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diff --git a/.gitattributes b/.gitattributes new file mode 100644 index 0000000..6833f05 --- /dev/null +++ b/.gitattributes @@ -0,0 +1,3 @@ +* text=auto +*.txt text +*.md text diff --git a/38584-8.txt b/38584-8.txt new file mode 100644 index 0000000..5b435db --- /dev/null +++ b/38584-8.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: ISO-8859-1 + +*** 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 + § 1. Climatal causes generally (whether dependent + upon 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 + + 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, _à priori_, to pronounce upon. + +The descriptive naturalist, whose primary object it is to register +what he sees (apart from the obscurer phænomena 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 _primâ-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 _à 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 _à 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 _à +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 _primâ facie_ appear to be the case: and +that, consequently, where opposite phænomena are displayed under +circumstances seemingly coincident, our first object should be (_not_ +to regard the phænomena 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 _à 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 phænomena 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 +phænomena 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 _primâ-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 minutiæ 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 _à 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 pleræque magis sunt lusus +naturæ, quam seriæ 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: illæ enim sunt, quæ 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 phænomena 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 _à +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 +phænomena 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. + + +§ 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 +phænomena which are due to the _creative_ force alone; yet we may, in +the contemplation of them, recognize such an amount of _design_, or a +primary adaptation to conditions from without, as shall afford, +through its permanence and method, fresh presumptive evidence that the +"conditions" _themselves_ may have some inherent modifying power of +their own on the aggressors from other districts, in which a contrary +influence may perchance prevail, and for the overspreading of which +they were, in the beginning, more peculiarly constituted and ordained. + +It has been already mentioned (and, despite the exceptional cases +which are to be found, it is in a _general_ sense true), that the +splendour and extravagance of the insect world attain their maximum +within the tropics; and that the nearer we approach the central heat, +the more and more unmistakeable is the existence of this law. It has +been also hinted, that when viewed on a very extensive scale, +we shall not derive much _direct_ assistance (whilst examining +insect-variation, with reference to climate) from the consideration of +a fact thus seemingly important,--since there are but few species +whose range is so comprehensive as to embrace, at the same time, the +equatorial and temperate regions of the earth; and since, as lately +suggested, it is not from a comparison of the _aborigines_ of +countries far removed that we can hope to derive much positive +information during our present inquiry. It may be useful however to +speculate, why the creative energy should have been thus lavished, as +it were, in the torrid zone, whilst the fauna of the cold north is so +unpretending and sombre. I believe that in the actual _number_, both +of individuals and species, which they contain, the difference is not +so great, between the two latitudes, as might be imagined; and that, +were the minims of Scandinavia to be suddenly magnified into the +giants of Brazil, the Laplanders and Swedes might stand a fair chance +of being temporarily alarmed: nevertheless, as regards the multitude +and eccentricity of her forms, there can be no question in which field +it is that Nature has ever delighted more particularly to sport. + +Laying aside, therefore, the numerical statistics from our account, is +not the exuberance of the tropics at once responsive to the conditions +imposed upon them? Do we ask why it is that the insect population is +there moulded upon a type comparatively so colossal?--let the +redundancy of the vegetation reply. Have not, also, more rapid laws of +putrefaction and decay been prescribed than in our cooler clime; and +can we imagine that it was _not_ in obedience to this decree, that +larger and more active scavengers were framed? The gaudy wings that +float idly on the breeze, and the coats of mail which glitter in the +light, have they nothing to tell of the local circumstances around +them; or, is it too much to infer, that a more glorious and +stimulating sun required creatures of superior brilliancy to bask in +its rays? A moderate degree of heat, and that only during a certain +portion of the year, may suffice in quiescent regions to keep up the +equilibrium of the organic world, the various members of which, +whether animals or plants, are ensured, in such countries, their +alternate seasons of activity and rest; but within the tropics, life, +in all its aspects, is ever vigorous; and, though the several species +may have their appointed times of partial repose, there is no such +thing as tranquillity for the mass. Hence it is, that to meet the +requirements of a Flora[5] such as there obtains, a less magnificent +Fauna would have been inadequate; and we cannot but recognize, that, +in the wonderful and almost endless modifications of the insect tribes +which people those zones, a special provision has been made to check +the overgrowth of other created things. + +But how, it may be asked, does this _primary adaptation_ to external +conditions affect the question of specific development? Perhaps not +much: nevertheless, as lately urged, it is well that such adaptations +should be borne in mind, not merely that due importance may be given +to influences in conformity with which the creative act was at the +first expressly regulated; but also that we may be prepared, if any +qualifying power be admitted to reside in those influences themselves, +for the _kind_ of aberration which reason and experience would seem +alike to imply that we should, in the various instances, anticipate. + +We have already stated, that climate, when taken alone, does not +appear to produce any very decided modifying effect on insect form, +seeing that there are vast numbers of species of a wide geographical +range which do not display, on their northern and southern limits, +differences sufficiently constant to be regarded as purely climatal +ones; and it is clear that, if climatal causes of themselves were of +real primary significance, we should probably seldom fail to trace +out, from their long-continued operation, some steady and positive +result. Yet when combined with other principles, there is evidence +that a considerable amount of influence must be conceded to the action +of mere heat and cold, working permanently and according to fixed +laws, on the members of the insect world. Such being the case, it is +perhaps not surprising that a slight difficulty should arise, through +our employment of separate sections under which to examine the causes +of variation; for, since it is ordinarily by the union of several +disturbing influences that aberrations are brought about, it is for +the most part impossible, to refer the results, however conspicuous +they may be, to a solitary controlling element. And hence, though we +may be able at times to point out perchance the _single_ reason for +certain phænomena with comparative precision, it will generally happen +that two or three agents must be appealed to before we can arrive at a +conclusion by any means satisfactory. I would desire, therefore, that +the examples hereafter to be noticed may be judged of in the mass; and +may not be considered as severally assigned, of necessity, to an +isolated deranging cause, through the fact of their being placed, for +the sake of convenience, and because of the _predominance_ which +special controlling principles have had in maturing them, under +sections, both, as it were, exclusive and particular. + +That climate of itself possesses but a limited modifying power on +insect development, is evident from the consideration (just alluded +to), that numerous species of comparatively wide distribution are +totally unaffected by it. Thus, for instance, the _Pissodes notatus_, +Fab., a weevil which occurs in pine forests from Lapland to Barbary, +and which has been naturalized even in the Madeira Islands, passes +through the alternations to which it is specifically subject, +irrespective of country. In like manner, the _Lixus angustatus_, Fab., +so abundant in Central and Southern Europe, the north of Africa, +Malta, Madeira, and the Canaries, and which has been detected in +Persia, would seem to be perfectly free from atmospheric control. The +_Coccinella 7-punctata_, Linn., which exists in nearly every portion +of the Old World, is apparently unacted upon geographically. +Numberless beetles which follow in the track of man, or at any rate +are liable to do so, almost everywhere (such as _Carpophilus +hemipterus_, Linn., _Trogosita mauritanica_, Linn., _Læmophloe us +pusillus_, Schönh., _Dermestes vulpinus_, Fab., _Anobium striatum_, +Oliv., _Rhizopertha pusilla_, Fab., _Sitophilus granarius_ and +_Oryzæ_, Linn., and _Tribolium ferrugineum_, Fab.), show little or no +tendency to variation. Nor is this independence of climate to be +observed less frequently in the aquatic forms, than in the terrestrial +ones: the _Agabus bipustulatus_, Linn., common in the streams and +pools of the whole of Europe, the north of Africa, and in Madeira, +although naturally somewhat inconstant, offers no aberration, _the +result of latitude_; as is equally the case with the _Hydroporus +confluens_, Fab., which is found from Sweden to the Canaries, and the +_Eunectes sticticus_, Linn.,--an insect literally cosmopolitan. The +Swallow-Tail Butterfly (_Papilio Machaon_, Linn.), the Clouded Yellow +(_Colias Edusa_, Fab.) and the Painted Lady (_Cynthia Cardui_, +Linn.),--the first and second of which occur throughout Europe, in +Siberia, Syria, Egypt, Barbary, Nepaul, and Cashmere; whilst the third +(so general in our own country) has been recorded from India, North +America, the Brazils, Africa, Java, and New South Wales,--however +irregular they may be, afford no indications[6] of undoubted +geographical instability. + +We need not however multiply examples, since our space will scarcely +admit of it, and numbers of them will be at once suggested to the +entomologist: what it mainly concerns us here to corroborate, is the +thesis, _that climatal operation_, although by no means invested with +a universal qualifying power, _has an amount of influence on certain +species, even whilst unconnected with other elements,--and therefore_, +à fortiori, _when in combination with them_. + +The two principal conditions on which climatal causes generally may be +said to rest, are latitude and altitude. As regards the former of +these, however, whilst the equatorial and arctic regions of the earth +will of course give us the extremes of heat and cold, we shall often +perceive differences of temperature (the result perhaps of local +circumstances) in areas but slightly removed from each other, +sufficient to affect very materially, though by what means it is +difficult to understand, the outward contour of the insect tribes. +Thus, to go no further than Ireland, we find that the specimens of +_Silpha atrata_, Linn., so abundant throughout England and the whole +of Europe, have put on (it may be from the moisture of the atmosphere, +or from some other obscure influence) the appearance of a distinct +race,--so distinct indeed as to have long received another name, _S. +subrotundata_, from British naturalists. I think it far from +improbable that the _Tachyporus nitidicollis_, Steph., an insect +eminently characteristic of that country (and one on which I have +lately offered some remarks[7]), is but a darker climatal modification +of the common _T. obtusus_: and it is well known that the examples of +_Pelophila borealis_, Payk., from Killarney and Loch Neagh are +permanently larger, and much more metallic, than those from the +Orkneys. The _Nebria complanata_, Linn., assumes a more pallid hue in +the neighbourhood of Bordeaux than it does on the sandy coasts of +Devonshire and Wales: and I have but little doubt that the _Omaseus +nigerrimus_, Dej., of Spain, the north of Africa, and Madeira, is a +geographical state of the _O. aterrimus_ of Central Europe. The +_Sitona gressoria_, Illig., so universal throughout the Mediterranean +districts, Madeira and the Canaries, may be but the subaustral form of +_S. grisea_. The _Bembidium obtusum_, Sturm, is shorter and less +parallel in our own latitude than it is in the Madeiran group and +along the Mediterranean shores: whilst the _Holoparamecus niger_, +Aubé, of Madeira and Sardinia is very much paler than the same beetle +when taken in Sicily. Specimens of _Pieris Brassicæ_, Linn. (the White +Cabbage-Butterfly,--an insect of widely acquired range), from Nepaul +and Japan, are recorded[8] to have differed so strongly from the +ordinary European type as to have been referred, by Boisduval, in +doubt to that species. Mr. Westwood has received the _Vanessa +Atalanta_, Linn., from North America, receding slightly from its +British analogue; but which he, nevertheless, does not regard as +specifically distinct: and such also (he adds) was the opinion of Mr. +Kirby, who has described his American examples under that name. The +common _Hipparchia_ of Madeira I believe to be a fixed geographical +modification of the _H. Semele_, Linn., of our own country,--in which +the paler bars of the upper surface are evanescent;--there are, +however, I imagine, but few entomologists who would concur with me in +this hypothesis. The Madeiran specimens of _Lycæna 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 _à +priori_ inclination to endorse. It must be recollected however that, +in the instances to which we would draw attention, _small_ aberrations +are all that can be usually looked for, since climate _of itself_ does +not appear to be very potent in its action. We should remember, also, +that the boundaries of insect instability are restricted; and, +although we would advocate freedom of development within limits which +are more or less comprehensive according to the species, to pass +beyond them would be confusion, and such as could result from a +_lapsus Naturæ_ only, rather than from a power of legitimate +variation. + +In exact conformity with what the above remarks will have prepared us +for, we find that the _Dromius obscuroguttatus_, Dufts., of Central +Europe, has undergone on the mountain summits of Madeira changes +precisely to that extent which we should have calculated upon; and +although they would seem in reality to be referable to climate _and +isolation_ combined, yet, since it is not always possible (as lately +stated) to treat the elements of disturbance separately, and it is my +object in this short treatise to bring forward a few prominent +examples in support of the considerations proposed, rather than to +accumulate a mass of material for the registry of which my space would +be inadequate, I will quote _in extenso_ the reflections which, during +the compilation of the 'Insecta Maderensia,' suggested themselves to +me. "The _Dromius obscuroguttatus_ is a common European insect, and +the Madeiran specimens recede from the ordinary ones in being slightly +larger, and in having their elytra more obscurely striated, with the +humeral patch less distinct: their entire surface, moreover, is of a +deeper black, a difference which is especially perceptible on the +legs. It occurs in the greatest profusion in Madeira proper, though +only from about 5000 to 6000 feet above the sea. Although so common +throughout Europe, it is perhaps, when geographically considered, one +of the most interesting of the Madeiran Coleoptera, as affording a +striking example, not only of the modification of form in a normally +northern insect when on its southern limit, but as showing likewise +how a species, abundant on the low sandy shores and sheltered +sea-cliffs of more temperate regions, finds its position here only on +the summits of the loftiest mountains. It is true that the aberration +from the typical state is not in the present instance very +considerable; yet when the circumstances producing it are taken into +account, I am persuaded that the difference is exactly of that nature +on which too great stress cannot possibly be placed, when discussing +the general question of geographical distribution as having a +tendency, more or less directly, to affect both colour and form. It is +well known to naturalists that a multitude of insects from the New +World, receding from their European analogues merely in certain +excessively minute characters, have usually been pronounced at once as +new to science, first because those differences are constant, and +secondly because the specimens have been received from the other side +of the Atlantic. And yet in instances like the present one,--in an +island which, while it belongs artificially to Europe, is yet +naturally sufficiently distinct from it as to form at any rate a +stepping-stone to the coast of Africa and the mountains of +Barbary,--species similarly circumstanced are not necessarily received +as new (and rightly so, I apprehend), though in every respect +affording differences not only _analogous_ to those already mentioned, +but in many instances positively identical with them. If, however, a +specific line of demarcation does of necessity exist between the +creatures of the Old and New Worlds, the problem yet remains unsolved, +so long as intermediate islands present parallel modifications, where +that line is to be drawn. Meanwhile, how far geographical varieties of +this kind, concerning the non-specific claims of which confessedly but +little doubt can exist, may lead to the explanation of the +Transatlantic ones just referred to, I will not venture to suggest. +Yet certain it is, that the one case bears directly on the other; and +that, if we can prove that common European insects, when isolated in +the ocean, become in nearly all cases more or less modified externally +in form, there is at least presumptive evidence that the law will hold +good on a wider scale, and may be extended, not only