikely that we shall eventually hail the latter as, after all +(at any rate to a certain extent), the more convenient of the two. +Look, for instance, at the great genus _Pterostichus_, which has +nearly 200 representatives in Europe alone: true it is that its +several sections (_Poe cilus_, _Argutor_, _Omaseus_, _Corax_, +_Steropus_, _Platysma_, _Cophosus_, _Pterostichus_ proper, _Abax_, +_Percus_, and _Molops_), although easily recognized in the mass, do +unquestionably blend into each other; yet I believe that it has arisen +from a too rigid promulgation of the generic theory that they have not +been retained as separate. And this opinion may be rendered somewhat +more plausible, from the knowledge that certain of the _Pterostichi_ +(the Argutors, for instance) approach so closely, in their trophi, to +_Calathus_, as to be hardly discernible from it; which latter genus is +scarcely distinguishable (structurally) from _Pristonychus_,--a form +which, in its turn, leads us on towards another type. Who would have +imagined, again, some fifty years ago, that the widely distributed +groups, _Calosoma_ and _Carabus_, were not thoroughly detached _inter +se_? yet what naturalist _now_ can draw an exact line of demarcation +between them? And so it is with numerous others, which it is needless +to recall. The practical inference, however, from the whole, is this: +_that if genera must be rejected because they are not homogeneous and +isolated throughout, the only ones that will remain are those which +have become abrupt from causes which are merely accidental_. + +Having now, however, examined the question in its broadest phasis, +that is to say, on the supposition that Nature is _complete_ in her +several links and parts; I shall perhaps be expected to offer a few +passing words on what I have already hinted at,--namely, the +possibility of genera being absolutely well-defined, even on their +outer limits, _from accident_. Briefly, then, it is through the +extinction of species that groups may, in some instances, be abruptly +expressed: but, as such contingences are at all times liable (whether +from natural or artificial causes) to happen; it would be unfair to +build up our generic _definition_ from examples which are the +exception, and not the rule,--and, _more_ than mere "exceptions" (as +commonly understood by that term), the result of positive disturbances +from without. Yet, that genera thus distinctly bounded, at either end, +do actually occur, must be self-evident to any one who has attempted +to study the distribution of organic beings with reference to the +geological changes which have taken place on the earth's surface; for +it is clear that a vast proportion of the creatures which inhabit our +globe came into existence at periods _anterior_ to many of those great +convulsions which altered finally the positions of sea and land, +apportioning to each the areas which they now embrace: so that, if +_generic provinces_ of radiation (no less than specific centres) be +more than a fancy or romance, it is certain that numerous members of +many geographical assemblages must have perished for ever during the +gigantic sinkings which have at various epochs been brought about. +From which it follows, _that those groups, or clusters, of which but +few representatives (comparatively) are extant, will be more or less +abruptly terminated, according as the original type to which they +severally belong was peculiar, and in proportion as the number of its +exponents has been reduced_. + +Although there are many means through which species may become +annihilated, yet, since the subsidence of a tract into the sea +involves the maximum of loss which a space of that magnitude can +sustain, the above conclusion gives rise to a corollary: _that it is +in islands that we should mainly look for genera which are to be +rigidly pronounced_. The question therefore naturally suggests +itself,--Is this in harmony with what we see; or, in other words, is +it consistent with experience, or not? I believe that it is; for I +think it will be found, on inquiry, _that the greater proportion of +those groups which are more especially isolated in their character_ (I +do not say, necessarily, the most anomalous; though this in some +measure follows from the fact of their detachment) _are peculiar to +countries which are insular_. + +But, however important an element, in the eradication of species, +submergence may be; we must not entirely omit to notice other methods +also, through the medium of which genera may become well-defined. We +should recollect that the removal of a _very few_ links from an +endemic cluster is sufficient to cause its disjunction from the type +to which it is next akin, and that where the creatures which unite in +composing it are of slow diffusive powers, or sedentary habits, the +elimination of such links is (through the smallness of the areas which +have been overspread) a comparatively easy operation. The accidental +introduction of organic beings amongst others to the interests of +which they are hostile, may be a powerful means, as Mr. Darwin has +suggested, of keeping the latter in check, and of finally destroying +them[83]. The gradual upheaval of a tract which has been well-stored +with specific centres of radiation, created expressly for itself, may +(through the climatal changes which have been brought about) succeed +in extirpating races innumerable,--those only surviving which are able +to adapt themselves to the altered conditions; and which would _now_ +be consequently looked upon as abrupt topographical assemblages. The +over-whelming effect of a volcanic eruption, in a region where the +aborigines of the soil have not wandered far from their primaeval +haunts, may, as Sir Charles Lyell has well remarked, put an end to +others, and so effect the separation of their allies from the central +stock. And, lastly, the intervention of man, with all the various +concomitants which civilization, art, and agriculture bring in his +train, is the most irresistible of every agency in the extensive +(though often accidental) demolition of a greater or less proportion +of the animate tribes. + +The whole of these ultimate assortments, however, are dependent, as it +were, for their outline, upon contingency or chance; and we must not +deduce our ideas of genera from the examples which _they_ supply. We +should rather reflect, that it is no matter of mere speculation, that +many organic links, now absent, have, through the crises and +occurrences to which we have just drawn attention, become lost. On the +contrary, indeed, we know that, in the common course of things, it +_must_ have been so; and therefore we are induced to regard those +cases as exceptional, and as in no way expository of Nature's +universal scheme. The more we look into the question, whether by the +light of analogy or the evidence of facts, the more are we convinced +that lines of rigid demarcation (either between genera or species, +though especially the former) do not anywhere, except through +accident, exist. And hence it is that we ascend, by degrees, to a +comprehension of that _unity_ at which I have already glanced; and +are led to believe that, could the entire living panorama, in all its +magnificence and breadth, be spread out before our eyes, with its +long-lost links (of the past and present epochs) replaced, it would be +found, from first to last, to be complete and continuous +throughout,--a very marvel of perfection, the work of a Master's +hand. + +FOOTNOTES: + +[78] "Nullo modo fieri potest, ut axiomata per argumentationem +constituta ad inventionem novorum operum valeant; quia subtilitas +naturae subtilitatem argumentandi multis partibus superat. Sed axiomata +a particularibus rite et ordine abstracta, nova particularia +rursus facile indicant et designant; itaque scientias reddunt +activas."--_Novum Organum_, Aphoris. xxiv. + +[79] In selecting this simple method to illustrate the _principle_ of +a binomial system of nomenclature, it is scarcely necessary to remind +the reader that I do not intend to imply that every man is +_specifically distinct_ from his neighbour! + +[80] Considerations sur un Nouveau Systeme de Nomenclature, par C. J. +B. Amyot (_Rev. Zool._, p. 133, A.D. 1838). + +[81] I may add, that this suggestion, as to the evenly balanced state +of generic types, is in accordance with the views of Mr. +Waterhouse,--whose extensive knowledge in the higher departments of +zoological science gives a value to his opinion, especially on +questions such as these, which I am glad to have an opportunity of +acknowledging. + +[82] Annals of Nat. Hist. (2nd series), xiv., p. 199. + +[83] A familiar example of this disappearance of a creature before the +aggressive powers of another, which is either hostile to or stronger +than itself, is presented by the Black Rat (_Mus rattus_) of our own +country,--which is said to have been extremely abundant formerly, but +which is now replaced by the common brown (or "Hanoverian") one of +Northern Europe. The British species, however, although it has become +extremely scarce, is not yet _quite_ exterminated: it has been +recorded (_vide_ 'Zoologist,' 611) in Essex, and in Devonshire +('Zoologist,' 2344); and it still swarms on a small rock off Lundy +Island, in the Bristol Channel. It is reported, moreover, to have been +lately re-introduced at Liverpool. + + + + +CHAPTER VII. + +CONCLUSION + +Deposita sarcina, levior volabo ad coe lum.--_S. Jerome._ + + +Having now completed the short task which I had undertaken to perform, +I will, in conclusion, offer a few brief comments on the results at +which we have arrived, and endeavour to realize to what extent the +consideration of them is likely to be found useful, during our +inquiries into the general subject of entomological geography. + +Commencing with the thesis, that specific variation, whether as a +matter of experience or as probable from analogy, does _ipso facto_ +exist; I have endeavoured to maintain that position, by evidence of +divers kinds; and I have sought to strengthen the inferences deduced, +by an appeal to some of those external agents and circumstances which +may be reasonably presumed (if not indeed actually demonstrated) to +have had a considerable share in bringing it about. I have also +suggested what the principal organs and characters are, in the +Insecta, which would appear to be more peculiarly sensitive to the +action of local influences; and I have then diverged to the question +of topographical distribution, in connection with the geological +changes on the earth's surface; and, lastly, to some practical hints +arising out of a proper interpretation of the generic theory. How far +I have succeeded in elucidating the several points which I proposed to +examine, is a problem which must be solved by others; meanwhile, if I +have failed at times to interpret what seems scarcely to admit of +positive proof, I shall at least have had the advantage of propounding +the enigmas for discussion, and of so paving the way for future +research. We must remember, however, that, where certainty is not to +be had, probability must be accepted in its stead; or, as an old +writer has well expressed it: "That we ought to follow probability +when certainty leaves us, is plain,--because it then becomes the only +light and guide that we have. For, unless it is better to wander and +fluctuate in _absolute_ uncertainty than to follow such a guide; +unless it be reasonable to put out our candle because we have not the +light of the sun, it _must_ be reasonable to direct our steps by +probability, when we have nothing clearer to walk by".[84] + +What my chief aim in the present treatise has been, will be easily +perceived,--namely, to substantiate, as such, those _elements of +disturbance_ (on the outward contour of the Annulose tribes) with +which the physical world does everywhere abound: and, thereupon, to +provoke the inquiry, whether entomologists, as a mass, have usually +taken them into sufficient account, when describing as "species," from +distant quarters of the globe, insects which recede in only minute +particulars from their ordinary states. My own impression is, that +they have not done so; and, moreover, that, if they had, our +catalogues would have worn a very different appearance to what they +now do: for, when once the subject is fairly looked into and analysed, +it is impossible not to be convinced, that the _prima-facie_ aspect of +these creatures is eminently beneath the control of the several +conditions to which they have been long exposed. But let me not be +misunderstood in the conclusion which I have been thus compelled to +endorse, or be supposed to ignore the fact that truly _representative +species_ may frequently occur in countries far removed from each +other; which cannot therefore be regarded as modifications of a common +type. I believe, however, that this doctrine of _representation_, +whatever truth it may contain, has been too much relied upon; and that +we have been over-ready to take advantage of it (unproved as it is) +for the multiplication of our, so called, "specific novelties." I +suspect, indeed, that _actual_ representative species (if they may be +thus expressed) are more often to be recognized on the isolated +portions of a formerly continuous tract, than in regions which have +been widely separated since the last creative epoch; and that, in the +instances where beings of a _nearly_ identical aspect are detected in +opposite divisions of the earth, it is more often the case that +members of them have been transported at a remote period (either by +natural or artificial means) from their primaeval haunts, and have +become gradually altered by the circumstances amongst which they have +been placed, than that the respective phases were produced _in situ_ +on patterns almost coincident. + +I have before announced my conviction, that _generic areas_ have a +real existence in Nature's scheme; and that, consequently, where +species which are so intimately allied that they can with difficulty +be distinguished, prevail, there is presumptive reason to suspect +(until at least the contrary is rendered probable) that the areas +which they now colonize were once connected by an intervening +land,--or, in other words, that the migrations of the latter were +brought about, through ordinary diffusive powers, from specific +centres within a moderate distance of each other. I say "_presumptive_ +reason," because there are undoubted exceptions to this law (as to +every other), and it can therefore be only judged of on a broad scale. +Still, I contend that in a wide sense it holds good; and that, +consequently, if closely related "species" are traceable in countries +which geology demonstrates to have been far asunder during the +_entire_ interval since the first appearance of the present animals +and plants upon our earth, there is at any rate an _a priori_ +probability that they are no _species_ at all,--but permanent +geographical states, which have been slowly matured since their casual +introduction beyond their legitimate bounds. + +If we except those forms which are in reality but modifications, from +climatal and other causes (and which have, therefore, been