to the Atlantic +itself, but even to countries beyond. The differences of the present +_Dromius_ from its more northern representatives are, as just stated, +small; nevertheless, since they are _fixed_, those naturalists who do +not believe in geographical influence might choose to consider them of +sufficient importance to erect a new species upon. But after a careful +comparison of this with other insects similarly circumstanced, I am +convinced that the modifications in question are merely local ones, +and such as may be reasonably accounted for by the combined agencies +of latitude and isolation, and the consequently altered habits of the +creature, which is thus compelled to seek alpine localities in lieu of +its natural ones[10]." + +In like manner the _Calathus fuscus_, Fab., the _Anchomenus +marginatus_, Linn., and the _Anthicus fenestratus_, Schmidt, which +occur almost exclusively in the _lower_ regions of northern +latitudes, are found in Madeira on the mountain tops; each, moreover, +possessing characters which are just sufficient (although slight) to +distinguish them from their European representatives. + +And if we inquire, on the other hand, into the _aboriginal_ species of +those islands,--or, at any rate, into such of them whose naturally +acquired range embraces the opposite extremes of atmosphere,--we shall +detect no less surely (albeit within a narrower space) the result of +climatal action on insect form. The _Helops confertus_, Woll., "varies +according to the altitude at which it is found; being usually deeply +striated and rugose on its lower, but subpicescent and much more +lightly sculptured on its upper limits. I have taken specimens indeed +on Pico Ruivo, and on the mountain-plain of the Fateiras, which are so +far diminished in roughness as almost to resemble, at first sight, the +_H. Pluto_[11]." The _Pecteropus Maderensis_, Woll., which ranges from +about 2500 feet above the sea to the summits of the loftiest hills, +although usually with pale legs, is distinguished by having its femora +almost invariably dusky when on its highest elevation; and, following +out the analogy with that beetle, the _Trechus alticola_, Woll., +should perhaps be regarded as an alpine state of the _T. custos_. The +_Calathus complanatus_, Koll., assumes along the upland heights a very +different aspect to what it does in the regions below, being generally +more piceous and convex, altogether broader (in proportion) and +shorter, and with _both_ sexes (though, of course, especially the +male) shining. + +Nor is this principle of topographical variability (the result of +climate) less apparent in other countries also. The _Notiophili_, for +instance, "are extremely unstable, both in their sculpture and hue, +being subject to considerable local modifications, though more +particularly affected, it would appear, by altitude. Thus, in our own +country, the _N. semipunctatus_, Fab., one of the common +representatives of the plains, is found likewise on the summits of the +mountains; but at that elevation it becomes liable to great +alternations of colour, ranging from pale brassy-brown, with the apex +testaceous, into deep black. The sculpture, however, perhaps is nearly +as much dependent on other circumstances for its modification as upon +altitude, since it seems tolerably clear that proximity to the +sea-shore, especially where the localities are saline, will frequently +produce a more faintly impressed surface[12]." It has indeed been +lately suggested, that the _Helobia nivalis_, Payk., may be perhaps, +after all, but a mountain variety of the _H. brevicollis_; the +_Leistus montanus_, Steph., of the _L. fulvibarbis_, and the _Patrobus +septentrionis_, Dej., of the _P. excavatus_; but of this I think +further proof is needed, seeing that certain species do appear to +exist which are _strictly_ alpine (that is to say, which have not +been, severally, detected in the lower regions of more northern +zones); and, in _most_ instances, where aberrations are to be met +with from the effect of _altitude_, we have a right to inquire +(provided the types from which they are supposed to have originally +sprung obtain in the less-elevated portions of the same country), +_where are the intermediate links_? Now I am not aware that any such +links have, in the examples above cited, ever been observed; whilst I +can vouch that in at any rate many districts where the _quasi_ variety +is found, the descendants of its assumed progenitor _do_ occur in the +plains beneath. I have remarked that the _Cicindelidæ_ often become +inconstant in colouring as they approach their maximum of +height above the sea; and I have but little doubt that the _C. +fasciatopunctata_[13], Germ., from Asia Minor and Turkey, is the _C. +sylvatica_ modified by a long residence in elevated regions. And so it +is with the _Chrysomelæ_, many of which become, in the loftiest +altitudes to which they ascend (as I have noticed at the head of the +St. Gotthard Pass of the Swiss Alps), subject to unusual changes, both +in lustre and hue. + +The above examples, although few and indiscriminately selected, will +serve to illustrate the principle which we have been contending +for,--that climatal influences generally, may (and in most instances +do) tend to affect, more or less directly, the outward contour of the +insect tribes. It will be remarked that, in the cases hitherto cited +no great disturbing power has been made evident,--the aberrations to +which we have appealed being, most of them, comparatively minute. +This, however, is simply in harmony with the belief which we have +already expressed, that climatal causes, when taken singly and alone, +are not of primary importance whilst discussing the question of +specific modification. It remains for us, in the following sections, +to inquire, whether there are any other elements at work from which +greater results are to be expected. Meanwhile, let us not forget that +differences _may_ be, in the strictest sense, significant, even whilst +small; and that it is their _constancy_, rather than their magnitude, +which more particularly concerns us in the present treatise, seeing +that it is with reference to those distinctions which are less +conspicuous that the greatest amount of misunderstanding (through the +fact of their being _fixed_) usually prevails; whilst it is our main +object to show that dissimilarities do not _necessarily_ imply the +specific isolation of the creatures which display them, merely because +they are, in their several localities, _permanent_. + + +§ 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 _Carabidæ_) 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 (_Chalcididæ_), +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 _Fulgoridæ_, +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. + + +§ 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 phænomena 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 _Harpalidæ_ and _Heteromera_, living beneath stones. The +vegetable-feeding _Chrysomelidæ_, 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 phænomena 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 phænomenon; 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, _à 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 +_Gymnaëtron Campanulæ_, 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 _Gymnaëtron_, 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. Veronicæ_, 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. Veronicæ_ 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 Ferté observes: "Il y a sculement lieu +de remarquer que les individus du bord de l'océan sont généralement +plus pâles que ceux des contrées orientales de l'Europe, et que ceux +des côtes de France et de Belgique sent entièrement dépourvus de tache +discoïdale[27]." And bearing, in much the same manner, on the subject +of variations, the _Anthicus humilis_, Germ., "est une des espèces le +plus généralement répandues en Europe; mais il lui faut le voisinage +de l'eau salée. Aussi on le rencontre non-seulement sur les rivages de +toutes les mers, même de la Baltique, mais encore aux bords des lacs +salés, tels que celui de Mannsfeld, en Saxe. _Ceux de cette dernière +localité sont généralement noirs_; ceux que j'ai pris à Perpignan sont +d'un rouge très-clair, ce qui me porte à croire que cette espèce est +dans le même cas que quelques autres _Anthicus_, dont les variétés les +plus foncées appartiennent au nord de l'Europe, et les plus pâles 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 phænomena 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_, _Aëpus 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_, Schönh., +is slightly shorter in Madeira, as also somewhat darker on its head +and prothoracic disk (and with its elytral striæ 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 +_Scydmænus 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 fasciæ, 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, _à 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 antennæ 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]." + + +§ 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 phænomena 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 _Curculionidæ_, +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 Callirhoë_, 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, _à 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 Chão. 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 Chão to somewhat less than half a line in length, being +literally of scarcely greater magnitude than some of the larger +representatives of the _Ptiliadæ_!"[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 _à 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, _primâ +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_ phænomena 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 phænomena +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 _à 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 phænomena (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. +Callirhoë_), 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 phænomena 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 aërial 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 Chão, the _Scarites abbreviatus_, Koll., +_Laparocerus morio_, Schön., 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_, Schön., _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_ phænomena, 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 causâ_ 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 phænomena, 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 _Carabidæ_," 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 _Chrysomelidæ_, 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. Callirhoë_ 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 phænomenon, I should have concluded myself: +nevertheless, recollecting how easy of transport the larvæ and pupæ of +Lepidoptera necessarily are (of which we have the plainest assurance +in the almost certain introduction of the _Pontia Brassicæ_, _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 _Callirhoë_ (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_ antennæ 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 _quæstio 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 aërial 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 quâ 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 æstivalis_ (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 pupæ or distinct species, +but as undeveloped specimens in the imago state[47]." And whilst +discussing the _Hydrometridæ_, 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 +_Lygæidæ_) "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]". _Lyæus +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_ phænomenon 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 arrêté le développement des organes +du vol[51]." And, again, when commenting upon the other tendency in a +representative of the _Reduviadæ_, he says ('Essai,' p. 96): "Je pense +que la présence des ailes et leur développement dépendent 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 +Chão) 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 _à 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 +_à 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 minutiæ 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 +striæ 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 _Pupæ_ 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 phænomena 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 +phænomena 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 phænomena 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 phænomena 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 primæval 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 primæval 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 _à 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_, Schön., 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 minutiæ, 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 +primæval 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 primæval 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 _début_ 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 versâ_); yet, with the analogy of +the three already mentioned before us, I am inclined _à 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 primæval 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 primæval radiating +forms. + +We must not however omit to notice, that some few of these endemic +_Helices_ appear to have been gifted (as we should _à 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 Caniçal),--and the _H. saccharata_, from the +São Lourenço promontory. + +In the same way we might pursue the _H. erubescens_, and show that in +the sylvan regions, and on the low barren Ponta São Lourenço of +Madeira, on the Pico de Facho of Porto Santo, on the Ilheo Chão, 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 phænomena, +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 phænomena 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, _primâ 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 phænomena +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 phænomena. + +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 primæval +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 phænomena 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 _Scarabæus_. 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 phænomena 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 primæval 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 _à priori_ bound to do), we must at least act consistently with +ourselves, and not anticipate phænomena 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 phænomena 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 +Caniçal 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 _Insulæ +Fortunatæ_ of Juba are almost universally identified with the present +Canarian Group (as indeed the accurate description of Pliny well nigh +demonstrates), and the _Purpurariæ_ 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 phænomena (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 phænomena 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 +personæ_, 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 'Méthode 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 'Méthode +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 _à 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 'Méthode 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 'Méthode 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 _à 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 _Carabidæ_, +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 Linnæan 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 primæval +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 +naturæ 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] Considérations sur un Nouveau Système 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 + +Depositâ sarcinâ, 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 _primâ-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 primæval 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 _à 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 primæval 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, +_à 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 minutiæ 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 +phænomenon, 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 _début_ 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 _à 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. + + _Aëpus marinus_, Ström., 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 'Méthode 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. + + Antennæ, 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 æstivalis_, 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. + + _Carabidæ_, 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. + + _Chrysomelæ_, vary from altitude, 41. + + _Chrysomelidæ_, almost absent in Tierra del Fuego, 47. + + _Cicindela fasciatopunctata_, Germ., a state of _C. sylvatica_ 41. + + _Cicindelidæ_, 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. + + _Gymnaëtron_, blood-red dashes characteristic of, 62. + + _---- Campanulæ_, Linn., its smallness on the Cornish coast, 58. + + _---- Veronicæ_, 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_, Fér., 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_, Küst., 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_, Aubé, 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. + + _Hydrometridæ_, 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. + + _Insulæ Fortunatæ_ 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. + + + _Læmophloe us pusillus_, Schönh., unaffected by climate, 31. + + _Lamprias chlorocephalus_, Ent. H., two distinct sizes of, frequently + indicated, 105. + + _Laparocerus morio_, Schönh., 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. + + _Lycæna 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. + + _Lygæus 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. + + 'Méthode 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. + + _Phlæophagus_, 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 Brassicæ_, 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 Brassicæ_, 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. + + _Purpurariæ_ 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. + + _Reduviadæ_, 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. + + _Scarabæus_, 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. + + _Scydmænus 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 oryzæ_, 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 _Reduviadæ_, 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_, Schönh., 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. + + _---- Callirhoë_, 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 _Lycæna 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 æstivalis_, 100; + on the development of the wings of the _Hydrometridæ_, 100; + on _Cimex apterus_, 100; + on _Prostemma guttala_ and _Coranus subapteras_, 101; + on the development of the wings of _Lygæus 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 £2 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 38584-8.txt or 38584-8.