wrongly +quoted as distinct); I believe that a vast proportion of the species +which have been usually considered to be "representative" ones, were +members, in the first instance, of the self-same assemblages,--which +had wandered to a distance from their primaeval haunts, and were +afterwards, through the submergence of the intervening land, cut off +from their allies. I have adduced, in a preceding chapter, some +remarkable examples in illustration of this hypothesis,--an hypothesis +which I believe to be the true clue to a very large item of the +"specific representation" theory. A considerable number of the +Madeiran _Helices_ may be cited (which I have already done[85]) as, in +the strictest sense, representative of each other,--and as therefore +specifically distinct: and I may add, that it is to island groups that +we must mainly look for this system in its full development. + +But, apart from the fact that I would not wish to resign _in toto_ the +doctrine of "specific representation," even as frequently understood +(that is to say, as recognizable in countries which have been +altogether disconnected since the last creative epoch), and therefore, +_a fortiori_, in what I conceive to be its truer meaning; there is yet +another point on which I would desire to be interpreted aright, whilst +endeavouring to substantiate the action of local influences on the +members of the insect world. It has been my aim, in the preceding +pages, to call attention to the importance of external circumstances +and conditions in regulating, within definite limits, the outward +aspect of the Articulate tribes. + +I do not, however, assert that _every_ species is liable to be +interfered with _ab extra_; that is a question which the greater or +less susceptibility of the several races, as originally constituted, +can alone decide; still less would I willingly lend a helping hand to +that most mischievous of dogmas, that they are _all_-important in +their operation,--or, in other words, that they possess within +themselves the inherent power (though it may not invariably be +exercised) of shaping out (provided a sufficient time be granted them, +and in conjunction with the advancing requirements of the creatures +themselves) those permanent organic states to which the name of +species (in a true sense) is now applied. Such a doctrine is in +reality nothing more than the transmutation theory, in all its +unvarnished fulness; and I do not see how it can be for a moment +maintained, so long as facts (and not reasoning only) are to be the +basis of our speculations. I repeat, that it is merely _within fixed +specific bounds_ that I would advocate a freedom of development, in +obedience to influences from without: only I would widen those limits +to a much greater extent than has been ordinarily done,--so as to let +in the controlling principle of physical agents, as a significant +adjunct for our contemplation. + +It does indeed appear strange that naturalists, who have combined +great synthetic qualities with a profound knowledge of minutiae and +detail, should ever have upheld so monstrous a doctrine as that of the +transmission of one species into another,--a doctrine, however, which +arises almost spontaneously,--if we are to assume that there exists in +every race the tendency to _an unlimited progressive improvement_. +There are certainly no observations on record which would, in the +smallest degree, countenance such an hypothesis. Many animals and +plants, it is true, are capable of considerable modifications and +changes, for the better,--very much more than is the case with others. +But what does this prove, except that their capacity for advancement +has a slightly wider compass than that of their allies? It touches not +the fact, that the boundaries of their respective ranges are +absolutely and critically defined. It is moreover a singular +phaenomenon, and one in which the strongest proofs of design (or a +primary adjustment of limits with a view to the future) may be +discerned, that the members of the organic creation which display the +greatest adaptive powers, are those which were apparently destined to +become peculiarly attendant upon man. "The best-authenticated +examples," says Sir Charles Lyell, "of the extent to which species can +be made to vary may be looked for in the history of domesticated +animals and cultivated plants. It usually happens that those species +which have the greatest pliability of organization, those which are +most capable of accommodating themselves to a great variety of new +circumstances, are most serviceable to man. These only can be carried +by him into different climates, and can have their properties or +instincts variously diversified by differences of nourishment and +habits. If the resources of a species be so limited, and its habits +and faculties be of such a confined and local character, that it can +only flourish in a few particular spots, it can rarely be of great +utility. We may consider, therefore, that in the domestication of +animals and the cultivation of plants, mankind have first selected +those species which have the most flexible frames and constitutions, +and have then been engaged for ages in conducting a series of +experiments, with much patience and at great cost, to ascertain what +may be the greatest possible deviation from a common type which can be +elicited in these extreme cases[86]." + +The fact, however, that all areas of aberration (however large they +may be) are positively circumscribed, need scarcely be appealed to, in +exposing the absurdity of the transmutation hypothesis. The whole +theory is full of inconsistencies from beginning to end; and from +whatever point we view it, it is equally unsound. How, for instance, +can any amount of local influences, or the progressive requirements of +the creatures themselves, give rise to the appearance of several +well-marked representatives of a genus on the self-same spot,--where +the physical conditions for each of them are absolutely the same? +Look, for example, at the _Tarphii_ (to which I have already +alluded[87]) of Madeira: I have detected about eighteen abundantly +defined species; and, as stated in a previous chapter, I have but +little doubt, from their sedentary habits, and the evident manner in +which they are adjusted to the peculiarities of the region in which +they obtain, that they are strictly an esoteric assemblage, inhabiting +the actual sites (or nearly so) of their original _debut_ upon this +earth. Here, then, we have a sufficient length of time for +developments to have taken place; they are all exposed to the +self-same agencies from without (for they live principally in +communion); yet, though I have examined carefully more than a thousand +specimens (a large proportion of them beneath the microscope), I have +never discovered a single intermediate link which could be regarded as +in a transition state between any of the remainder. But how is +this?