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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Vernon Wollaston, M.A., F.L.S. + </title> + + <style type="text/css"> + + body { + margin-left:10%; + margin-right:10%; + } + + div.toc { + width:30em; + margin-left:auto; + margin-right:auto; + text-align:left; + } + + div.inset14, div.inset22 { + margin-top:1em; + margin-bottom:1em; + margin-left:auto; + margin-right:auto; + } + + div.inset14 { + width:14em + } + + div.inset22 { + width:22em + } + + div.inset14 p, p, div.inset22 { + text-indent:0; + } + + .footnote { + font-size:0.9em; + margin-right:10%; + margin-left:10%; + } + + .footnote .label { + position:absolute; + right:84%; + text-align:right; + } + + .footnotes { + border:dashed 1px; + } + + .fnanchor { + vertical-align:super; + font-size:.8em; + text-decoration: + none; + } + + .h1 { + font-size:2em; + margin:.67em 0; + } + + .h1, .h2, .h3, .h4, .h5, .h6 { + font-weight:bolder; + text-align:center; + text-indent:0; + } + + h1, h2, h3, h4, h5, h6 { + text-align:center; + } + + .h2 { + font-size:1.5em; + margin:.75em 0; + } + + .h3 { + font-size:1.17em; + margin:.83em 0; + } + + .h4 { + margin:1.12em 0 ; + } + + .h5 { + font-size:.83em; + margin:1.5em 0 ; + } + + h5 { + margin-bottom:1%; + margin-top:1%; + } + + .h6 { + font-size:.75em; + margin:1.67em 0; + } + + hr.chap { + margin-top:6em; + margin-bottom:4em; + clear:both; + } + + hr.tb { + margin:2em 25%; + width:50%; + } + + p { + text-align:justify; + margin-top:.75em; + margin-bottom:.75em; + text-indent:0; + } + + p.index { + margin:0; + padding-left:3em; + text-indent:-3em; + } + + p.right { + text-align:right; + } + + p.right1 { + text-align:right; + margin-right:5em; + margin-bottom:-.8em; + } + + p.right2 { + text-align:right; + margin-right:10em; + margin-bottom:-.8em; + } + + p.spacer { + margin-top:2em; + margin-bottom:3em; + } + + .pagenum { +/* visibility:hidden; remove comment out to hide page numbers */ + position:absolute; + right:2%; + font-size:75%; + color:gray; + background-color:inherit; + text-align:right; + text-indent:0; + font-style:normal; + font-weight:normal; + font-variant:normal; + } + + .smcap { + font-variant:small-caps; + } + + span.m2 { + font-size: .8em; + } + + span.m3 { + font-size: .5em; + } + + span.m4 { + font-size: .3em; + } + + span.in2 { + margin-left:2em; + } + + span.in3 { + margin-left:3em; + } + + span.right { + float:right; + } + + .trnote { + margin-left:15%; + margin-right:15%; + margin-top:5%; + margin-bottom:5%; + padding:1em; + background-color:#f6f2f2; + color:black; + border:1px dotted black; + } + + ul.index {list-style-type: none;} + li.ifrst {margin-top: 2em;} + li.indx {margin-top: .5em;} + li.isub1 {text-indent: 1em;} + + </style> + +</head> + +<body> + + +<pre> + +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: ISO-8859-1 + +*** 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) + + + + + + +</pre> + + +<h1><span class="m3">ON<br /></span><br /> + +THE VARIATION OF SPECIES<br /> + +<span class="m4"><br />WITH ESPECIAL REFERENCE TO<br /></span><br /> + +<span class="m2">THE INSECTA;</span><br /> + +<span class="m4"><br />FOLLOWED BY<br /></span><br /> + +<span class="m3">AN INQUIRY INTO<br /></span><br /> + +THE NATURE OF GENERA.</h1> + +<p class="h3"><br />BY<br /> +T. VERNON WOLLASTON, M.A., F.L.S.</p> + +<p class="spacer"> </p> + +<div class="inset14"> +<p>"No compound of this earthly ball<br /> +Is like another, all in all."<br /> +<span class="right smcap">Tennyson.</span></p> +</div> + +<p class="spacer"> </p> + +<p class="h4">LONDON:<br /> +JOHN VAN VOORST, PATERNOSTER ROW.<br /> +1856.</p> + +<p class="spacer"> </p> + +<p>"I do not enter so far into the province of the logicians as to take notice of the difference +there is between the <i>analytic</i> and <i>synthetic</i> 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 <i>proper media</i> 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."—<i>Religion +of Nature Delineated</i>, p. 45 (<span class="smcap">a.d.</span> 1722).</p> + +<p class="spacer"> </p> + +<p>PRINTED BY TAYLOR AND FRANCIS, RED LION COURT, FLEET STREET.</p> + +<p class="spacer"> </p> + +<p class="h3">TO<br /> +<br /> +CHARLES DARWIN, <span class="smcap">Esq.</span>, M.A., V.P.R.S.,</p> + +<p>Whose researches, in various parts of the world, have added +so much to our knowledge of Zoological geography,</p> + +<p class="right2">this short Treatise</p> +<p class="right1">is dedicated.</p> + +<hr class="chap" /> + +<p><span class="pagenum"><a id="Page_v">[Pg v]</a></span></p> + +<h2>PREFACE.</h2> + +<p>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.</p> + +<p>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 +<span class="pagenum"><a id="Page_vi">[Pg vi]</a></span> +on the volume a more practical interest, for the +general naturalist.</p> + +<p>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.</p> + +<p>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.</p> + +<p>10 Hereford Street, Park Lane, London.<br /> +May 10th, 1856.</p> + +<hr class="chap" /> + +<p><span class="pagenum"><a id="Page_vii">[Pg vii]</a></span></p> + +<div class="toc"> + +<h2>CONTENTS.</h2> + +<p class="h4">CHAPTER I. +<span class="right m2">Page</span><br /></p> + +<p>Introductory Remarks<span class="right"><a href="#Page_1">1</a></span></p> + +<p class="h4">CHAPTER II.</p> + +<p>Fact of Variation<span class="right"><a href="#Page_7">7</a></span><br /> +<span class="in2">As a matter of experience</span> + <span class="right"><a href="#Page_8">8</a></span><br /> +<span class="in2">As probable from analogy</span> + <span class="right"><a href="#Page_10">10</a></span></p> + +<p class="h4">CHAPTER III.</p> + +<p>Causes of Variation<span class="right"><a href="#Page_19">19</a></span><br /> +<span class="in2">§ 1. Climatal causes generally (whether dependent</span><br /> +<span class="in3">upon latitude or upon altitude)</span> + <span class="right"><a href="#Page_23">23</a></span><br /> +<span class="in2">§ 2. Temporary heat or cold, of an unusual degree</span> + <span class="right"><a href="#Page_42">42</a></span><br /> +<span class="in2">§ 3. Nature of the country, and of the soil</span> + <span class="right"><a href="#Page_46">46</a></span><br /> +<span class="in2">§ 4. Isolation; and exposure to a stormy atmosphere</span> + <span class="right"><a href="#Page_70">70</a></span></p> + +<p class="h4">CHAPTER IV.</p> + +<p>Organs and Characters of Variation<span class="right"><a href="#Page_95">95</a></span></p> + +<p class="h4">CHAPTER V.</p> + +<p>Geological Reflections<span class="right"><a href="#Page_111">111</a></span></p> + +<p class="h4">CHAPTER VI.</p> + +<p>The Generic Theory<span class="right"><a href="#Page_157">157</a></span></p> + +<p class="h4">CHAPTER VII.</p> + +<p>Conclusion<span class="right"><a href="#Page_181">181</a></span></p> +</div> + +<hr class="chap" /> + +<p><span class="pagenum"><a id="Page_viii">[Pg viii]</a></span></p> + +<h2>CORRIGENDUM.</h2> + +<p class="h4">Page 90, for <i>Pecteropus Maderensis</i> read <i>Pecteropus rostratus</i>.</p> + +<hr class="chap" /> + +<p><span class="pagenum"><a id="Page_1">[Pg 1]</a></span></p> + +<p class="h2">SPECIFIC VARIATION<br /> +<br /> +<span class="m2">IN THE</span><br /> +<br /> + INSECTA.</p> + +<hr class="chap" /> + +<h2>CHAPTER I.<br /> +<br /> +<span class="m2">INTRODUCTORY REMARKS.</span></h2> + +<p>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<span class="pagenum"><a id="Page_2">[Pg 2]</a></span> +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, <i>à priori</i>, to pronounce upon.</p> + +<p>The descriptive naturalist, whose primary object it is +to register what he sees (apart from the obscurer phænomena +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 <i>circumstances</i> 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, <i>inter alia</i>, to +throw out a few general hints; first, on the fact of aberration,<span class="pagenum"><a id="Page_3">[Pg 3]</a></span> +as a mere matter of experience; and, secondly, +on some of the <i>causes</i> to which the physiologist would, +in many instances, endeavour to refer it.</p> + +<p>The <i>former</i> of these considerations (namely, the <i>fact</i> +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 <i>latter</i> 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 <i>are</i> 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)<span class="pagenum"><a id="Page_4">[Pg 4]</a></span> +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.</p> + +<p>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 <i>former</i> to involve that ideal <i>relationship +amongst all its members</i> which the descent +from a common parent can alone convey: whilst the +<i>latter</i> 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 <i>appear</i>, at first sight, to be distinct from it.</p> + +<p>The <i>first</i> 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<span class="pagenum"><a id="Page_5">[Pg 5]</a></span> +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 +<i>postulate, assumed to illustrate the doctrine of species</i>, +than as a problem capable of satisfactory demonstration.</p> + +<p>The <i>second</i> 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 <i>primâ-facie</i> elements of stability,—and to +an extent moreover that, without the intermediate links +(which, although rarer than the variety itself, <i>must +nevertheless exist</i>) 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 <i>extreme</i> aberration is of more common +occurrence than the intermediate ones. Here then is a +case in point: there is but a <i>single</i> 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<span class="pagenum"><a id="Page_6">[Pg 6]</a></span> +variety, <i>every</i> separate degree of colour which could +possibly be found between the outer limits,—seeing that +(increasing, as they did, in an even ratio) no <i>one</i> could +be tabulated in preference to another.</p> + +<p>This however is an example in which the rate of alteration +(so far as colour is concerned) is <i>equal</i>; and one +therefore in which the extreme end of the series can be +alone singled out as <i>the</i> 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.</p> + +<p>As a corollary arising out of these remarks, it would +seem to follow that even small differences <i>should be regarded +as specific ones</i> 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 <i>abrupt</i> modifications may be <i>à priori</i> 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.</p> + +<hr class="chap" /> + +<p><span class="pagenum"><a id="Page_7">[Pg 7]</a></span></p> + +<h2>CHAPTER II.<br /> +<br /> +<span class="m2">FACT OF VARIATION.</span></h2> + +<p>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, <i>variability</i> +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.</p> + +<p>The <i>fact</i> of aberration, to which we would briefly +allude in this chapter, requires but little comment; it is +patent <i>à priori</i>. As a matter of experience, every observer +who has spent a week in the field of Nature<span class="pagenum"><a id="Page_8">[Pg 8]</a></span> +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 <i>latter</i>, 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.</p> + +<p>In the vegetable world this proneness to variation is +self-evident; and botanists innumerable, who have investigated +the <i>causes</i> 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 <i>change</i> absolutely indispensable +to the existence of animal and vegetable life +upon the earth's surface, and that <i>variety of conditions</i> +by which is ensured a fitting abode for each kind of its +multifarious and diversified inhabitants."</p> + +<p>Whilst exploring the barren moor, or bleak upland<span class="pagenum"><a id="Page_9">[Pg 9]</a></span> +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 <i>à priori</i> 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.</p> + +<p>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,<span class="pagenum"><a id="Page_10">[Pg 10]</a></span> +why, we may ask, are the lower members of the animal +creation to be denied analogous effects from the same +causes?</p> + +<p>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 <i>senses</i> 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.</p> + +<p>"Creation," says one of our most intelligent writers +of modern times, "<i>is full of analogies</i>, 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<span class="pagenum"><a id="Page_11">[Pg 11]</a></span> +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 <i>different</i> +effects may chance, therefore, to be occasionally brought +about by causes which may possibly <i>seem</i> 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 <i>inter se</i> 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 <i>liable</i> to be +differently compounded from what may <i>primâ facie</i> +appear to be the case: and that, consequently, where +opposite phænomena are displayed under circumstances +seemingly coincident, our first object should be (<i>not</i> to +regard the phænomena as indicative, that no constant +result can be anticipated from causes which are similar, +but), to inquire whether the circumstances in question +<i>are</i> 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.</p> + +<p>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<span class="pagenum"><a id="Page_12">[Pg 12]</a></span> +effects being observed (amongst the members of the +organic world) from conditions which <i>we assume to be</i> +co-ordinate, but which in fact are not so; we may, on +the other, run a similar risk (and thus fail to discern a +<i>corresponding modus operandi</i> in the maturation of like +results), from a mere <i>à priori</i> 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.</p> + +<p>"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 <i>the effect of extraneous disturbing +causes</i>. Thus furnished, he will be prepared to seize on +any of those minute indications which often connect +phænomena 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 <i>not</i> to happen; for these are +the facts which serve as clews to new discoveries<a id="FNanchor_1_1"></a><a href="#Footnote_1_1" class="fnanchor">[1]</a>."</p> + +<p>There can be no doubt that amongst a large proportion +of our naturalists, <i>differences</i>, 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 +<span class="pagenum"><a id="Page_13">[Pg 13]</a></span>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 phænomena 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 +<i>chief part of which</i> 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<a id="FNanchor_2_2"></a><a href="#Footnote_2_2" class="fnanchor">[2]</a>."</p> +<p><span class="pagenum"><a id="Page_14">[Pg 14]</a></span></p> +<p>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, <i>if</i> analogy with +creatures of a more perfect development be not altogether +disallowed us, during our researches into the insect +tribes, or <i>if</i> similar causes may be presumed to have +somewhat similar effects in opposite sections of the +<span class="pagenum"><a id="Page_15">[Pg 15]</a></span>animate world, an enlargement of our prescribed limits, +for specific variation, ought in reality to follow (sooner +or later) as an inevitable consequence.</p> + +<p>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 <i>it does, ipso facto, exist</i>. +"There is no similitude in Nature that owneth not <i>also +to a difference</i>;" 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.</p> + +<p>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<span class="pagenum"><a id="Page_16">[Pg 16]</a></span> +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.</p> + +<p>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,<span class="pagenum"><a id="Page_17">[Pg 17]</a></span> +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 +<i>may</i> exist likewise, despite our <i>primâ-facie</i> 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 <i>do</i>.</p> + +<p>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 minutiæ of each of its component<span class="pagenum"><a id="Page_18">[Pg 18]</a></span> +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 <i>à priori</i> though it be), that <i>individual +variations</i>, even to the extent which I have ventured to +suggest, are not incompatible with <i>specific similitudes</i>.</p> + +<div class="footnotes"><h3>FOOTNOTES:</h3> + +<div class="footnote"><p><a id="Footnote_1_1"></a><a href="#FNanchor_1_1"><span class="label">[1]</span></a> Preliminary Discourse on the Study of Natural Philosophy +(London, 1830), p. 132.</p></div> + +<div class="footnote"><p><a id="Footnote_2_2"></a><a href="#FNanchor_2_2"><span class="label">[2]</span></a> "Magna enim hucusque atque adeo curiosa fuit hominum industria, +in notanda rerum varietate, atque explicandis accuratis +animalium, herbarum, et fossilium differentiis; quarum pleræque +magis sunt lusus naturæ, quam seriæ 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: illæ enim sunt, quæ naturam uniunt, et constituere +scientias incipiunt."—<i>Novum Organum</i>, lib. ii. 27.</p> +</div></div> + +<hr class="chap" /> + +<p><span class="pagenum"><a id="Page_19">[Pg 19]</a></span></p> + +<h2>CHAPTER III.<br /> +<br /> +<span class="m2">CAUSES OF VARIATION.</span></h2> + +<p>"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 <i>particular cases</i> +(at least many of them) may have been provided for +without innovations in the course of Nature<a id="FNanchor_3_3"></a><a href="#Footnote_3_3" class="fnanchor">[3]</a>." 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.</p> + +<p>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 phænomena 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 +<span class="pagenum"><a id="Page_20">[Pg 20]</a></span>looked upon as a matter of course, and as therefore +beneath the notice of an intelligent mind; yet the man +who regards <i>truth</i> as valuable, for its own sake, under +whatever aspect it may come, and who can rise to the +appreciation of <i>results</i>, 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 <i>observation</i>," 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 <i>reason</i>. Both together may support opinion and +practice, in the absence of knowledge and certainty."</p> + +<p>In the last chapter we offered a few passing remarks +on insect-aberration generally, whether regarded as a +<i>universal fact</i> (which, however, even supposing such to +be true, it is not the object of the present treatise to +substantiate), or as an <i>occasional</i> one,—that is to say, +as existing at all times to that extent (as an hereditary<span class="pagenum"><a id="Page_21">[Pg 21]</a></span> +principle), that it is <i>liable</i> 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 <i>former</i> of these resultant conditions +(namely, that in which "varieties," technically so called, +though <i>more or less</i> isolated in their character, are apparent) +will be especially discussed; since my principal +desire is, to point out the influence of <i>local disturbing +causes</i> 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 <i>same</i> 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 +<i>à priori</i> 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 phænomena which should be constant (and +in some instances, also, conspicuous) <i>in proportion as</i> +the conditions which unite in bringing them about are<span class="pagenum"><a id="Page_22">[Pg 22]</a></span> +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 <i>may</i>, 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.</p> + +<p>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:—</p> + +<p>1. Climatal causes <i>generally</i> (whether dependent on +latitude or upon altitude).</p> + +<p>2. Temporary heat or cold, of an unusual degree.</p> + +<p>3. Nature of the country and of the soil.</p> + +<p>4. Isolation, and exposure to a stormy atmosphere.</p><p><span class="pagenum"><a id="Page_23">[Pg 23]</a></span></p> + +<h3>§ I. <i>Climatal causes generally, whether dependent on +latitude or altitude.</i></h3> + +<p>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<a id="FNanchor_4_4"></a><a href="#Footnote_4_4" class="fnanchor">[4]</a>."</p> + +<p>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 +<span class="pagenum"><a id="Page_24">[Pg 24]</a></span>universal; and whatever, therefore, be our ideas on a +subject which might perchance <i>seem</i> 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 <i>none</i> 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.</p><p><span class="pagenum"><a id="Page_25">[Pg 25]</a></span></p> + +<p>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 <i>have reached +it</i>, 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 <i>latter</i> +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 <i>must</i> 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 <i>imply</i> a transportation of them (from their +primeval centres) beyond the limits of a naturally +acquired range. Moreover, the +αὐτόχθονες +of the soil +(if we may be excused the idiom) are in all instances +adjusted to the peculiarities of the region in which they +were formed; and, consequently, where they have not +(as very frequently happens) diffused themselves to a +sufficient distance from the birthplace of their kind to +be acted upon in two opposite manners from without, +the date <i>they</i> supply, during our inquiry into +specific modifications as dependent on external disturbing +elements, cannot be very considerable.</p><p><span class="pagenum"><a id="Page_26">[Pg 26]</a></span></p> + +<p>In spite of this severe distinction, however, which I +would urge between the insect <i>aborigines</i> of a country +and <i>those which</i> (whether by compulsion or not) <i>have +colonized it</i>, 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 <i>in toto</i> 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 <i>physical results</i> amongst phænomena +which are due to the <i>creative</i> force alone; yet we may, +in the contemplation of them, recognize such an amount +of <i>design</i>, or a primary adaptation to conditions from +without, as shall afford, through its permanence and +method, fresh presumptive evidence that the "conditions" +<i>themselves</i> 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.</p> + +<p>It has been already mentioned (and, despite the exceptional +cases which are to be found, it is in a <i>general</i> +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 <i>direct</i><span class="pagenum"><a id="Page_27">[Pg 27]</a></span> +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 <i>aborigines</i> 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 <i>number</i>, 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.</p> + +<p>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<span class="pagenum"><a id="Page_28">[Pg 28]</a></span> +than in our cooler clime; and can we imagine +that it was <i>not</i> 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<a id="FNanchor_5_5"></a><a href="#Footnote_5_5" class="fnanchor">[5]</a> 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.</p> +<p><span class="pagenum"><a id="Page_29">[Pg 29]</a></span></p> +<p>But how, it may be asked, does this <i>primary adaptation</i> +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 <i>kind</i> of +aberration which reason and experience would seem +alike to imply that we should, in the various instances, +anticipate.</p> + +<p>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 +<span class="pagenum"><a id="Page_30">[Pg 30]</a></span>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 <i>single</i> reason for certain phænomena +with comparative precision, it will generally happen that +two or three agents must be appealed to before we can +arrive at a conclusion by any means satisfactory. I +would desire, therefore, that the examples hereafter to +be noticed may be judged of in the mass; and may not +be considered as severally assigned, of necessity, to an +isolated deranging cause, through the fact of their being +placed, for the sake of convenience, and because of the +<i>predominance</i> which special controlling principles have +had in maturing them, under sections, both, as it were, +exclusive and particular.</p> + +<p>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 <i>Pissodes notatus</i>, Fab., a weevil<span class="pagenum"><a id="Page_31">[Pg 31]</a></span> +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 <i>Lixus angustatus</i>, 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 <i>Coccinella 7-punctata</i>, 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 <i>Carpophilus +hemipterus</i>, Linn., <i>Trogosita mauritanica</i>, Linn., <i>Læmophlœus +pusillus</i>, Schönh., <i>Dermestes vulpinus</i>, Fab., <i>Anobium +striatum</i>, Oliv., <i>Rhizopertha pusilla</i>, Fab., <i>Sitophilus +granarius</i> and <i>Oryzæ</i>, Linn., and <i>Tribolium ferrugineum</i>, +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 <i>Agabus +bipustulatus</i>, 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, <i>the result of latitude</i>; as is equally the case +with the <i>Hydroporus confluens</i>, Fab., which is found from +Sweden to the Canaries, and the <i>Eunectes sticticus</i>, +Linn.,—an insect literally cosmopolitan. The Swallow-Tail +Butterfly (<i>Papilio Machaon</i>, Linn.), the Clouded +<span class="pagenum"><a id="Page_32">[Pg 32]</a></span>Yellow (<i>Colias Edusa</i>, Fab.) and the Painted Lady (<i>Cynthia +Cardui</i>, 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<a id="FNanchor_6_6"></a><a href="#Footnote_6_6" class="fnanchor">[6]</a> +of undoubted geographical instability.</p> + +<p>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, <i>that climatal +operation</i>, although by no means invested with a +universal qualifying power, <i>has an amount of influence on +certain species, even whilst unconnected with other elements,—and +therefore</i>, á; fortiori, <i>when in combination +with them</i>.</p> + +<p>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 +<span class="pagenum"><a id="Page_33">[Pg 33]</a></span>contour of the insect tribes. Thus, to go no further +than Ireland, we find that the specimens of <i>Silpha +atrata</i>, 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, <i>S. subrotundata</i>, +from British naturalists. I think it far from +improbable that the <i>Tachyporus nitidicollis</i>, Steph., an +insect eminently characteristic of that country (and one +on which I have lately offered some remarks<a id="FNanchor_7_7"></a><a href="#Footnote_7_7" class="fnanchor">[7]</a>), is but a +darker climatal modification of the common <i>T. obtusus</i>: +and it is well known that the examples of <i>Pelophila +borealis</i>, Payk., from Killarney and Loch Neagh are permanently +larger, and much more metallic, than those +from the Orkneys. The <i>Nebria complanata</i>, 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 <i>Omaseus +nigerrimus</i>, Dej., of Spain, the north of Africa, and +Madeira, is a geographical state of the <i>O. aterrimus</i> of +Central Europe. The <i>Sitona gressoria</i>, Illig., so universal +throughout the Mediterranean districts, Madeira and +the Canaries, may be but the subaustral form of <i>S. grisea</i>. +The <i>Bembidium obtusum</i>, Sturm, is shorter and less +parallel in our own latitude than it is in the Madeiran +group and along the Mediterranean shores: whilst the +<i>Holoparamecus niger</i>, Aubé, of Madeira and Sardinia is +<span class="pagenum"><a id="Page_34">[Pg 34]</a></span>very much paler than the same beetle when taken in +Sicily. Specimens of <i>Pieris Brassicæ</i>, Linn. (the White +Cabbage-Butterfly,—an insect of widely acquired range), +from Nepaul and Japan, are recorded<a id="FNanchor_8_8"></a><a href="#Footnote_8_8" class="fnanchor">[8]</a> 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 <i>Vanessa Atalanta</i>, 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 <i>Hipparchia</i> +of Madeira I believe to be a fixed geographical modification +of the <i>H. Semele</i>, 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 <i>Lycæna Phlœas</i>, 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<a id="FNanchor_9_9"></a><a href="#Footnote_9_9" class="fnanchor">[9]</a>."</p> + +<p>Few observers can have failed to remark, that increased +<span class="pagenum"><a id="Page_35">[Pg 35]</a></span><i>altitude</i> frequently corresponds, both in its fauna and +flora, to a higher <i>latitude</i>; 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 <i>à priori</i> inclination +to endorse. It must be recollected however that, in the +instances to which we would draw attention, <i>small</i> aberrations +are all that can be usually looked for, since climate +<i>of itself</i> 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 <i>lapsus Naturæ</i> only, rather than from a +power of legitimate variation.</p><p><span class="pagenum"><a id="Page_36">[Pg 36]</a></span></p> + +<p>In exact conformity with what the above remarks will +have prepared us for, we find that the <i>Dromius obscuroguttatus</i>, +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 <i>and isolation</i> 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 <i>in extenso</i> +the reflections which, during the compilation of the +'Insecta Maderensia,' suggested themselves to me. "The +<i>Dromius obscuroguttatus</i> 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<span class="pagenum"><a id="Page_37">[Pg 37]</a></span> +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 +<i>analogous</i> 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<span class="pagenum"><a id="Page_38">[Pg 38]</a></span> +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 <i>Dromius</i> from its more northern +representatives are, as just stated, small; nevertheless, +since they are <i>fixed</i>, 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<a id="FNanchor_10_10"></a><a href="#Footnote_10_10" class="fnanchor">[10]</a>."</p> + +<p>In like manner the <i>Calathus fuscus</i>, Fab., the <i>Anchomenus +marginatus</i>, Linn., and the <i>Anthicus fenestratus</i>, +Schmidt, which occur almost exclusively in the <i>lower</i> +<span class="pagenum"><a id="Page_39">[Pg 39]</a></span>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.</p> + +<p>And if we inquire, on the other hand, into the <i>aboriginal</i> +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 <i>Helops confertus</i>, +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 <i>H. Pluto</i><a id="FNanchor_11_11"></a><a href="#Footnote_11_11" class="fnanchor">[11]</a>." The <i>Pecteropus +Maderensis</i>, 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 <i>Trechus alticola</i>, Woll., should +perhaps be regarded as an alpine state of the <i>T. custos</i>. +The <i>Calathus complanatus</i>, 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 +<span class="pagenum"><a id="Page_40">[Pg 40]</a></span>with <i>both</i> sexes (though, of course, especially the male) +shining.</p> + +<p>Nor is this principle of topographical variability (the +result of climate) less apparent in other countries also. +The <i>Notiophili</i>, 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 <i>N. semipunctatus</i>, 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<a id="FNanchor_12_12"></a><a href="#Footnote_12_12" class="fnanchor">[12]</a>." It has indeed been lately +suggested, that the <i>Helobia nivalis</i>, Payk., may be perhaps, +after all, but a mountain variety of the <i>H. brevicollis</i>; +the <i>Leistus montanus</i>, Steph., of the <i>L. fulvibarbis</i>, +and the <i>Patrobus septentrionis</i>, Dej., of the <i>P. excavatus</i>; +but of this I think further proof is needed, seeing that +certain species do appear to exist which are <i>strictly</i> +alpine (that is to say, which have not been, severally, +detected in the lower regions of more northern zones); +and, in <i>most</i> instances, where aberrations are to be met +<span class="pagenum"><a id="Page_41">[Pg 41]</a></span>with from the effect of <i>altitude</i>, 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), <i>where are the +intermediate links</i>? 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 <i>quasi</i> variety is found, the descendants +of its assumed progenitor <i>do</i> occur in the plains +beneath. I have remarked that the <i>Cicindelidæ</i> often +become inconstant in colouring as they approach their +maximum of height above the sea; and I have but +little doubt that the <i>C. fasciatopunctata</i><a id="FNanchor_13_13"></a><a href="#Footnote_13_13" class="fnanchor">[13]</a>, Germ., from +Asia Minor and Turkey, is the <i>C. sylvatica</i> modified by +a long residence in elevated regions. And so it is with +the <i>Chrysomelæ</i>, 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.</p> + +<p>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 +<span class="pagenum"><a id="Page_42">[Pg 42]</a></span>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 <i>may</i> be, in the strictest sense, significant, +even whilst small; and that it is their <i>constancy</i>, 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 <i>fixed</i>) usually prevails; whilst it is our +main object to show that dissimilarities do not <i>necessarily</i> +imply the specific isolation of the creatures which +display them, merely because they are, in their several +localities, <i>permanent</i>.</p> + +<h3>§ II. <i>Temporary heat or cold, of an unusual degree.</i></h3> + +<p>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<span class="pagenum"><a id="Page_43">[Pg 43]</a></span> +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 <i>accidentally</i> 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 <i>are</i> 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.</p> + +<p>I believe that almost the only deviation from the +typical state, in insect form, which has been observed to +originate, <i>par excellence</i>, 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 <i>Carabidæ</i>) +are subject to great inconstancy in their metathoracic +organs of flight. Many species, as the common <i>Calathus +mollis</i> of our own country (to which my attention has +been more particularly drawn by the Rev. J. F. Dawson),<span class="pagenum"><a id="Page_44">[Pg 44]</a></span> +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 <i>permanently</i> +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<a id="FNanchor_14_14"></a><a href="#Footnote_14_14" class="fnanchor">[14]</a>), 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 <i>heat</i> +does (in the main) favour, and <i>cold</i> 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.</p> + +<p>Speaking of certain representatives of the Hymenoptera +(<i>Chalcididæ</i>), Mr. Westwood observes: "A +curious peculiarity exists in one at least of these +apterous species, which has been noticed by no previous +author, namely, <i>Choreius ineptus</i>, Westw., which, +although ordinarily found in an apterous state, was +discovered by me in considerable numbers during the +hot summer of 1835, with wings<a id="FNanchor_15_15"></a><a href="#Footnote_15_15" class="fnanchor">[15]</a>". And, touching +the irregularity of the alary organs in the Homopterous +<i>Fulgoridæ</i>, he remarks: "Other instances, in which the +wings undergo a deficiency of development, occur in the +<span class="pagenum"><a id="Page_45">[Pg 45]</a></span>genus <i>Delphax</i>, 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, <i>as well as the hind wings</i>. In certain +seasons, however, especially hot ones, the wings are +fully developed<a id="FNanchor_16_16"></a><a href="#Footnote_16_16" class="fnanchor">[16]</a>". Mr. Curtis has indeed formed the +undeveloped specimens into a different genus, <i>Criomorphus</i>.</p> + +<p>Although the result of a more stimulating sun may +be often neutralized by that of <i>isolation</i> (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 <i>heat</i>, when freed from +counter influences, may be traced in its <i>permanent</i> 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 (<i>Cimex +lectularius</i>, Linn.) is almost invariably apterous, or with +very short rudimental hemelytra; yet Scopoli (<i>Ent. +Carn.</i> 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 +<span class="pagenum"><a id="Page_46">[Pg 46]</a></span>as temporarily applied (in an unusual degree), or +through the accidental transportation of the insect into +a naturally warmer atmosphere.</p> + +<h3>§ III. <i>Nature of the country and of the soil.</i></h3> + +<p>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 <i>increase</i> +and <i>diffusion</i> 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.</p> + +<p>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 <i>climate alone</i>, however, +that I wished them to be understood: and it is not +until now that I have ventured to urge the necessity of<span class="pagenum"><a id="Page_47">[Pg 47]</a></span> +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 <i>many</i> 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 phænomena as a +whole, or to reconcile the apparent inconsistencies which +they are accustomed to display.