--Is it possible to account for differences so decided, yet each +of such amazing constancy, amongst the several creatures of a central +type which have been exposed to identical conditions through, at any +rate, generations innumerable? They clearly cannot be explained on the +doctrine of transmutation: yet they are no exceptions to the ordinary +rule,--occupying an analogous position to the members of every other +endemic group. + +But I will not occupy more space on the transmutation theory: suffice +it to have shown that, in thus conceding a legitimate power of +self-adaptation, in accordance with external circumstances, to the +members of the insect world; and in suggesting the inquiry, whether +the action of physical influences has been adequately allowed for by +entomologists generally (or, in other words, whether the small shades +of difference which have often, because permanent, been at once +regarded as specific, may not be _sometimes_ rendered intelligible by +a knowledge of the localities in which the creatures have been +matured), I do not necessarily open the door to the disciples of +Lamarck, or infringe upon the strict orthodoxy of our zoological +creed. On the contrary, indeed, I believe that the actual reverse is +nearer the truth; and, moreover, that those very hyper-accurate +definers who recognize a "species" wheresoever the minutest decrepancy +is shadowed forth, will be found eventually (however unaware of it +themselves) to have been the most determined abettors of that +dogma,--seeing that their species, if such they be, do most assuredly +pass into each other. + +We must not, however, omit to notice, briefly, how this perversion of +Nature's economy took its rise. It was from the desire, which is +almost inherent within us, to account for everything by physical laws; +and to dispense with that constant intervention of the direct creative +act which the successive races of animals and plants, such as are +proved by geology to have made their appearance at distinct epochs +upon this earth, would seem to require. Or, which amounts to the same +thing, it resulted through an endeavour to explain by material +processes what is placed beyond their reach. But, if this be the case, +it may be reasonably asked,--Are material laws then not to be inquired +into, and should the various influences which operate in the organic +world around us be debarred from analysis? Unquestionably not. Truth +is truth, under whatever aspect it may come; and cannot possibly +contradict another truth. To exercise our intellectual faculties, by +tracing out, through slow, inductive methods, the _modus operandi_ of +even a single natural law, is an honourable task; nor should the +apparent smallness of the media which we are at times compelled to +employ, render it less so (else would this present treatise, like many +others of a kindred stamp, have been best unwritten): but it is from +the conceit that our own imperfect interpretations have left nothing +more to be found out, that the great danger is to be anticipated. An +effect may be literally dependent upon a certain proximate cause; and +if we be so fortunate as to ascertain that cause, we have done +something; but it does not necessarily follow that we have done +_much_. On the contrary, it often happens that, in so doing, we have +achieved wonderfully little,--seeing that the problem may be +self-evident. Behind that "cause," we should recollect, others lie +concealed, of a far deeper nature, each depending upon the next in +succession to it; until, in the order of causation, we are at length +led back, step by step, to the Final One,--with which alone the mind +can be thoroughly content. "We make discovery after discovery," says +Dr. Whewell, "in the various regions of science; each, it may be, +satisfactory, and in itself complete, but none final. Something always +remains undone. The last question answered, the answer suggests still +another question. The strain of music from the lyre of Science flows +on, rich and sweet, full and harmonious; but never reaches a close: +no cadence is heard with which the intellectual ear can feel +satisfied[88]." + +As regards that most obscure of questions, _what the limits of species +really are_, observation alone can decide the point. It frequently +happens indeed that even observation itself is insufficient to render +the lines of demarcation intelligible,--therefore, how much more mere +dialectics! To attempt to argue such a subject on abstract principles, +would be simply absurd; for, as Lord Bacon has remarked, "the subtilty +of Nature far exceeds the subtilty of reasoning:" but if, by a careful +collation of _facts_, and the sifting of minute particulars gathered +from without, the problem be fairly and deliberately surveyed, the +various disturbing elements which the creatures have been severally +exposed to having been duly taken into account, the boundaries will +not often be difficult to define. Albeit, we must except those races +of animals and plants which, through a long course of centuries, have +become modified by man,--the starting-points of which will perhaps +continue to the last shrouded in mystery and doubt. It would be +scarcely consistent indeed to weigh tribes which have been thus +unnaturally tampered with by the same standard of evidence as we +require for those which have remained for ever untouched and +free,--especially so, since (as we have already observed) it does +absolutely appear, that those species, the external aspects of which +have been thus artificially controlled, are by constitution more +tractile (and possess, therefore, more decided powers for aberration) +than the rest. Whether traces of design may be recognized in this +circumstance, or whether those forms were originally selected by man +_on account_ of their pliability, it is not for me to conjecture; +nevertheless, the first of these inferences is the one which I should, +myself, be _a priori_ inclined to subscribe to. + +In examining, however, this enigma, _of the limits within which +variation is_ (as such) _to be recognized_; it should never be +forgotten, that it is possible for those boundaries to be absolutely +and critically marked out even where we are not able to discern them: +so that the difficulty which a few domesticated creatures of a +singularly flexible organization present, should not unnecessarily +predispose us to dispute the question in its larger and more general +bearings. Nor should we be unmindful that (as Sir Charles Lyell has +aptly suggested) "some mere varieties present greater differences, +_inter se_, than do many individuals of distinct species;" for it is a +truth of considerable importance, and one which may help us out of +many an apparent dilemma. + +But, whatever be the several ranges within which the members of the +organic creation are free to vary; we are positively certain that, +_unless the definition of a species, as involving relationship, be +more than a delusion or romance_, their circumferences are of +necessity real, and must be indicated _somewhere_,--as strictly, +moreover, and rigidly, as it is possible for anything in Nature to be +chalked out. The whole problem, in that case, does in effect resolve +itself to this,--Where, and how, are the lines of demarcation to be +drawn? No amount of inconstancy, provided its limits be fixed, is +irreconcilable with the doctrine of specific similitudes. Like the +ever-shifting curves which the white foam of the untiring tide +describes upon the shore, races may ebb and flow; but they have their +boundaries, in either direction, beyond which they can never pass. And +thus in every species we may detect, to a greater or less extent, the +emblem of instability and permanence combined: although perceived, +when inquired into, to be fickle and fluctuating in their component +parts, in their general outline they remain steadfast and unaltered, +as of old,-- + +"Still changing, yet unchanged; still doom'd to feel _Endless +mutation, in perpetual rest_." + +FOOTNOTES: + +[84] Religion of Nature Delineated, p. 103. + +[85] Vide _supra_, p. 128. + +[86] Principles of Geology, 9th edition, pp. 583, 584. + +[87] Vide _supra_, p. 121. + +[88] Indications of the Creator (London, 1845), p. 163. + + + + +INDEX. + + + Aberration, perhaps indicated universally, 16, 17, 18. + + Aborigines, insect, unimportant for climatal modifications, 25, 26, + 27. + + _Acalles_, the Canarian type of, apparent on the Salvages and + Dezertas, 124. + + _---- Neptunus_, Woll., perhaps a state of _A. argillosus_, 124. + + _Achatina Eulima_, Lowe, its extinction in Porto Santo, 131. + + _Achenium Hartungii_, Heer, a form of _A. depressum_, 65. + + _Acherontia Atropos_, Linn., its introduction into Madeira perhaps + recent, 74. + + _Adimonia_, the capture of, out at sea, 150. + + _Aepus marinus_, Stroem., pallid hue of, 64. + + _---- Robinii_, Lab., pallid hue of, 64. + + _Agabus bipustulatus_, Linn., unaffected by climate, 31. + + Alligators, their peculiarity to S. America, 143. + + Alpine species, some peculiarly so, 40. + + Altitude and latitude, sometimes reciprocal, 35, 114. + + _Amyeterus_, its concentration in Australia, 143. + + Amyot, M., his 'Methode Mononomique,' 164. + + Analogies, Lord Bacon on the importance of, 13; + why necessary to be studied, 14. + + Analogy, argument from, 10, 11, 12. + + _Anchomenus marginatus_, Linn., slightly modified in Madeira, 38. + + Andes, dissimilarity of the fauna on the opposite sides of the, 146. + + _Anobium striatum_, Oliv., unaffected by climate, 31. + + Antennae, joints of, said occasionally to vary, 96. + + _Anthicus bimaculatus_, Illig., variability of, near the sea, 63. + + _---- fenestratus_, Schmidt, slightly modified in Madeira, 38. + + _---- humilis_, Germ., variability of in salt places, 63. + + _---- instabilis_, Hoffm., pallid hue of, 64. + + _Anthonomus ater_, Mshm, very small in Lundy Island, 58, 73. + + _Aphelocheirus aestivalis_, Fabr., the hemelytra of, sometimes fully + developed, 100. + + _Aphodius nitidulus_, Fabr., paler in Madeira than in Europe + generally, 65. + + _Aphodius plagiatus_, Linn., usually black in England, 61; + two distinct states of, indicated, 105. + + _Apocyrtus_, its concentration in the Philippine Islands, 143. + + _Apotomus_, common to Madeira and Sicily, 139. + + _Argutor_, always apterous in Madeira, 82; + trophi of, almost identical with those of _Calathus_, 175. + + Armadillos, their peculiarity to S. America, 143. + + Armitage, Mr., on _Cicindela fasciatopunctata_ from Mount Olympus, 41. + + Arrangement, a lineal one is not indicated in Nature, 163. + + Atlantic continent, Prof. E. Forbes on the former existence of, 137. + + Atlantis of the ancients, the impossibility of its being identified + with a former Atlantic region, 140; + perhaps the New World, 141. + + _Atlantis_, the genus, a modification of _Laparocerus_, 143. + + Azores, the colonization of, by two Madeiran _Helices_, 133. + + + Bacon, Lord, on the importance of analogies, 13; + on the Atlantis of the ancients, 141; + on the necessity of observation for forming science, 159. + + Banksias, their concentration in Australia, 142. + + Barriers, natural, the difference between primary and recent, 145; + their hindrance to insect diffusion, 145. + + _Bembidium Atlanticum_, Woll., paler in Porto Santo than in Madeira, + 66; + the variations to which it is subject, 107, 108. + + _---- bistriatum_, Dufts., paler in saline districts, 62. + + _---- ephippium_, Mshm, pallid hue of, 64. + + _---- obtusum_, Sturm, varies in southern latitudes, 33. + + _---- pallidipenne_, Illig., pallid hue of, 64. + + _---- saxatile_, Gyll., variety of, on the south coast of England, 60. + + _---- Schmidtii_, Woll., perhaps a state of _B. callosum_, 66. + + _---- scutellare_, Germ., pallid hue of, 64. + + _---- tabellatum_, Woll., perhaps a state of _B. tibiale_, 66. + + _Berginus_, common to Madeira and Sicily, 139. + + Black Rat, nearly exterminated in England, 178. + + _Blemus areolatus_, Creutz., paler in brackish places, 62. + + _Bolitochara assimilis_, Kby, smallness of, in the Scilly Islands, 73. + + _Boromorphus_, common to Madeira and Sicily, 139. + + _Brachinus crepitans_, Linn., two distinct sizes of, frequently + indicated, + 105. + + _Bradycellus fulvus_, Mshm, apterous in Madeira, 85. + + Bread-fruit Trees, their peculiarity to the South Sea Islands, 142. + + + _Calathus_, apterous in Madeira, 82; its trophi almost identical with + those of _Pristonychus_, 175. + + _---- complanatus_, Koll., varies from altitude, 39; + variety of, on one of the Madeira Islands, 88. + + _---- fuscus_, Fabr., slightly modified in Madeira, 38, 85. + + _Calathus melanocephalus_, Linn., smallness of, in the Scilly Islands, + 73. + + _---- mollis_, Mshm, variable in its wings, 43; + lurid colour of, 64. + + Calcareous soils, effect of, on the aspect of insects, 66. + + Calceolarias, their concentration on the Andes, 142. + + _Calosoma_, a species of, ten miles from shore, 147; + the genus, mergescgradually into _Carabus_, 175. + + _---- Syncophanta_, Linn., its power of crossing the sea, 147. + + Canary Islands, migratory direction of their insect population, 119. + + _Carabidae_, inconstant in their organs of flight, 43; + family of, nearly similar throughout in its oral organs, 174. + + _Carpophilus hemipterus_, Linn., unaffected by climate, 31. + + _Caulotrupis conicollis_, Woll., large size of, on one of the Madeira + Islands, 88, 109. + + _---- lucifugus_, Woll., varies from isolation, 90, 109. + + Causes, never final ones which we investigate, 191. + + _Centrinus_, its concentration in S. America, 143. + + _Ceutorhynchus contractus_, Mshm, smallness of, in Lundy Island, 59, + 73. + + _Cholovocera_, common to Madeira and Sicily, 139. + + _Choreius ineptus_, Westw., on a winged state of, 44. + + _Chorosoma miriforme_, the development of the wings of, 100. + + _Chrysomela_, apterous in Madeira, 82. + + _Chrysomelae_, vary from altitude, 41. + + _Chrysomelidae_, almost absent in Tierra del Fuego, 47. + + _Cicindela fasciatopunctata_, Germ., a state of _C. sylvatica_ 41. + + _Cicindelidae_, often variable, 41. + + _Cillenum laterale_, Sam., lurid hue of, 64. + + _Cimex apterus_, Linn., the development of the wings of, 100. + + _---- lectularius_, Linn., on the development of the wings of, 45. + + _Cistela sulphurea_, Linn., its variability near the sea, 60. + + _Clausilia deltostoma_, Lowe, a Porto-Santan form of, 134. + + Climatal modifications significant, although small, 42. + + Climate, not important as a disturbing cause, 23, 24, 31, 32, 42. + + Clouded-yellow Butterfly, unaffected by climate, 31. + + _Clypeaster pusillus_, Gyll., differs slightly in Madeira, 65. + + Coast, inconstancy of insects in the vicinity of the, 57. + + _Coccinella 7-punctata_, Linn., unaffected by climate, 31. + + _Colias Edusa_, Fabr., unaffected by climate, 31. + + Colour, its inconstancy in insects found near the sea, 57, 58. + + ---- of insects, affected by isolation, 88. + + _Colymbetes_, a species of, captured forty-five miles from shore, 149, + 150. + + Compensation, generally apparent when an insect is deprived of an + organ or sense, 81. + + _Coranus subapterus_, Curt., the development of the wings of, 101. + + Cordillera, Mr. Darwin on the fauna of the, 145. + + _Corylophus_, apterous in Madeira, 82. + + _Criomorphus_, Curtis, referable to the genus _Delphax_, 45. + + _Cyclostoma lucidum_, Lowe, its extinction in Porto Santo, 131. + + _Cynthia Cardui_, Linn., unaffected by climate, 32. + + _Cynucus_, a species of, seventeen miles from shore, 150. + + _Cyrtonota_, its concentration in S. America, 143. + + + Darwin, Mr., on the fauna of the Galapagos, 23; + relative proportions of the insect tribes in the tropics, 28, 29; + on the insects of Tierra del Fuego, 47; + on the natural features of Tierra del Fuego, 50; + on the insects of Keeling Island, 55; + on the insects of