</p> + +<p>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 <i>Harpalidæ</i> +and <i>Heteromera</i>, living beneath stones. The vegetable-feeding +<i>Chrysomelidæ</i>, 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<span class="pagenum"><a id="Page_48">[Pg 48]</a></span> +insects is widely dissimilar<a id="FNanchor_17_17"></a><a href="#Footnote_17_17" class="fnanchor">[17]</a>." 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.</p> + +<p>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 <i>was</i> Madeira, in its normal +state<a id="FNanchor_18_18"></a><a href="#Footnote_18_18" class="fnanchor">[18]</a>; and such it still is throughout a large district +<span class="pagenum"><a id="Page_49">[Pg 49]</a></span>towards the northern coast. I cannot indeed refrain +<span class="pagenum"><a id="Page_50">[Pg 50]</a></span>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<a id="FNanchor_19_19"></a><a href="#Footnote_19_19" class="fnanchor">[19]</a>."</p> + +<p>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 +<span class="pagenum"><a id="Page_51">[Pg 51]</a></span>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.</p> + +<p>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 <i>exuberance</i> in their Faunas,—seeing that in both +the regions just appealed to, we not only perceive a vast +difference in the <i>numbers</i> 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 <i>resemblance of physical +conditions</i> (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.</p><p><span class="pagenum"><a id="Page_52">[Pg 52]</a></span></p> + +<p>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 <i>apart from the direct action of heat +and cold</i>:—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 <i>fact</i> 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<span class="pagenum"><a id="Page_53">[Pg 53]</a></span> +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 phænomena which we +see; for Ireland is not only remarkable for the paucity +of its <i>species</i>, but also for the paucity of its <i>individuals</i>,—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 <i>latitude</i>) which must be regarded +as adverse to the general prosperity of the insect +races.</p> + +<p>And so it is with <i>localities</i> (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<span class="pagenum"><a id="Page_54">[Pg 54]</a></span> +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<a id="FNanchor_20_20"></a><a href="#Footnote_20_20" class="fnanchor">[20]</a>."</p> + +<p>There can be no doubt that islands are, for the most +part, more unproductive (even in proportion) than continents; +<span class="pagenum"><a id="Page_55">[Pg 55]</a></span>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<a id="FNanchor_21_21"></a><a href="#Footnote_21_21" class="fnanchor">[21]</a>. It is however to geological causes that +we must mainly look for the explanation of this phænomenon; +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-<i>diffusion</i>, which, at the opening of this section, we +proposed to consider, along with that of insect <i>productiveness</i> +(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.</p> +<p><span class="pagenum"><a id="Page_56">[Pg 56]</a></span></p> +<p>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) +<i>that certain countries and spots are by constitution more +favourable than others for the increase</i> (apart from the after +dissemination) <i>of the insect tribes</i>,—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 <i>nature of the several districts</i> 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 <i>per se</i> 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 <i>topo</i>graphical one.</p><p><span class="pagenum"><a id="Page_57">[Pg 57]</a></span></p> + +<p>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:—</p> + +<p>"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, +<i>à priori</i>, we have strong presumptive evidence that the<span class="pagenum"><a id="Page_58">[Pg 58]</a></span> +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 <i>Mesites Tardii</i>, 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 <i>Olisthopus rotundatus</i> +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 +<i>Gymnaëtron Campanulæ</i>, none of which exceeded three-quarters +of a line,—the usual length being from a line +to a line and three-quarters. <i>Anthonomus ater</i>, the +average length of which is two lines, I have taken a +series of in Lundy Island, none of which exceeded one.<span class="pagenum"><a id="Page_59">[Pg 59]</a></span> +In the same locality, also, the common <i>Ceutorhynchus +contractus</i> 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 <i>Haltica +rufipes</i><a id="FNanchor_22_22"></a><a href="#Footnote_22_22" class="fnanchor">[22]</a>, captured by him on the coast, in which the +entire insect is of a uniform brownish-red hue. Of the +rare <i>Mantura Chrysanthemi</i> 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 <i>Telephorus +testaceus</i>. In like manner, I might enumerate +other species equally remarkable; but I trust that those +<span class="pagenum"><a id="Page_60">[Pg 60]</a></span>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<a id="FNanchor_23_23"></a><a href="#Footnote_23_23" class="fnanchor">[23]</a>."</p> + +<p>Apparently dependent, in a large measure, on the +same circumstance (namely proximity to the coast), the +<i>Bembidium saxatile</i>, 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<a id="FNanchor_24_24"></a><a href="#Footnote_24_24" class="fnanchor">[24]</a>." I have +taken it in profusion on the coasts of the Isle of Wight, +Dorsetshire, and Devon. And so with the <i>Cistela sulphurea</i>, +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 <i>in situ</i>), it might stand a fair chance, +occasionally, of being mistaken for a separate species. +A <i>Psylliodes</i> in Lundy Island, allied to (if not identical +<span class="pagenum"><a id="Page_61">[Pg 61]</a></span>with) the <i>chrysocephala</i>, Linn., found in abundance on +a <i>Brassica</i> along the ascent from the eastern landing-place, +varies "in every consecutive shade between the +limits of light yellow and dark metallic-green<a id="FNanchor_25_25"></a><a href="#Footnote_25_25" class="fnanchor">[25]</a>," 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.</p> + +<p>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 <i>Aphodius plagiatus</i>, 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 <i>as the variety</i>, is in +England evidently the typical one,—for out of about +sixty specimens which I captured [at Tenby in South +Wales], only <i>two</i> 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 +<span class="pagenum"><a id="Page_62">[Pg 62]</a></span>and Norwich present a larger proportion of the ordinary +European state. The <i>blood-red dashes</i>, 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 <i>Gymnaëtron</i>, either that its representatives should +be thus ornamented typically, or else that those which +are normally black should, <i>when they vary</i>, keep in view, +as it were, <i>this principle</i> for their wanderers to subscribe +to. Thus, I have no doubt that the <i>G. Veronicæ</i>, Germ., +is but a variety of the <i>G. niger</i>,—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 <i>G. Veronicæ</i> being really distinct from +the <i>G. niger</i>, 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<a id="FNanchor_26_26"></a><a href="#Footnote_26_26" class="fnanchor">[26]</a>."</p> + +<p>The <i>Bembidium bistriatum</i>, 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 <i>Blemus areolatus</i>, Creutz., I have frequently remarked +is similarly affected in brackish places: and I +think it far from improbable that the <i>Stenolophus Skrim<span class="pagenum"><a id="Page_63">[Pg 63]</a></span>shiranus</i>, +Steph., is but a local modification (though not +altogether, perhaps, through marine influences) of the +<i>S. Teutonus</i>, Schr. The <i>Dromius fasciatus</i>, Gyll., not +being <i>exclusively</i> 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 <i>Anthicus bimaculatus</i>, Illig., M. de la +Ferté observes: "Il y a sculement lieu de remarquer que +les individus du bord de l'océan sont généralement plus +pâles que ceux des contrées orientales de l'Europe, et que +ceux des côtes de France et de Belgique sent entièrement +dépourvus de tache discoïdale<a id="FNanchor_27_27"></a><a href="#Footnote_27_27" class="fnanchor">[27]</a>." And bearing, in much +the same manner, on the subject of variations, the <i>Anthicus +humilis</i>, Germ., "est une des espèces le plus généralement +répandues en Europe; mais il lui faut le voisinage +de l'eau salée. Aussi on le rencontre non-seulement sur +les rivages de toutes les mers, même de la Baltique, mais +encore aux bords des lacs salés, tels que celui de Mannsfeld, +en Saxe. <i>Ceux de cette dernière localité sont généralement +noirs</i>; ceux que j'ai pris à Perpignan sont d'un +rouge très-clair, ce qui me porte à croire que cette espèce +est dans le même cas que quelques autres <i>Anthicus</i>, dont +les variétés les plus foncées appartiennent au nord de +l'Europe, et les plus pâles au midi<a id="FNanchor_28_28"></a><a href="#Footnote_28_28" class="fnanchor">[28]</a>."</p> + +<p>Whilst touching on this immediate question of variability +<i>as dependent to a great extent</i>, in numerous cases, +<span class="pagenum"><a id="Page_64">[Pg 64]</a></span><i>on proximity to the sea</i>, we may just notice the marked tendency +which even the insects <i>peculiar to</i> 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 <i>strictly maritime</i>,—seeing +that we have no means of judging in such +instances whether similar phænomena 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 <i>Nebria complanata</i> and <i>livida</i>, <i>Calathus +mollis</i>, <i>Pogonus luridipennis</i>, <i>Trechus lapidosus</i>, <i>Aëpus +marinus</i> and <i>Robinii</i>, <i>Cillenum laterale</i>, <i>Bembidium scutellare</i>, +<i>ephippium</i> and <i>pallidipenne</i>, <i>Ochthebius marinus</i>, +<i>Psylliodes marcida</i>, <i>Phaleria cadaverina</i>, <i>Helops testaceus</i>, +and <i>Anthicus instabilis</i>, 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.</p> + +<p>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<span class="pagenum"><a id="Page_65">[Pg 65]</a></span> +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 +<i>either</i> well-defined <i>races</i> have been gradually shaped out, +by means of the physical influences to which they have +been exposed, or else that the <i>species themselves</i> (as +witnessed by the intermediate geographical links, which, +although sometimes rare, are in all instances to be found) +do assuredly merge into each other.</p> + +<p>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 <i>Tarus lineatus</i>, Schönh., is slightly shorter +in Madeira, as also somewhat darker on its head and +prothoracic disk (and with its elytral striæ less deeply +impressed), than it is in Algeria and Spain. The +Madeiran specimens of the <i>Aphodius nitidulus</i>, Fabr., +are usually a little paler, and more distinctly punctulated, +than their northern analogues; as are also, in +the latter respect, those of the <i>Clypeaster pusillus</i>, Gyll. +The <i>Scydmænus Helferi</i>, Schaum, is permanently smaller +in the Madeiran group than it is in Sicily; and I +believe that the <i>Achenium Hartungii</i>, Heer, of those<span class="pagenum"><a id="Page_66">[Pg 66]</a></span> +islands, is but a local state of the <i>A. depressum</i>, Grav., +of Central Europe. The <i>Bembidium tabellatum</i> and +<i>Schmidtii</i>, Woll., may be in reality but geographical +modifications of the <i>B. tibiale</i> and <i>callosum</i> of higher +latitudes; and the <i>Malthodes Kiesenwetteri</i>, Woll., of +the common European <i>M. brevicollis</i>. 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 <i>Dromius +arenicola</i>, Woll., is the Portosantan representative of +the <i>D. obscuroguttatus</i>, 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 +<i>Hadrus illotus</i>, Woll., may be specifically identical with +the Madeiran <i>H. cinerascens</i>. In like manner, the <i>Bembidium +Atlanticum</i>, 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<span class="pagenum"><a id="Page_67">[Pg 67]</a></span> +more prolific in varieties, generally, than others. The +neighbourhood of Ipswich, in our own country, has +been cited by Mr. Curtis<a id="FNanchor_29_29"></a><a href="#Footnote_29_29" class="fnanchor">[29]</a> as possessing this peculiarity; +and I have remarked a similar tendency in certain parts +of Ireland. The common <i>Haliplus obliquus</i>, 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 fasciæ, especially +the hinder ones, being occasionally evanescent.</p> + +<p>One more example must satisfy us under this section,—namely, +the <i>Harpalus vividus</i>, 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, +<span class="pagenum"><a id="Page_68">[Pg 68]</a></span><i>à priori</i>, 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<span class="pagenum"><a id="Page_69">[Pg 69]</a></span> +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, +<i>ore rotundo</i>, 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 antennæ 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<a id="FNanchor_30_30"></a><a href="#Footnote_30_30" class="fnanchor">[30]</a>."</p> + +<p><span class="pagenum"><a id="Page_70">[Pg 70]</a></span></p> + +<h3>§ IV. <i>Isolation; and exposure to a stormy atmosphere.</i></h3> + +<p>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.</p> + +<p>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 phænomena can be satisfactorily +explained. Laying aside those forms which +are manifestly endemic (the numerical proportion of +which usually accords with the <i>distance</i> from the nearest +mainland), again and again are we baffled by the near +resemblance of the various creatures to continental +types,—whilst the minute <i>differences</i> 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 <i>effects</i> +where there are fair grounds for anticipating them, will +not be slow to perceive, that, in the small deviations<span class="pagenum"><a id="Page_71">[Pg 71]</a></span> +which we are so often accustomed under such circumstances +to behold, <i>the results of isolation itself</i> (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.</p> + +<p>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 <i>self-dispersion, +as occasionally interrupted by subsidence</i>.</p> + +<p>If we except a few of the <i>Heteromera</i> and apterous +<i>Curculionidæ</i>, 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<span class="pagenum"><a id="Page_72">[Pg 72]</a></span> +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.</p> + +<p>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 (<i>inter alia</i>) 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), <i>that</i><span class="pagenum"><a id="Page_73">[Pg 73]</a></span> +<i>where there is a law there must be an exception to it</i>; +and that, consequently, exceptional cases, if not exceedingly +numerous, should never pervert our belief from an +otherwise presumptive truth.</p> + +<p>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 <i>Anthonomus ater</i>, Mshm, and <i>Ceutorhynchus +contractus</i>, 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 +<i>Calathus melanocephalus</i>, Linn., and <i>Olisthopus +rotundatus</i>, 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 <i>Bolitochara +assimilis</i>, Kby, is invariably smaller in those +islands than it is in the neighbourhood of Penzance<a id="FNanchor_31_31"></a><a href="#Footnote_31_31" class="fnanchor">[31]</a>. +The <i>Vanessa Callirhoë</i>, Fabr. (a geographical analogue of +the Red Admiral Butterfly<a id="FNanchor_32_32"></a><a href="#Footnote_32_32" class="fnanchor">[32]</a>, so common in our own +<span class="pagenum"><a id="Page_74">[Pg 74]</a></span>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 <i>Ptini</i> 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 +<i>species</i> (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<a id="FNanchor_33_33"></a><a href="#Footnote_33_33" class="fnanchor">[33]</a>." That "one," Protean, representative is the +<i>Ptinus albopictus</i>, Woll.; and it is so eminently a case +in point, that it may be admissible to quote, <i>in extenso</i>, +a few of the observations which I have already published +concerning it:—</p> + +<p><span class="pagenum"><a id="Page_75">[Pg 75]</a></span></p><p>"The <i>P. albopictus</i> is the commonest of the Madeiran +<i>Ptini</i>, 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 +<i>extremes</i>, 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 <i>var.</i> α, +which reaches its maximum in the sheltered ravines of +the central mass, the bulk is usually large, and the tints +comparatively intense. <i>Var.</i> β. 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,<span class="pagenum"><a id="Page_76">[Pg 76]</a></span> +we might have expected, <i>à priori</i>, 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, <i>in +situ</i>, becomes extremely important; since such does at +once convince us that its almost exclusive attachment to +the interior of the stalks of the <i>Silybum Marianum</i>, +Grtn. (the <i>Holy Thistle</i> 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 <i>stature</i> (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 <i>var.</i> γ. +of the diagnosis), we immediately perceive that <i>both</i> 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 <i>minimum</i> +is to be obtained:—which, true to the <i>modus operandi</i> 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 Chão. When we bear<span class="pagenum"><a id="Page_77">[Pg 77]</a></span> +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 <i>as</i> +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 +Chão to somewhat less than half a line in length, being +literally of scarcely greater magnitude than some of the +larger representatives of the <i>Ptiliadæ</i>!"<a id="FNanchor_34_34"></a><a href="#Footnote_34_34" class="fnanchor">[34]</a></p> + +<p>I stated above, that, although this diminution of +stature is a very general accompaniment of isolation, +amongst insects which have been <i>long</i> 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 <i>immense</i> 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 +<span class="pagenum"><a id="Page_78">[Pg 78]</a></span>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 <i>à priori</i> 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, <i>which has not yet +shown any matured traces of its action</i>, 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 <i>geological</i> interval, it +would be absurd to look for them in the mere settlers +(as it were) of yesterday.