St. Helena, 55; + on the insects of Ascension, 55; + on the apterous condition of insular species, 86; + on the fauna of the Cordillera, 145; + on a _Calosoma_ captured at sea, 147; + on insects captured in the sea, 149, 150; + on the disappearance of animals before more powerful ones than + themselves, 178. + + Dawson, Rev. J. F., on a variety of _Bembidium saxatile_, 60. + + Definition of the term 'species,' 4; + of the term 'variety,' 4. + + _Delphax_, on the development of the wings of, 45. + + _Dermestes vulpinus_, Fabr., unaffected by climate, 31. + + _Deucalion_, its occurrence on the Salvages and Dezertas, 125. + + _---- Desertarum_, Woll., its sedentary nature, 125, 126, 127. + + _Dichelus_, its concentration in S. Africa, 143. + + Differences, when to be regarded as specific, 6; + too exclusively studied, 12. + + Diffusion, various means of, which operate on the insect tribes, 148. + + Disturbing agents, Prof. Henfrey on, 8. + + _Ditylus_, the same type of, indicated in the Canaries and Salvages, + 124. + + Domesticated animals, pliable nature of, 187, 192. + + _Dromius arenicola_, Woll., representative of _D. obscuroguttatus_, + 66. + + _---- fasciatus_, Gyll., its paleness near the sea, 63. + + _---- negrita_, Woll., perhaps an ultimate state of _D. glabratus_, + 85. + + _---- obscuroguttatus_, Dufts., its changes in Madeira, 36, 37, 38; + apterous in Madeira, 84. + + _---- sigma_, Rossi, its colour affected by isolation, 88, 89. + + + Elevation, sometimes corresponds with latitude, 35, 114. + + _Ellipsodes glabratus_, Fabr., singular variety of, on one of the + Madeira Islands, 88, 109. + + Elytra, connateness of, a variable character, 96. + + 'Endemic,' to what species the term is applicable, 118. + + Entomology, the study of, does not necessarily cramp the mind, 111. + + _Ephistemus_, apterous in Madeira, 82. + + _Eucalypti_, their concentration in Australia, 142. + + _Eunectes sticticus_, Linn., unaffected by climate, 31. + + Euphorbias, their concentration in Southern Africa, 142. + + _Eurygnathus Latreillei_, Lap., variety of, on one of the Madeira + Islands, 88, 109. + + Exceptions, not be allowed to negative a law, 72, 73. + + Extinction of species, as indicated in the Madeiran _Helices_, 131; + the only cause by which genera may be abruptly defined, 176. + + + Forbes, Prof. E., on the origin of the British animals and plants, + 130; + his epochs of migration of the British animals and plants, 136; + on the existence of a former Atlantic continent, 137. + + Forests, the hindrance which they offer to insect-diffusion, 154. + + "Fortunate Islands" of the ancients, probably the Canarian group, 141. + + + Galapagos, fauna of, 23. + + Genera, the nature of, often misunderstood, 160; + a familiar explanation of, 160, 161, 162; + cannot be abrupt except from accident, 169; + how to be defined, 169; + the types of, usually situated towards the centres of the several + groups, 170; + the types of, usually evenly balanced in their structural + characters, 171, 172; + may be abruptly defined from accidental causes, 176, 177. + + Generic areas, an important feature throughout Nature, 130, 141, 184. + + Geology, a necessary item in the study of insect-diffusion, 113. + + Germanic plains, the, probably a primary area of diffusion, 130. + + _Gerris_, on the development of the wings of, 100. + + Gould, Mr., on the Swallows of Malta, 102. + + _Gymnaetron_, blood-red dashes characteristic of, 62. + + _---- Campanulae_, Linn., its smallness on the Cornish coast, 58. + + _---- Veronicae_, Germ., a variety of _G. niger_, 62. + + + _Hadrus illotus_, Woll., perhaps a form of _H. cinerascens_, 66. + + _Haliplus obliquus_, Gyll., dark state of, in Ireland, 67. + + _Haltica exoleta_, Fabr., its variability on the coast, 59. + + Harcourt, Mr., on the discovery of Madeira, 49, 50. + + _Harpalus vividus_, Dej., changes to which it is subject, 67, 68, 69; + variable in the connateness of its elytra, 96, 97. + + _Hegeter_, its maximum attained in the Canaries, 120. + + _---- elongatus_, Oliv., its migration from the Canaries, 120; + of a more adaptive nature than its allies, 121. + + _---- latebricola_, Woll., its occurrence in the Salvages, 120. + + _Helices_, have often two distinct states, 106; + many of them representative in the Madeira Islands, 128, 129; + those in the Madeiras chiefly of slow migratory powers, 130, 131. + + _Helix attrita_, Lowe, its local character, 132. + + _---- Bowdichiana_, Fer., perhaps a gigantic state of _H. punctulata_, + 106. + + _---- calculus_, Lowe, sedentary nature of, 132. + + _Helix commixta_, Lowe, sedentary nature of, 132. + + _---- coronata_, Desh., its peculiarity to Porto Santo, 128; + its occurrence beneath the surface of the ground, 131. + + _---- coronula_, Lowe, its peculiarity to the Southern Dezerta, 128. + + _---- Delphinula_, Lowe, the Madeiran representative of _H. + tectiformis_ in Porto Santo, 129. + + _---- discina_, Lowe, a form of _H. polymorpha_, 133. + + _---- erubescens_, Lowe, its powers of diffusion greater than those of + its allies, 133; + sensitive to external influences, 134. + + _---- fluctuosa_, Lowe, its extinction in Porto Santo, 131. + + _---- hirsuta_, Say, two distinct states of, 106. + + _---- lapicida_, Linn., its extinction in Porto Santo, 131. + + _---- latens_, Lowe, the Madeiran representative of _H. obtecta_ in + Porto Santo, 129. + + _---- lincta_, Lowe, the common Madeiran form of _H. polymorpha_, 134. + + _---- Lowei_, Pfr., perhaps a gigantic state of _H. Portosanctana_, + 106. + + _---- papilio_, Lowe, a form of _H. polymorpha_, 133. + + _---- paupercula_, Lowe, its powers of diffusion greater than those of + its allies, 133. + + _---- polymorpha_, Lowe, sensitive to external influences, and of + great diffusive powers, 133. + + _---- Portosanctana_, Sow., its peculiarity to Porto Santo, 129. + + _---- pulvinata_, Lowe, a form of _H. polymorpha_, 133. + + _---- saccharata_, Lowe, a local state of _H. polymorpha_, 134. + + _---- senilis_, Lowe, the Dezertan form of _H. polymorpha_, 134. + + _---- squalida_, Lowe, the Madeiran representative of _H. depauperata_ + in Porto Santo, 129. + + _---- tiarella_, Webb, its sedentary nature, 128. + + _---- undata_, Lowe, its peculiarity to Madeira proper, 129. + + _---- Vulcania_, Lowe, its peculiarity to the Dezertas, 129. + + _---- Wollastoni_, Lowe, sedentary nature of, 132. + + _Helobia nivalis_, Payk., perhaps a state of _H. brevicollis_, 40. + + _Helops_, always apterous in Madeira, 82. + + _---- confertus_, Woll., varies from altitude, 39. + + _---- futilis_, Woll., varies from isolation, 109. + + _---- testaceus_, Kuest., pallid hue of, 64. + + _---- Vulcanus_, Woll., large size of, on one of the Madeira Islands, + 88. + + Henfrey, Prof., on disturbing agents, 8. + + Herschel, Sir John, on the requisites for an observer, 12. + + _Hipparchia Semele_, Linn., has a distinct aspect in Madeira, 34. + + _Hipporhinus_, its concentration in S. Africa, 143. + + Holme, Mr., on _Olisthopus rotundatus_ in the Scilly Islands, 58, 102; + on a winged state of _Phosphuga atrata_, 102. + + _Holoparamecus_, common to Madeira and Sicily, 139. + + _---- Niger_, Aube, different in Madeira and Sicily, 33. + + Hooker, Dr., on the insects of Kerguelen's Land, 86. + + Humboldt, his notice of Sphinxes and flies high up on the Andes, 149. + + Humming-Birds, their peculiarity to S. America and the W. Indies, 142. + + _Hydrobius_, apterous in Madeira, 82; + the capture of, out at sea, 150. + + _Hydrometridae_, on the development of the wings of, 100. + + _Hydroporus_, the capture of, out at sea, 150. + + _---- confluens_, Fabr., unaffected by