</p> + +<p>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<span class="pagenum"><a id="Page_79">[Pg 79]</a></span> +would seem, <i>primâ facie</i>, 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 <i>always</i> 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, <i>if</i> 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 <i>most commonly</i> 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, <i>if in the example before us there doth +not appear something particular,—some reason for exception</i><a id="FNanchor_35_35"></a><a href="#Footnote_35_35" class="fnanchor">[35]</a>." +Hence it is, that, even amongst the <i>opposite</i> +phænomena 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, +<span class="pagenum"><a id="Page_80">[Pg 80]</a></span>that it is not from <i>its failure</i>, 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.</p> + +<p>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, <i>in some shape or other</i>, +is absolute and real.</p> + +<p>After the above remarks, we shall not be surprised +that the phænomena 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<span class="pagenum"><a id="Page_81">[Pg 81]</a></span> +presumptive truth; on a narrower one it does not always +apply; for species are differently constituted <i>ab ovo</i>, 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<a id="FNanchor_36_36"></a><a href="#Footnote_36_36" class="fnanchor">[36]</a>, 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 +<span class="pagenum"><a id="Page_82">[Pg 82]</a></span>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.</p> + +<p>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 <i>à priori</i> 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, <i>endemic</i>, 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<a id="FNanchor_37_37"></a><a href="#Footnote_37_37" class="fnanchor">[37]</a>, or whether they were thus created severally +<span class="pagenum"><a id="Page_83">[Pg 83]</a></span>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.</p> + +<p>I alluded, in a previous section, to the <i>Dromius +obscuroguttatus</i>, 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 phænomena +(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 <i>isolation</i> that we must mainly look, if we would understand +those changes aright. But what <i>are</i> the distinctive +<span class="pagenum"><a id="Page_84">[Pg 84]</a></span>features, it may be asked, which the <i>D. obscuroguttatus</i> has +adopted, since its first arrival from more northern latitudes +over an unbroken<a id="FNanchor_38_38"></a><a href="#Footnote_38_38" class="fnanchor">[38]</a> continent? It has not altered much, +after all: it is, however, the <i>nature</i> 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) <i>its wings are evanescent</i>. +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 +<i>vast</i> interval has elapsed since the Madeiran islands were +portions of a continuous whole, we have at once a sufficient +<i>time</i> 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.</p> + +<p>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 +<span class="pagenum"><a id="Page_85">[Pg 85]</a></span>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 <i>increase</i> in +their bulk. The common <i>Bradycellus fulvus</i>, 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 +<i>sufficient time</i>, is satisfied; and what is the result? The +insect is a trifle more robust than its ordinary European +representatives, and it is <i>invariably apterous</i>. The <i>Calathus +fuscus</i>, 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 <i>Dromius negrita</i>, Woll., may be +the ultimate phasis (from isolation) of the common <i>D. +glabratus</i>, 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 <i>Vanessa Atalanta</i> (when considered with +respect to the <i>V. Callirhoë</i>), may it not be of more recent<span class="pagenum"><a id="Page_86">[Pg 86]</a></span> +importation from the European continent, and as yet in +a transition state?—an idea which the <i>smallness</i> of its +wing, as compared with those of its British analogues, +would seem rather to corroborate.</p> + +<p>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 <i>primary</i> 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.</p> + +<p>Why it is, in the Insecta, that <i>islands</i><a id="FNanchor_39_39"></a><a href="#Footnote_39_39" class="fnanchor">[39]</a> 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 +<span class="pagenum"><a id="Page_87">[Pg 87]</a></span>should in reality be gifted with <i>stronger</i> powers of flight +(rather than weaker ones), to fortify them against such +disasters; and that, consequently, the above phænomena +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 <i>flight</i> 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 +aërial 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.</p><p><span class="pagenum"><a id="Page_88">[Pg 88]</a></span></p> + +<p>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 Chão, the <i>Scarites abbreviatus</i>, Koll., +<i>Laparocerus morio</i>, Schön., and the <i>Helops Vulcanus</i>, +Woll., attain a gigantic size; yet it is on that very island +that the <i>Ptinus albopictus</i>, Woll., finds its minimum of +development,—scarcely exceeding in dimensions some of +the larger members of the <i>Trichopterygia</i>. The Deserta +Grande has some special modifying capability of its +own,—the <i>Eurygnathus Latreillei</i>, Lap., <i>Notiophilus geminatus</i>, +Dej., <i>Zargus pellucidus</i>, Woll., <i>Calathus complanatus</i>, +Koll., <i>Olisthopus Maderensis</i>, Woll., <i>Caulotrupis +conicollis</i>, Woll., <i>Laparocerus morio</i>, Schön., <i>Omias Waterhousei</i>, +Woll., <i>Helops Vulcanus</i>, Woll., and the <i>Ellipsodes +glabratus</i>, Fab., being also larger on that rock +than is typical: all of them, however, with the exception +of <i>Notiophilus geminatus</i>, are there, as elsewhere, +apterous.</p> + +<p>Other qualifying results, from isolation, are equally +apparent. Take <i>colour</i>, for instance; and we shall perceive +that in the <i>Dromius sigma</i>, 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 <i>inter se</i>; nevertheless, +being constant, I would lay particular stress upon them,<span class="pagenum"><a id="Page_89">[Pg 89]</a></span> +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<a id="FNanchor_40_40"></a><a href="#Footnote_40_40" class="fnanchor">[40]</a>." The <i>Olisthopus Maderensis</i>, 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 +<span class="pagenum"><a id="Page_90">[Pg 90]</a></span>obtained from thence as specifically distinct; nevertheless, +with the knowledge both of the modifying effects of +isolation, and also of the <i>kind</i> 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<a id="FNanchor_41_41"></a><a href="#Footnote_41_41" class="fnanchor">[41]</a>." +The <i>Pecteropus Maderensis</i>, Woll., is of a greenish-brassy +tinge in Porto Santo, and much acuminated in +front; whereas on the Dezerta Grande it is almost +invariably <i>coppery</i>, and less narrowed anteriorly. The +<i>Caulotrupis lucifugus</i>, 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<a id="FNanchor_42_42"></a><a href="#Footnote_42_42" class="fnanchor">[42]</a>." The <i>Psylliodes +vehemens</i>, 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 +<span class="pagenum"><a id="Page_91">[Pg 91]</a></span>undergone a change since the remote period of its isolation +on a comparatively calcareous soil<a id="FNanchor_43_43"></a><a href="#Footnote_43_43" class="fnanchor">[43]</a>." The <i>Scarites +abbreviatus</i>, 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 +<i>species</i>, 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 <i>of necessity</i> specific. Rejecting therefore this hypothesis +as utterly untenable, and as contrary to all experience, +we are driven to acknowledge that isolation <i>does</i>, +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 +<span class="pagenum"><a id="Page_92">[Pg 92]</a></span>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<a id="FNanchor_44_44"></a><a href="#Footnote_44_44" class="fnanchor">[44]</a>."</p> + +<hr class="tb" /> + +<p>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 +<span class="pagenum"><a id="Page_93">[Pg 93]</a></span>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 +<i>permanent</i> 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 <i>conceded</i> 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 +<i>insular</i> phænomena, for I am convinced that a due allowance +is seldom, if ever, made for the qualifying power of +isolation, <i>per se</i>,—the most significant perhaps of all the +conditions which we have attempted in the preceding +pages to examine.</p> + +<p>"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 <i>reasons</i> 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<span class="pagenum"><a id="Page_94">[Pg 94]</a></span> +system of created things is, from first to last, replete +with design. <i>Natura nil agit sine causâ</i> 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 phænomena, 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 <i>seeming</i> deviations from the +common rule. These are generally the effects of that +influence which free agents, and various circumstances, +have upon natural productions<a id="FNanchor_45_45"></a><a href="#Footnote_45_45" class="fnanchor">[45]</a>."</p> + +<div class="footnotes"><h3>FOOTNOTES:</h3> + +<div class="footnote"><p><a id="Footnote_3_3"></a><a href="#FNanchor_3_3"><span class="label">[3]</span></a> Religion of Nature Delineated, p. 103.</p></div> + +<div class="footnote"><p><a id="Footnote_4_4"></a><a href="#FNanchor_4_4"><span class="label">[4]</span></a> Journal of Researches (London, 1852), p. 381.</p></div> + +<div class="footnote"><p><a id="Footnote_5_5"></a><a href="#FNanchor_5_5"><span class="label">[5]</span></a> 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 <i>Carabidæ</i>," +says Mr. Darwin, "appear in extremely few numbers within the +tropics. The carrion-feeders and <i>Brachelytra</i> are very uncommon; +on the other hand, the <i>Rhynchophora</i> and <i>Chrysomelidæ</i>, all of +which depend on the vegetable world for subsistence, are present in +astonishing numbers. The orders <i>Orthoptera</i> and <i>Hemiptera</i> are +peculiarly numerous; as is, likewise, the stinging division of the +<i>Hymenoptera</i>, the bees, perhaps, being excepted."—Journal of +Researches, p. 34.</p></div> + +<div class="footnote"><p><a id="Footnote_6_6"></a><a href="#FNanchor_6_6"><span class="label">[6]</span></a> Mr. Westwood states that he possesses an individual of the +<i>Papilio Machaon</i> from the Himalayan Mountains, captured by Professor +Royle, "which scarcely exhibits the slightest differences +when compared with English specimens."—<i>The Butterflies of Great +Britain</i>, p. 4.</p></div> + +<div class="footnote"><p><a id="Footnote_7_7"></a><a href="#FNanchor_7_7"><span class="label">[7]</span></a> Zoologist, xiii. p. 4655.</p></div> + +<div class="footnote"><p><a id="Footnote_8_8"></a><a href="#FNanchor_8_8"><span class="label">[8]</span></a> The Butterflies of Great Britain (London, 1855), p. 17.</p></div> + +<div class="footnote"><p><a id="Footnote_9_9"></a><a href="#FNanchor_9_9"><span class="label">[9]</span></a> <i>Id.</i> p. 94.</p></div> + +<div class="footnote"><p><a id="Footnote_10_10"></a><a href="#FNanchor_10_10"><span class="label">[10]</span></a> Insecta Maderensia (London, 1854), pp. 7, 8, 9.</p></div> + +<div class="footnote"><p><a id="Footnote_11_11"></a><a href="#FNanchor_11_11"><span class="label">[11]</span></a> Insecta Maderensia, p. 516.</p></div> + +<div class="footnote"><p><a id="Footnote_12_12"></a><a href="#FNanchor_12_12"><span class="label">[12]</span></a> Insecta Maderensia, p. 17.</p></div> + +<div class="footnote"><p><a id="Footnote_13_13"></a><a href="#FNanchor_13_13"><span class="label">[13]</span></a> 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 <i>C. sylvatica</i>, +Linn.</p></div> + +<div class="footnote"><p><a id="Footnote_14_14"></a><a href="#FNanchor_14_14"><span class="label">[14]</span></a> Introduction to the Modern Classification of Insects (London, +1840), ii. p. 473.</p></div> + +<div class="footnote"><p><a id="Footnote_15_15"></a><a href="#FNanchor_15_15"><span class="label">[15]</span></a> Id. ii. p. 158.</p></div> + +<div class="footnote"><p><a id="Footnote_16_16"></a><a href="#FNanchor_16_16"><span class="label">[16]</span></a> Introduction to the Modern Classification of Insects, ii. +p. 431.</p></div> + +<div class="footnote"><p><a id="Footnote_17_17"></a><a href="#FNanchor_17_17"><span class="label">[17]</span></a> Journal of Researches, p. 238.</p></div> + +<div class="footnote"><p><a id="Footnote_18_18"></a><a href="#FNanchor_18_18"><span class="label">[18]</span></a> 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 <i>Madeira</i>,—and, more-over, +of Madeira <i>as it was</i>, 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 <i>now</i> 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 <i>an entire year</i> +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 <i>Santo</i>, 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 <i>Madeira</i>."—<i>A Sketch of Madeira</i>, London, +1851, p. 16.</p></div> + +<div class="footnote"><p><a id="Footnote_19_19"></a><a href="#FNanchor_19_19"><span class="label">[19]</span></a> Journal of Researches, pp. 209, 210.</p></div> + +<div class="footnote"><p><a id="Footnote_20_20"></a><a href="#FNanchor_20_20"><span class="label">[20]</span></a> Zoologist, x. 3616.</p></div> + +<div class="footnote"><p><a id="Footnote_21_21"></a><a href="#FNanchor_21_21"><span class="label">[21]</span></a> Considering that I have already detected more than one thousand +species in those islands, it may perhaps be questioned whether +the same truth <i>is</i> 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 <i>as contrasted with countries in a +similar latitude</i>; and, secondly, that its <i>real</i> 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, <i>are</i> occurring nearly every season), having been dismissed +from our present inquiry.</p></div> + +<div class="footnote"><p><a id="Footnote_22_22"></a><a href="#FNanchor_22_22"><span class="label">[22]</span></a> I perceive, on reference to the original examples, still in my +collection, that this was wrongly quoted as the <i>Haltica rufipes</i>. It +is the <i>H. exoleta</i>, 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.</p></div> + +<div class="footnote"><p><a id="Footnote_23_23"></a><a href="#FNanchor_23_23"><span class="label">[23]</span></a> Zoologist, iv. pp. 1283, 1284.</p></div> + +<div class="footnote"><p><a id="Footnote_24_24"></a><a href="#FNanchor_24_24"><span class="label">[24]</span></a> Geodephaga Britannica (London, 1854), p. 186.</p></div> + +<div class="footnote"><p><a id="Footnote_25_25"></a><a href="#FNanchor_25_25"><span class="label">[25]</span></a> Zoologist, iii. p. 900.</p></div> + +<div class="footnote"><p><a id="Footnote_26_26"></a><a href="#FNanchor_26_26"><span class="label">[26]</span></a> Zoologist, v. p. 1941.</p></div> + +<div class="footnote"><p><a id="Footnote_27_27"></a><a href="#FNanchor_27_27"><span class="label">[27]</span></a> Monographie des <i>Anthicus</i> (Paris, 1848), p. 149.</p></div> + +<div class="footnote"><p><a id="Footnote_28_28"></a><a href="#FNanchor_28_28"><span class="label">[28]</span></a> <i>Id.</i> pp. 127, 128.</p></div> + +<div class="footnote"><p><a id="Footnote_29_29"></a><a href="#FNanchor_29_29"><span class="label">[29]</span></a> Proceedings of the Entomological Society of London (Part 3. +New Series), p. 4.</p></div> + +<div class="footnote"><p><a id="Footnote_30_30"></a><a href="#FNanchor_30_30"><span class="label">[30]</span></a> Insecta Maderensia, pp. 55, 56.</p></div> + +<div class="footnote"><p><a id="Footnote_31_31"></a><a href="#FNanchor_31_31"><span class="label">[31]</span></a> Trans. of the Ent. Soc. of London, ii. pp. 59, 62.</p></div> + +<div class="footnote"><p><a id="Footnote_32_32"></a><a href="#FNanchor_32_32"><span class="label">[32]</span></a> Considering that the true <i>Vanessa Atalanta</i>, of more northern +latitudes, <i>does</i> occasionally occur around Funchal, it may be reasonably +contended that the fact of its coexistence (on the same spot) +with the <i>V. Callirhoë</i> 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 phænomenon, I should have concluded myself: nevertheless, +recollecting how easy of transport the larvæ and pupæ of +Lepidoptera necessarily are (of which we have the plainest assurance +in the almost certain introduction of the <i>Pontia Brassicæ</i>, +<i>Sphinx Convolvuli</i>, <i>Acherontia Atropos</i>, &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 <i>Atalanta</i> is a comparatively recent one, whilst that of the +<i>Callirhoë</i> (which, unlike the typical <i>Red Admiral</i>, 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 <i>slowness</i> 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.</p></div> + +<div class="footnote"><p><a id="Footnote_33_33"></a><a href="#FNanchor_33_33"><span class="label">[33]</span></a> Insecta Maderensia, p. 260.</p></div> + +<div class="footnote"><p><a id="Footnote_34_34"></a><a href="#FNanchor_34_34"><span class="label">[34]</span></a> Insecta Maderensia, pp. 268, 269.</p></div> + +<div class="footnote"><p><a id="Footnote_35_35"></a><a href="#FNanchor_35_35"><span class="label">[35]</span></a> Religion of Nature Delineated, p. 99.</p></div> + +<div class="footnote"><p><a id="Footnote_36_36"></a><a href="#FNanchor_36_36"><span class="label">[36]</span></a> 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 <i>or</i> antennæ 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.</p></div> + +<div class="footnote"><p><a id="Footnote_37_37"></a><a href="#FNanchor_37_37"><span class="label">[37]</span></a> This is certainly rendered <i>probable</i>, however, from the fact that +a large proportion of these apterous species are members of <i>genera</i> +which are usually winged,—such as <i>Tarus</i>, <i>Loricera</i>, <i>Calathus</i>, +<i>Olisthopus</i>, <i>Argutor</i>, <i>Trechus</i>, <i>Hydrobius</i>, <i>Ephistemus</i>, <i>Syncalypta</i>, +<i>Phlœophagus</i>, <i>Tychius</i>, <i>Longitarsus</i>, <i>Chrysomela</i>, <i>Scymnus</i>, <i>Corylophus</i>, +<i>Helops</i>, and <i>Othius</i>,—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.</p></div> + +<div class="footnote"><p><a id="Footnote_38_38"></a><a href="#FNanchor_38_38"><span class="label">[38]</span></a> I do not think it necessary to apologize for the apparent disposal +of this <i>quæstio vexata</i>; because, from the wildness of the +upland ridges to which the <i>D. obscuroguttatus</i> is in Madeira exclusively +confined, I deem it an absolute impossibility that it could +ever have been <i>introduced</i>, through any chance agencies whatsoever. +And hence, unless we reject the doctrine of specific centres <i>in toto</i>, +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.