climate, 31. + + _Hypsonotus_, its concentration in S. America, 143. + + + Influence of climate not important, 23. + + Insect-aberration, perhaps a universal fact, 16, 17, 18. + + _Insulae Fortunatae_ of Juba, probably the Canarian Group, 141. + + Ireland, poverty of the fauna of, 52, 53; + the south-west of, has something in common with Madeira, 139. + + Islands, faunas of, often too greatly magnified, 70; + the species of, generally more isolated in their structure than + those of continents, 177. + + Isolation, effects of, on insect-stature, 71. + + Ixias, their concentration in Southern Africa, 142. + + + Kangaroos, their concentration in Australia, 142. + + Kerguelen's Land, insects of, 86. + + Kirby, Rev. W., on insects washed up on the Suffolk coast, 147. + + + _Laemophloe us pusillus_, Schoenh., unaffected by climate, 31. + + _Lamprias chlorocephalus_, Ent. H., two distinct sizes of, frequently + indicated, 105. + + _Laparocerus morio_, Schoenh., large size of, on one of the Madeira + Islands, 88. + + Latitude and altitude, sometimes reciprocal, 35. + + _Leistus montanus_, Steph., has been supposed to be equal to _L. + fulvibarbis_, 40. + + _Lemur_, its peculiarity to Madagascar, 143. + + _Litargus_, common to Madeira and Sicily, 139. + + _Lixus angustatus_, Fabr., unaffected by climate, 31. + + Localities, some naturally more productive than others, 53, 54. + + _Longitarsus_, the native species of, apterous in Madeira, 82. + + _Loricera_, apterous in Madeira, 82. + + Lowe, Rev. R. T., his capture of the _Deucalion Desertarum_, 127. + + Lundy Island, smallness of the insects in, 58, 59; + occurrence of the Black Rat in, 178. + + _Lycaena Phloe as_, Linn., darker in Madeira than in England, 34. + + Lyell, Sir Charles, on _Helix hirsuta_, 106; + on the fossil period of the Madeiran _Helices_, 129; + on insects washed up on the shore, 148; + on the effect of gales in the transportation of insects, 148; + on the effects of a volcanic eruption in destroying species, 179; + on the flexible nature of certain animals and plants, 187; + on the greater differences which varieties often present than do + species, 193. + + _Lygaeus brevipennis_, Latr., on the development of the wings of, 101. + + + _Macronota_, its peculiarity to Java, 143. + + Madeira, has some features in common with Tierra del Fuego, 48, 49, + 50, 51; + former state of, 48, 49; + great fire on the southern side of, 49; + origin of the name of, 50; the insects of, 55; + the tendency of its insects to become apterous, 82; + the migratory direction of its insect population, 119; + the local nature of its various species, 152, 153. + + Magnolias, their concentration in Central America, 142. + + Malta, Mr. Gould on the birds of, 102. + + _Malthodes Kiesenwetteri_, Woll., perhaps a state of _M. + brevicollis_, 66. + + Man, agency of, in the destruction of species, 179. + + _Mantura Chrysanthemi_, Ent. H., variability of, in Lundy Island, 59. + + _Marsupialia_, their concentration in Australia, 142. + + Mesembryanthemums, their concentration in Southern Africa, 142. + + _Mesites_, a modification of _Cossonus_, 144. + + _---- Maderensis_, Woll., its near relationship to the _M. Tardii_, + 141. + + _---- Tardii_, Curtis, its variability near the coast, 58. + + 'Methode Mononomique,' the unsoundness of, 164-168. + + Migratory powers, slowness of, in the Madeiran _Helices_, 130-132. + + ---- progress, direction of, in the Madeiran animals, 120, 135. + + Mimosas, their concentration in Australia, 142. + + Mollusca, Terrestrial, often present two distinct states, 106. + + _Moluris_, its concentration in S. Africa, 143. + + _Monochelus_, its concentration in S. Africa, 143. + + Mountain-chains, their hindrance to insect-diffusion, 145. + + Mountain-tops, either very prolific in insect life, or else barren, + 115. + + _Mus Rattus_, almost exterminated in England, 178. + + _Mycetoporus pronus_, Erichs., two distinct states of, indicated, 106. + + Myrtles, their concentration in Australia, 142. + + + Naturalist, the, what his province to investigate, 158. + + Nature, not irregular because presenting occasional anomalies, 94. + + _Naupactus_, its concentration in S. America, 143. + + _Nebria complanata_, Linn., unusually pale near Bordeaux, 33; + pallid hue of, 64. + + New World, some of its insects perhaps but states of those of the Old, + 37. + + Nomenclature, a binomial system the only true one, 164, 168. + + _Notaphus_, the capture of, out at sea, 150. + + _Notiophili_, extremely variable, 40. + + _Notiophilus geminatus_, Dej., large size of, on one of the Madeira + Islands, 88. + + + Observation, indispensable in natural science, 20, 159, 192. + + Ocean, the, its hindrance to insect-diffusion, 145. + + _Ochthebius marinus_, Payk., lurid hue of, 64. + + _Olisthopus_, apterous in Madeira, 82. + + _---- Maderensis_, Woll., large state of, on one of the Madeira + Islands, 88, 89. + + _---- rotundatus_, Payk., very small in the Scilly Islands, 58, 73; + subapterous in the Scilly Islands, 102. + + _Omaseus nigerrimus_, Dej., a form of _O. aterrimus_, 33. + + _Omias Waterhousei_, Woll., large state of, on one of the Madeira + Islands, 88, 109. + + _Oncocephalus griseus_, development of the wings of, 101. + + _Othius_, apterous in Madeira, 82. + + Ourangs, their peculiarity to the Indian Islands, 143. + + _Oxyomus_, a modification of _Aphodius_, 144. + + + _Pachymerus brevipennis_, the development of the wings of, 100. + + _Pachyrhynchus_, its concentration in the Philippine islands, 143. + + Painted-Lady Butterfly, unaffected by climate, 32. + + _Papilio Machaon_, Linn., unaffected by climate, 31. + + _Paropsis_, its concentration in Australia, 143. + + Patagonia, insects of, distinct from those of Tierra del Fuego, 47, + 48. + + _Patrobus septentrionis_, Dej., has been supposed to be a state of _P. + excavatus_, 40. + + _Pecteropus_, its maximum attained in the Canaries, 124. + + _---- Maderensis_, Woll., varies from altitude, 39. + + _---- rostratus_, Woll., varies from isolation, 90. + + Pelargoniums, their concentration in Southern Africa, 142. + + _Pelophila borealis_, Payk., larger in Ireland than in the Orkneys, + 33. + + _Phaleria cadaverina_, Fabr., pallid hue of, 64. + + _Philhydrus melanocephalus_, Oliv., two states of, frequently + indicated, 105. + + _Phlaeophagus_, apterous in Madeira, 82. + + _Phosphuga atrata_, Linn., taken with the wings developed, 102. + + _---- subrotundata_, Leach, the Irish form of the _P. atrata_, 33. + + _Phytophaga_, preponderance of, in the tropics, 28, 29. + + _Pieris Brassicae_, Linn., varies in Nepaul and Japan, 34. + + _Pissodes notatus_, Fabr., unaffected by climate, 30. + + _Platyomus_, its concentration in S. America, 143. + + _Platyrrhini_, their peculiarity to S. America, 143. + + _Pogonus luridipennis_, Germ., lurid hue of, 64. + + _Pontia Brassicae_, Linn., its introduction into Madeira probably + recent, 74. + + Porto Santo, origin of the name of, 49; + a generic area of radiation for certain _Helices_, 130. + + Predacious insects, less numerous in the tropics, 28, 29. + + _Prostemma guttula_, Fabr., the development of the wings of, 100, 101. + + _Psylliodes_, a variable species of, in Lundy Island, 60. + + _---- erythrocephala_, Linn., two distinct states of, frequently + indicated, 105. + + _---- marcida_, Illig., pallid hue of, 64. + + _---- nigricollis_, Mshm, a pale state of the _P. erythrocephala_, + 105. + + _---- vehemens_, Woll., varies from isolation, 90. + + _Pterostichus_, its various divisions are natural ones, 175. + + _Ptini_, their stature affected by isolation, 74; + which characters of, are the most constant, 104. + + _Ptinus