</p></div> + +<div class="footnote"><p><a id="Footnote_39_39"></a><a href="#FNanchor_39_39"><span class="label">[39]</span></a> 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.</p></div> + +<div class="footnote"><p><a id="Footnote_40_40"></a><a href="#FNanchor_40_40"><span class="label">[40]</span></a> Insecta Maderensia, p. 6.</p></div> + +<div class="footnote"><p><a id="Footnote_41_41"></a><a href="#FNanchor_41_41"><span class="label">[41]</span></a> Insecta Maderensia, p. 36.</p></div> + +<div class="footnote"><p><a id="Footnote_42_42"></a><a href="#FNanchor_42_42"><span class="label">[42]</span></a> <i>Id.</i> p. 310.</p></div> + +<div class="footnote"><p><a id="Footnote_43_43"></a><a href="#FNanchor_43_43"><span class="label">[43]</span></a> Insecta Maderensia, p. 452.</p></div> + +<div class="footnote"><p><a id="Footnote_44_44"></a><a href="#FNanchor_44_44"><span class="label">[44]</span></a> Insecta Maderensia, p. 11.</p></div> + +<div class="footnote"><p><a id="Footnote_45_45"></a><a href="#FNanchor_45_45"><span class="label">[45]</span></a> Religion of Nature Delineated, p. 84.</p> +</div></div> + +<hr class="chap" /> + +<p><span class="pagenum"><a id="Page_95">[Pg 95]</a></span></p> + +<h2>CHAPTER IV.<br /> +<br /> +<span class="m2">ORGANS AND CHARACTERS OF VARIATION.</span></h2> + +<p>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 <i>members</i> +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.</p> + +<p>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<span class="pagenum"><a id="Page_96">[Pg 96]</a></span> +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 <i>sub</i>-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 <i>Harpalus</i> (the +<i>H. vividus</i>, 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 <i>Harpalus</i> 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<span class="pagenum"><a id="Page_97">[Pg 97]</a></span> +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 <i>sub</i>-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 <i>remains</i> 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.<a id="FNanchor_46_46"></a><a href="#Footnote_46_46" class="fnanchor">[46]</a>"</p> + +<p>But by far the greatest amount of variability to which +insect structure is liable, is presented by the <i>wings</i>,—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 +<span class="pagenum"><a id="Page_98">[Pg 98]</a></span>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 <i>latter</i> 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 +<i>principle</i> 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 +<i>proneness</i> 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?</p> + +<p>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<span class="pagenum"><a id="Page_99">[Pg 99]</a></span> +is everywhere inclined (if at all) to be especially stimulated: +whilst, as regards the second, it will be sufficient +to state, that in <i>continents</i>, when any decided alteration +takes place in the organs of flight, it for the most part +comes to pass that an <i>increased</i> (rather than diminished) +action is the result; whereas in <i>islands</i>, provided that +the species are not absolutely dependent on aërial 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 +<i>reverse</i> is nearer the truth. So that the <i>second</i> problem,—the +<i>reason why</i> 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 <i>sine quâ non</i> 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.</p> + +<p>Mr. Westwood, in his admirable <i>Introduction to the +Modern Classification of Insects</i>, 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 <i>enlargement</i> of the erratic powers. Speaking +of the <i>Aphelocheirus æstivalis</i> (a member of the<span class="pagenum"><a id="Page_100">[Pg 100]</a></span> +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 <i>the wings are fully developed</i>. I do not, however, +on that account, regard the former either as pupæ +or distinct species, but as undeveloped specimens in the +imago state<a id="FNanchor_47_47"></a><a href="#Footnote_47_47" class="fnanchor">[47]</a>." And whilst discussing the <i>Hydrometridæ</i>, +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, <i>acquire +their full size</i>. Hence, I consider that the apterous +specimens of <i>Hydrometra stagnorum</i>, 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<a id="FNanchor_48_48"></a><a href="#Footnote_48_48" class="fnanchor">[48]</a>." And, again, in his +reflections on the Hemiptera, Mr. Westwood says (and +most entomologists are aware of the fact): "The species +of <i>Gerris</i>, <i>Hydrometra</i>, and <i>Velia</i> are mostly found perfectly +apterous, though <i>occasionally with full-sized wings</i>. +<i>Chorosoma miriforme</i>, <i>Prostemma guttula</i>, <i>Pachymerus +brevipennis</i>, &c., are generally found with very short +wing-covers, but sometimes with full-sized wings<a id="FNanchor_49_49"></a><a href="#Footnote_49_49" class="fnanchor">[49]</a>." +In like manner, the <i>Cimex apterus</i>, Linn. (one of the +<span class="pagenum"><a id="Page_101">[Pg 101]</a></span><i>Lygæidæ</i>) "exhibits, in an eminent degree, the ordinary +occurrence of an imperfect perfect-state; whilst individuals +are occasionally found <i>with fully developed organs +of flight</i><a id="FNanchor_50_50"></a><a href="#Footnote_50_50" class="fnanchor">[50]</a>". <i>Lyæus brevipennis</i>, Lat., also ordinarily +occurs with abbreviated hemelytra; but it has been +found with them perfect by Westwood, as well as with +metathoracic wings.</p> + +<p>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 <i>causes</i> of it. Whether +the specimens of <i>Oncocephalus griseus</i>, to which Spinola +called attention, were insular ones, I cannot say; but he +seems to have noted an example in which an <i>opposite</i> +phænomenon 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 arrêté le développement +des organes du vol<a id="FNanchor_51_51"></a><a href="#Footnote_51_51" class="fnanchor">[51]</a>." And, again, when commenting +upon the other tendency in a representative of +the <i>Reduviadæ</i>, he says ('Essai,' p. 96): "Je pense que +la présence des ailes et leur développement dépendent +du climat." Whilst treating of two British species of +the same family, Mr. Westwood observes: "The <i>Prostemma +guttula</i>, Fab., and <i>Coranus subapterus</i>, Curt., are +interesting on account of their being generally found in +<span class="pagenum"><a id="Page_102">[Pg 102]</a></span>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<a id="FNanchor_52_52"></a><a href="#Footnote_52_52" class="fnanchor">[52]</a>." The common <i>Phosphuga +atrata</i> 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<a id="FNanchor_53_53"></a><a href="#Footnote_53_53" class="fnanchor">[53]</a>, that he has +several times taken it on the wing, during the hot sunshine. +And, concerning the <i>Olisthopus rotundatus</i>, he +states<a id="FNanchor_54_54"></a><a href="#Footnote_54_54" class="fnanchor">[54]</a> that every specimen which he captured in the +Scilly Islands was subapterous.</p> + +<p>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 <i>the length of the wings</i>,—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 <i>all</i> the sylvan birds in Malta, such as the Black-caps, +Willow-wrens, &c., though unquestionably of the +<span class="pagenum"><a id="Page_103">[Pg 103]</a></span>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, <i>inter alia</i>, for a slight shortening of their +organs of transit.</p> + +<p>If, however, the members of the insect tribes are +capable of but small variation in actual <i>structure</i>, 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<span class="pagenum"><a id="Page_104">[Pg 104]</a></span> +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), <i>selected</i>, to be acted +upon.</p> + +<p>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 <i>Ptinus +albopictus</i>,—a species which, whilst it averages more +than a line in length on the central island of the group, +is reduced to <i>less than half</i> that bulk on a small and +weather-beaten rock (the Ilheo Chão) at a distance from +it. Judging indeed from many hundred specimens of +the <i>Ptini</i> 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<a id="FNanchor_55_55"></a><a href="#Footnote_55_55" class="fnanchor">[55]</a>." +I have in fact generally noticed, that size and +colour are more peculiarly liable to be affected <i>together</i>. +This, however, is nothing more than what we should +anticipate, since the same causes which have stunted the +<span class="pagenum"><a id="Page_105">[Pg 105]</a></span>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.</p> + +<p>There are many insects which appear to have <i>two +distinct states</i>, 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<a id="FNanchor_56_56"></a><a href="#Footnote_56_56" class="fnanchor">[56]</a>, we nevertheless feel an <i>à priori</i> conviction +that they are by no means specifically dissimilar <i>inter se</i>. +Such phases, as regards <i>stature</i>, are presented by the +<i>Brachinus crepitans</i> and <i>Lamprias chlorocephalus</i> of our +own country; whilst, as regards <i>colour</i>, the <i>Philhydrus +melanocephalus</i>, <i>Aphodius plagiatus</i>, and the <i>Psylliodes +<span class="pagenum"><a id="Page_106">[Pg 106]</a></span>erythrocephala</i> (constituting in its paler garb the <i>P. nigricollis</i>, +Mshm) may be quoted, as cases in point. Thus, +also, in Madeira, the <i>Mycetoporus pronus</i>, 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 <i>Stenus Heeri</i>, Woll., and the +<i>Saprinus nitidulus</i>, Fab.<a id="FNanchor_57_57"></a><a href="#Footnote_57_57" class="fnanchor">[57]</a></p> + +<p>As regards the instability displayed by <i>colour</i>, 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, +<span class="pagenum"><a id="Page_107">[Pg 107]</a></span>that it is unnecessary to repeat it here. True it is that +it was then my sole province to discuss the <i>causes</i> which +would appear to regulate, in a large measure, the external +aspect of the Annulosa; yet the <i>existence</i> 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 <i>disturbing element</i> was demonstrated, +the mere fact that the thing (whatsoever it +may have been) <i>was interfered with</i>, was surely proved +<i>á fortiori</i>. 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 <i>Bembidium Atlanticum</i> 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 <i>Bembidium</i> to which it belongs is +essentially a variable one, yet I am not acquainted with +any <i>Peryphus</i> 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,<span class="pagenum"><a id="Page_108">[Pg 108]</a></span> +as are those of the <i>B. Atlanticum</i>: 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<a id="FNanchor_58_58"></a><a href="#Footnote_58_58" class="fnanchor">[58]</a>".</p> + +<p>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 +<span class="pagenum"><a id="Page_109">[Pg 109]</a></span>already insisted upon. Like most of the minutiæ 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 <i>Caulotropis lucifugus</i> 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 <i>C. conicollis</i>, 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 <i>Omias Waterhousei</i>, 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 <i>Ellipsodes glabratus</i> 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 <i>Helops confertus</i>, we have +intimated at a previous page, is less coarsely sculptured +in the lofty regions of Madeira, than in the lower ones: +and the <i>H. futilis</i> has its elytral tubercles apparent in +Madeira proper, but evanescent on the Dezerta Grande. +The <i>Eurygnathus Latreillei</i> assumes a permanent variety +on the Dezerta, the insect having become modified +<span class="pagenum"><a id="Page_110">[Pg 110]</a></span>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 striæ of its elytra somewhat more evidently +punctate<a id="FNanchor_59_59"></a><a href="#Footnote_59_59" class="fnanchor">[59]</a>.</p> + +<p>Such examples, however, might be multiplied <i>ad infinitum</i>; +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 <i>enumerate</i> 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.</p> + +<div class="footnotes"><h3>FOOTNOTES:</h3> + +<div class="footnote"><p><a id="Footnote_46_46"></a><a href="#FNanchor_46_46"><span class="label">[46]</span></a> Insecta Maderensia, pp. 56, 57.</p></div> + +<div class="footnote"><p><a id="Footnote_47_47"></a><a href="#FNanchor_47_47"><span class="label">[47]</span></a> Introduction to the Modern Classification of Insects, ii. p. 466.</p></div> + +<div class="footnote"><p><a id="Footnote_48_48"></a><a href="#FNanchor_48_48"><span class="label">[48]</span></a> <i>Id.</i> ii. p. 469.</p></div> + +<div class="footnote"><p><a id="Footnote_49_49"></a><a href="#FNanchor_49_49"><span class="label">[49]</span></a> <i>Id.</i> ii. p. 454.</p></div> + +<div class="footnote"><p><a id="Footnote_50_50"></a><a href="#FNanchor_50_50"><span class="label">[50]</span></a> Introduction to the Modern Classification of Insects, ii. p. 480.</p></div> + +<div class="footnote"><p><a id="Footnote_51_51"></a><a href="#FNanchor_51_51"><span class="label">[51]</span></a> Essai, p. 103.</p></div> + +<div class="footnote"><p><a id="Footnote_52_52"></a><a href="#FNanchor_52_52"><span class="label">[52]</span></a> Introduction to the Modern Classification of Insects, ii. p. 473.</p></div> + +<div class="footnote"><p><a id="Footnote_53_53"></a><a href="#FNanchor_53_53"><span class="label">[53]</span></a> Trans. of the Ent. Soc. of London, ii. p. 60.</p></div> + +<div class="footnote"><p><a id="Footnote_54_54"></a><a href="#FNanchor_54_54"><span class="label">[54]</span></a> <i>Id.</i> ii. p. 59.</p></div> + +<div class="footnote"><p><a id="Footnote_55_55"></a><a href="#FNanchor_55_55"><span class="label">[55]</span></a> Insecta Maderensia, pp. 260, 261.</p></div> + +<div class="footnote"><p><a id="Footnote_56_56"></a><a href="#FNanchor_56_56"><span class="label">[56]</span></a> Vide <i>supra</i>, p. 5.</p></div> + +<div class="footnote"><p><a id="Footnote_57_57"></a><a href="#FNanchor_57_57"><span class="label">[57]</span></a> 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 <i>Pupæ</i> have at least two abruptly-marked forms,—a +larger and smaller one. Many of the <i>Helices</i> also exhibit this tendency +in an eminent degree: I have indeed been shown specimens +by Sir Charles Lyell of the <i>Helix hirsuta</i>, 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 <i>have</i> not yet been observed to connect the extremes. May +not therefore the gigantic <i>H. Lowei</i> and <i>Bowdichiana</i>, which are +now extinct in the Madeira Islands, have been but forms of the +<i>H. Portosanctana</i> and <i>punctulata</i>, 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 <i>latter</i> are extremely rare in the fossil deposits, whilst they +themselves literally abound.</p></div> + +<div class="footnote"><p><a id="Footnote_58_58"></a><a href="#FNanchor_58_58"><span class="label">[58]</span></a> Insecta Maderensia, p. 78.</p></div> + +<div class="footnote"><p><a id="Footnote_59_59"></a><a href="#FNanchor_59_59"><span class="label">[59]</span></a> Insecta Maderensia, pp. 21, 22.</p></div> +</div> + +<hr class="chap" /> + +<p><span class="pagenum"><a id="Page_111">[Pg 111]</a></span></p> + +<h2>CHAPTER V.<br /> +<br /> +<span class="m2">GEOLOGICAL REFLECTIONS.</span></h2> + +<p>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,<span class="pagenum"><a id="Page_112">[Pg 112]</a></span> +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 phænomena 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.</p> + +<p>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<span class="pagenum"><a id="Page_113">[Pg 113]</a></span> +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.</p> + +<p>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 +<i>good</i> 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<span class="pagenum"><a id="Page_114">[Pg 114]</a></span> +many of the occult phænomena 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.</p> + +<p>First and foremost, perhaps, in importance, of all the +changes which it is self-evident have happened, may be +mentioned <i>subsidence</i>. 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 <i>existence</i> 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 <i>elevation</i> 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 <i>summits</i> 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 <i>must</i> have occurred in instances innumerable. +But, what would be the many results of a diminution in<span class="pagenum"><a id="Page_115">[Pg 115]</a></span> +the level of our imaginary range? It needs no argument +to prove, that <i>one</i> 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 +<i>somewhat modified from their normal states</i>,—states +which, be it recollected (for this is an instructive lesson), +would still exist in more northern zones.</p> + +<p>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 phænomena 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<span class="pagenum"><a id="Page_116">[Pg 116]</a></span> +undue accumulation of life on its loftiest pinnacles,—for, +even allowing a certain number of species (which +<i>even formerly</i> 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 <i>from the ancient elevations +above it</i>. But, if, on the other hand, an area, already +peopled, be in parts greatly upheaved, there will be +<i>either</i> 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.</p> + +<p>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 +<i>time</i> 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,<span class="pagenum"><a id="Page_117">[Pg 117]</a></span> +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 <i>general</i> sinking should +take place, causing its higher points to be alone visible +above the ocean, or merely a <i>partial</i> 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<span class="pagenum"><a id="Page_118">[Pg 118]</a></span> +animal population of this tract, the main phænomena are +almost self-evident. Should any of its isolated fragments +chance to contain a portion of one of <i>those limited +areas</i> 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 <i>endemic</i>. 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 <i>inter se</i>. 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 <i>except a few</i>.