albopictus_, Woll., its changes on the islands of the Madeiran + Group, 75-77. + + _Pupa_, often two distinct states of, 106. + + _Purpurariae_ of the ancients, probably the Madeiran Group, 141. + + Pyrenean region, the, perhaps a primary area of diffusion, 130. + + + Reasoning, not sufficient of itself for the formation of science, 159. + + Red-Admiral Butterfly, its introduction into Madeira perhaps recent, + 74. + + _Reduviadae_, on the development of the wings of a representative of + the, 101. + + Representative species, exemplified by the Madeiran _Helices_, 128, + 129, 185; + where frequently to be recognized, 183. + + _Rhyzopertha pusilla_, Fabr., unaffected by climate, 31. + + Rivers, their power of transporting insects along their course, 149. + + + Saline spots, variation of insects in, 57. + + Salvages, occurrence of a Canarian form on the, 120, 124. + + _Saprinus_, a modification of _Hister_ proper, 143. + + _---- nitidulus_, Fabr., two distinct states of, indicated, 106. + + _Scarabaeus_, the capture of, out at sea, 150. + + _Scarites abbreviatus_, Koll., large size of, on one of the Madeira + Islands, 88; + varies both from isolation and altitude, 91. + + Sciences, the, should assist rather than oppose each other, 155, 156. + + _Scydmaenus Helferi_, Schaum, smaller in Madeira than in Sicily, 65. + + _Scymnus_, an apterous species of, in Porto Santo, 82. + + Sea, inconstancy of insects in the vicinity of the, 57. + + Sicily, the fauna of, has much in common with that of Madeira, 139. + + _Silpha atrata_, Linn., presents a distinct state in Ireland, 33. + + _Silybum Marianum_, Grtn., its stalks the food of a _Ptinus_, 76. + + Similitudes, Lord Bacon on the importance of, 13. + + _Sitonia gressoria_, Illig., perhaps a form of the _S. grisea_, 33. + + _Sitophilus granarius_, Linn., unaffected by climate, 31. + + _Sitophilus oryzae_, Linn., unaffected by climate, 31. + + Sloths, their peculiarity to S. America, 143. + + Species, definition of the term, 4; + familiar explanation concerning the nature of, 161, 162; + limitation of, how to be attempted, 192; + limits of, real, though often difficult to trace out, 193; + in a certain sense both unstable and permanent, 194. + + Specific centres of creation, 5. + + _Sphinx Convolvuli_, Linn., its introduction into Madeira probably + recent, 74. + + Spinola, on one of the _Reduviadae_, 101; + on _Oncocephalus griseus_, 101. + + Stapelias, their concentration in Southern Africa, 142. + + States, large and small ones indicated in some insects, 105. + + Stature of insects, smaller in islands than on continents, 70. + + _Stenolophus Skrimshiranus_, Steph., perhaps a state of _S. Teutonus_, + 63. + + _Stenus Heeri_, Woll., two distinct states of, indicated, 106. + + Structural characters, seldom variable in the Insecta, 95. + + Subsidences, the effect of, on insect life, 114. + + Swallow-Tail Butterfly, unaffected by climate, 31. + + _Syncalypta_, apterous in Madeira, 82. + + + _Tachyporus nitidicollis_, Steph., perhaps a state of _T. obtusus_, + 33. + + _Tarphii_, their economy in the Madeira Group, 121. + + _Tarphius_, its maximum attained in Madeira proper, 121; + common to Madeira and Sicily, 139. + + _---- gibbulus_, Germ., the Sicilian exponent of the genus, 123. + + _---- Lowei_, Woll., of a more adaptive nature than its allies, 122. + + _Tarus_, always apterous in Madeira, 82. + + _---- lineatus_, Schoenh., assumes a distinct state in Madeira, 65. + + _Telephorus testaceus_, Linn., its variability in Lundy Island, 59. + + Thompson, Mr., on the reptiles of Ireland, England, and Belgium, 136. + + _Thorictus_, common to Madeira and Sicily, 139. + + Tierra del Fuego, insects of, 47; + has many characters in common with Madeira, 48-51. + + Time, an important item in the question of modifications, 77. + + Toucans, their peculiarity to S. America and the W. Indies, 142. + + Transmutation-theory, unsoundness of the, 186-189; + how it took its rise, 190. + + _Trechus_, always apterous in Madeira, 82. + + _---- alticola_, Woll., perhaps a state of _T. custos_, 39. + + _---- lapidosus_, Daws., pallid hue of, 64. + + Tree-Porcupines, their peculiarity to S. America, 143. + + _Tribolium ferrugineum_, Fabr., unaffected by climate, 31. + + _Trogosita mauritanica_, Linn., unaffected by climate, 31. + + Tropics, exuberance of the, 27, 28; + relative proportions of the insect tribes within the, 28, 29. + + _Tychius_, always apterous in Madeira, 82. + + + Unity, indicated in the organic creation, 179, 180. + + + _Vanessa Atalanta_, Linn., has a different aspect in N. America, 34; + perhaps a recent introduction into Madeira, 74. + + _---- Callirhoe_, Fabr., smaller in Porto Santo than in Madeira, 73. + + Variation in the Insecta, a matter of experience, 7, 8, 15; + probable from analogy, 15; + perhaps indicated in every individual, 16, 17, 18; + restricted, 35. + + Variety, definition of the term, 4. + + _Velia_, on the development of the wings of, 100. + + + Waterhouse, Mr., his opinion concerning generic types, 172. + + Westwood, Mr., on _Papilio Machaon_ from the Himalayas, 32; + on American specimens of _Lycaena Phloe as_, 34; + on the effect of heat in developing the wings of insects, 44; + on a winged state of _Choreius ineptus_, 44; + on the development of the wings in _Delphax_, 45; + on a winged state of _Cimex lectularius_, 45; + on _Aphelocheirus aestivalis_, 100; + on the development of the wings of the _Hydrometridae_, 100; + on _Cimex apterus_, 100; + on _Prostemma guttala_ and _Coranus subapteras_, 101; + on the development of the wings of _Lygaeus brevipennis_, 101. + + Whewell, Dr., on the natural causes which science has to investigate, + 191. + + White-Cabbage Butterfly, varies in Nepaul and Japan, 34. + + Winds, the effects of, in the diffusion of insects, 148. + + Wings of insects, subject to undue development in hot seasons, 43; + liable to become gradually obsolete in islands, 81; + more variable than other organs, 97. + + + _Xenostrongylus_, its geographical distribution, 124; + common to Madeira and Sicily, 139. + + + _Zargus pellucidus_, Woll., variety of, on one of the Madeira Islands, + 88. + + +FINIS. + + + + +Printed by Taylor and Francis, Red Lion Court, Fleet Street. + + +Lately published, by the same Author, in large 4to (with Thirteen +Coloured Plates), price L2 2_s._, + + + INSECTA MADERENSIA; + BEING + AN ACCOUNT OF THE INSECTS + OF + THE ISLANDS + OF + THE MADEIRAN GROUP. + + +London: JOHN VAN VOORST, 1, Paternoster Row. + + + + +Transcriber's Notes: + + +Inconsistent/archaic spelling and punctuation left as in original. + + + + + + +End of the Project Gutenberg EBook of On the Variation of Species, with +Especial Reference to the Insecta ; Followed by an Inquiry into the Nature of Genera, by Thomas Vernon Wollaston + +*** END OF THIS PROJECT GUTENBERG EBOOK VARIATION OF SPECIES *** + +***** This file should be named 38584.txt or 38584.zip ***** +This and all associated files of various formats will be found in: + http://www.gutenberg.org/3/8/5/8/38584/ + +Produced by Charlene Taylor, Bryan Ness, Matthew Wheaton +and the Online Distributed Proofreading Team at +http://www.pgdp.net (This file was produced from images +generously made available by The Internet Archive) + + +Updated editions will replace the previous one--the old editions +will be renamed. + +Creating the works from public domain print editions means that no +one owns a United States copyright in these works, so the Foundation +(and you!) can copy and distribute it in the United States without +permission and without paying copyright royalties. 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