</p> + +<p>Such are the primary facts which suggest themselves, +whilst discussing the question of isolation as regulating +the <i>distribution</i> of the Annulose tribes. Its <i>after effects</i>, +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), <i>apart from</i> 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<span class="pagenum"><a id="Page_119">[Pg 119]</a></span> +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.</p> + +<p>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 <i>slowness</i> +and the <i>direction</i> 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, <i>which they do not</i>. +The latter of the above conclusions, namely, the <i>direction</i> +of the migratory current, will become apparent in the<span class="pagenum"><a id="Page_120">[Pg 120]</a></span> +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 <i>northerly</i> +one.</p> + +<p>As regards the slowness, and the direction, of the +<i>quondam</i> 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 <i>Hegeter</i>; 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 +primæval 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 <i>H. latebricola</i>, +Woll.) had arrived at what now constitutes the +rocks of the Salvages; another (the <i>H. elongatus</i>, Oliv.), +at least, if not two, had colonized the Madeiras, and is +said (though I believe incorrectly) to have even reached<span class="pagenum"><a id="Page_121">[Pg 121]</a></span> +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.</p> + +<p>In like manner, the genus <i>Tarphius</i> 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 <i>Tarphii</i> 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 primæval +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<span class="pagenum"><a id="Page_122">[Pg 122]</a></span> +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 <i>so</i> 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 <i>had not extended +elsewhere</i>. But I stated, that two of the above-mentioned +<i>Tarphii</i> 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 <i>T. Lowei</i>, Woll. (an insect of a +different, and more active, nature than the rest) which +has violated that <i>local exclusiveness</i> 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 <i>in +situ</i> on those elevated tracts, it hides within the bunches +of <i>Evernia scopulorum</i> and <i>prunastri</i> which clothe the +trunks of living trees, and fill up the crevices of the +weather-beaten peaks. Hence, when contrasted with<span class="pagenum"><a id="Page_123">[Pg 123]</a></span> +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 +<i>human</i> 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 <i>à priori</i>, in favour of the <i>T. Lowei</i> having overspread, +whether by natural or artificial means, a wider +area than its congeners. I believe that there is no such +thing as a <i>Tarphius</i> in the Canarian Group: nevertheless, +singularly enough, a representative, which is more +akin to the <i>T. Lowei</i> than to any other hitherto discovered +(and which was imagined until lately to have +been the sole exponent of the genus), namely, the +<i>T. gibbulus</i>, 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 +<i>Tarphii</i>, 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.</p> + +<p>The genus <i>Acalles</i> presents a nucleus of species in the +Canaries, moulded on a very large pattern. A closely<span class="pagenum"><a id="Page_124">[Pg 124]</a></span> +allied member, the <i>A. Neptunus</i>, Woll. (which may perhaps +be in reality but an insular modification of the +<i>A. argillosus</i>, Schön., 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 <i>Pecteropus</i>, 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 <i>Xenostrongylus</i>, +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 +<i>Ditylus</i> 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 <i>D. fulvus</i>, Woll.) is found on the +'Great Piton' of the Salvages, so nearly resembling, +except in its smaller size, one of those from the Canaries<span class="pagenum"><a id="Page_125">[Pg 125]</a></span> +that I think it far from improbable that it is a fixed +insular state of that insect. <i>Deucalion</i>, 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 <i>D. oceanicum</i>, Woll.) on +one of the rocks of the Salvages, whilst another (the +<i>D. Desertarum</i>, 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 <i>in extenso</i> the few reflexions which +then suggested themselves to my mind. "There is no +genus, perhaps, throughout all the Madeiran Coleoptera, +more truly indigenous than <i>Deucalion</i>. 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 +<span class="pagenum"><a id="Page_126">[Pg 126]</a></span>structure and sedentary<a id="FNanchor_60_60"></a><a href="#Footnote_60_60" class="fnanchor">[60]</a> 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 <i>Parmena</i> and <i>Dorcadion</i>, 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 minutiæ, yet +agreeing to an identity in everything generic, they offer +conjointly the strongest presumptive evidence to the +<i>quondam</i> 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 <i>Deucalion +<span class="pagenum"><a id="Page_127">[Pg 127]</a></span>Desertarum</i> is of the utmost rarity, the only two<a id="FNanchor_61_61"></a><a href="#Footnote_61_61" class="fnanchor">[61]</a> 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 +<i>whole area</i> of its actual primæval 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<a id="FNanchor_62_62"></a><a href="#Footnote_62_62" class="fnanchor">[62]</a>."</p> + +<p>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 primæval +continent. The researches of the Rev. R. T. Lowe, and +<span class="pagenum"><a id="Page_128">[Pg 128]</a></span>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 +<i>Helix tiarella</i>, <i>coronata</i>, and <i>coronula</i>,—which in all +probability still occupy the positions (or nearly so) of +their original <i>début</i> 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<span class="pagenum"><a id="Page_129">[Pg 129]</a></span> +fact that two of them (for the third has apparently +become extinct) have not altered one iota since the <i>fossil +period</i>, 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 +<i>Tarphii</i>, 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.</p> + +<p>In a similar manner, the <i>H. undata</i> in Madeira proper, +the <i>H. Vulcania</i> on the Dezertas, and the <i>H. Porto-sanctana</i> +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 <i>vice versâ</i>); yet, with +the analogy of the three already mentioned before us, I +am inclined <i>à priori</i> 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 <i>H. latens</i> +in Madeira, and the <i>H. obtecta</i> in Porto Santo; the +<i>H. squalida</i> in Madeira, and the <i>H. depauperata</i> in +Porto Santo; the <i>H. Delphinula</i> in Madeira, and the +<span class="pagenum"><a id="Page_130">[Pg 130]</a></span><i>H. tectiformis</i> in Porto Santo), which are no less representative +<i>inter se</i>. From which we are driven to conclude;—first, +that this <i>quondam</i> continent was densely +stocked at the beginning with foci of radiation created +expressly for itself<a id="FNanchor_63_63"></a><a href="#Footnote_63_63" class="fnanchor">[63]</a>; 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 <i>their migratory +progress was unusually slow</i>.</p> + +<p>Touching the two-fold question, of the <i>local engagement</i> +of this Atlantic district with specific centres of +diffusion, and the <i>extreme slowness of their diffusive progress</i>, +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 <i>generic area of radiation</i>. +<span class="pagenum"><a id="Page_131">[Pg 131]</a></span>Nor does this primæval 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<a id="FNanchor_64_64"></a><a href="#Footnote_64_64" class="fnanchor">[64]</a> 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 +<span class="pagenum"><a id="Page_132">[Pg 132]</a></span>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 <i>H. Wollastoni</i> 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 <i>H. Wollastoni</i> 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.</p> + +<p>In like manner, we might recall many others which +are peculiar, <i>recent and fossil</i>, to the self-same precincts. +Such, for example, are the <i>H. calculus</i> and <i>commixta</i>, +which swarm on the summit of the Ilheo de Baixo, in +both states. The <i>H. attrita</i>, again, is the Pico d'Anna +Ferreira modification of the <i>H. polymorpha</i>; and it is +only in the beds towards the base of that mountain that +its fossil homologue is found. But what do these facts<span class="pagenum"><a id="Page_133">[Pg 133]</a></span> +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 +primæval radiating forms.</p> + +<p>We must not however omit to notice, that some few +of these endemic <i>Helices</i> appear to have been gifted (as +we should <i>à priori</i> anticipate) with more rapid capabilities +for diffusion than the rest. Thus, the <i>H. erubescens</i> +and <i>paupercula</i> 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 +<i>H. polymorpha</i> has also penetrated the Madeiran region +throughout; and being, like the <i>H. erubescens</i>, 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 <i>H. attrita</i> has been applied; +when on the Ilheo de Baixo, it is the <i>H. papilio</i>; at the +Zimbra d'Areia, on the Pico de Conseilho, and in the +Ribeira da Coxinha, it is the <i>H. pulvinata</i>; and, in +many other situations widely removed <i>inter se</i>, it puts +on the shape (variable, both in size and hue) to which +the title of <i>H. discina</i> has been given. But, if we leave<span class="pagenum"><a id="Page_134">[Pg 134]</a></span> +Porto Santo, and follow this Protean <i>Helix</i> into the +other divisions of the group; we meet with it on the +Dezertas as the <i>H. senilis</i> (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 <i>H. lincta</i> (with an +additional pale variety for the calcareous district of +Caniçal),—and the <i>H. saccharata</i>, from the São Lourenço +promontory.</p> + +<p>In the same way we might pursue the <i>H. erubescens</i>, +and show that in the sylvan regions, and on the low +barren Ponta São Lourenço of Madeira, on the Pico +de Facho of Porto Santo, on the Ilheo Chão, 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 <i>Clausilia deltostoma</i> +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 <i>insects</i> to which I have lately called attention, +that where sufficient areas had been overspread (before +the periods of subsidence) for the creatures to have<span class="pagenum"><a id="Page_135">[Pg 135]</a></span> +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.</p> + +<p>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 <i>general direction</i> of the migratory current +over that ancient tract, and the <i>extreme slowness of its +progress</i>,—the former of which I consider probable +from the north-easterly course in which creatures <i>generically +identical</i> were, if we may so express it, "given-off;" +whilst the circumstance of their being for the +most part <i>specifically dissimilar</i> (or, in other words, of +the islands harbouring, many of them, species which +are endemic) would seem as it were to establish the +latter.</p> + +<p>We must not however forget, that it is only to the +<i>aborigines</i> of this <i>quondam</i> 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 <i>in an opposite direction</i> to the living<span class="pagenum"><a id="Page_136">[Pg 136]</a></span> +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 phænomena, 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<a id="FNanchor_65_65"></a><a href="#Footnote_65_65" class="fnanchor">[65]</a>, 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 +<i>at least two</i> 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 <i>westward progress</i> 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, <i>before</i> +<span class="pagenum"><a id="Page_137">[Pg 137]</a></span>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.</p> + +<p>As regards Professor Forbes's views of the creation of +a vast continent (reaching far into the Atlantic<a id="FNanchor_66_66"></a><a href="#Footnote_66_66" class="fnanchor">[66]</a>) 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, <i>per +se</i>, be not altogether worthless, I would point to the +conclusions (lately adverted to) which my Madeiran +researches have forced upon me, concerning the <i>direction</i> +of the former insect migrations,—inferences which +are, from first to last, of necessity erroneous, if the +<span class="pagenum"><a id="Page_138">[Pg 138]</a></span>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 +phænomena 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 <i>fact</i> (if such in reality it be) remains +untouched, that <i>the land itself is</i> at any rate <i>there</i>. 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<span class="pagenum"><a id="Page_139">[Pg 139]</a></span> +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.</p> + +<p>"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 <i>Apotomus</i>, <i>Xenostrongylus</i>, <i>Tarphius</i>, <i>Cholovocera</i>, +<i>Holoparamecus</i>, <i>Berginus</i>, <i>Litargus</i>, <i>Thorictus</i>, and <i>Boromorphus</i>. +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 +<i>Mesites</i>. Whether or not this partial parallelism may +be employed to further Professor E. Forbes's theory of +the <i>quondam</i> approximation, by means of a continuous +land, of the Kerry and Gallician hills, and of a huge<span class="pagenum"><a id="Page_140">[Pg 140]</a></span> +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 <i>northern tendency</i> 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 <i>subsidences</i> +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 <i>veritable</i> Atlantis +from beneath the waves of time<a id="FNanchor_67_67"></a><a href="#Footnote_67_67" class="fnanchor">[67]</a>."</p> +<p><span class="pagenum"><a id="Page_141">[Pg 141]</a></span></p> +<p>The <i>Mesites Maderensis</i>, Woll., to which I alluded in +the above quotation, is undoubtedly a strong case in +point. Although specifically dissimilar from the <i>M. +Tardii</i>, its Irish counterpart, it nevertheless approaches +it so closely, that it might be literally mistaken, <i>primâ +facie</i>, 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 +<i>inter se</i>; or, at all events, if distant, connected by an +intervening land:—in other words, that <i>generic areas</i>, +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 <i>as a law</i> +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 +<span class="pagenum"><a id="Page_142">[Pg 142]</a></span>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 phænomena 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 +<i>for the most part</i> 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 <i>Eucalypti</i>, in Australia; and of the +Bread-fruit Trees in the South Sea Islands: the ornithologist +points, <i>inter alia</i>, 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 +<i>Marsupialia</i>, except the genus <i>Didelphys</i>) as peculiar to +Australia and a few islands to the north of it; of <i>Lemur</i><span class="pagenum"><a id="Page_143">[Pg 143]</a></span> +<i>proper</i> to Madagascar; of the Sloths, Armadillos, Tree +Porcupines, and of Alligators, and of the <i>Platyrrhini</i> +(amongst the Monkeys), to South America; and of the +Ourangs to the islands of the Indian Archipelago.</p> + +<p>And so it is with the Insecta; many of the larger +groups of which (as <i>Amycterus</i> and <i>Paropsis</i>, in Australia; +<i>Pachyrhynchus</i> and <i>Apocyrtus</i>, in the Philippine Islands; +<i>Hipporhinus</i>, <i>Monochelus</i>, <i>Dichelus</i>, and <i>Moluris</i>, in +Southern Africa; <i>Macronota</i>, in Java; and <i>Naupactus</i>, +<i>Hypsonotus</i>, <i>Centrinus</i>, <i>Platyomus</i>, and <i>Cyrtonota</i>, 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 +<i>structural</i> characteristics are not uniform throughout, but +afford fixed geographical modifications (<i>not</i>, 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<a id="FNanchor_68_68"></a><a href="#Footnote_68_68" class="fnanchor">[68]</a> is <i>Saprinus</i>, as compared with +<i>Hister</i>; <i>Atlantis</i> with <i>Laparocerus</i>; and <i>Oxyomus</i> with +<span class="pagenum"><a id="Page_144">[Pg 144]</a></span><i>Aphodius</i>; and, I might also add, <i>Mesites</i> with <i>Cossonus</i>. +I believe indeed that <i>Mesites</i> 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 <i>M. Tardii</i> and <i>Maderensis</i>,—one of which +has been given off in the direction of Ireland, and the +other of the Madeiran Archipelago.</p> + +<p>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 <i>subsidences</i>,—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 <i>assumed +consequences</i> 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 +phænomena.</p><p><span class="pagenum"><a id="Page_145">[Pg 145]</a></span></p> + +<p>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), +<i>natural barriers</i> 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 <i>primary</i> +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 <i>ocean</i> (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 <i>subsidences</i> +are discussed,—subsidences which have had the +effect of letting it in over portions of an <i>already tenanted</i>, +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;<span class="pagenum"><a id="Page_146">[Pg 146]</a></span> +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<a id="FNanchor_69_69"></a><a href="#Footnote_69_69" class="fnanchor">[69]</a>."</p> + +<p>Conceding, therefore, this distinction between barriers +of a primæval 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 phænomena 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) +<span class="pagenum"><a id="Page_147">[Pg 147]</a></span>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 <i>them</i>. Others, as the +<i>Calosoma Syncophanta</i> of Europe, have been stated to +traverse the ocean unhurt<a id="FNanchor_70_70"></a><a href="#Footnote_70_70" class="fnanchor">[70]</a>; 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 <i>rejectamenta</i>, 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, +<span class="pagenum"><a id="Page_148">[Pg 148]</a></span>and then falling had been brought by the waves, cannot +certainly be ascertained; but Kalm's observation inclines +me to the latter opinion<a id="FNanchor_71_71"></a><a href="#Footnote_71_71" class="fnanchor">[71]</a>." And Sir Charles +Lyell remarks:—"Exotic beetles are sometimes thrown +on our shore, which revive after being drenched in salt +water<a id="FNanchor_72_72"></a><a href="#Footnote_72_72" class="fnanchor">[72]</a>." 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<a id="FNanchor_73_73"></a><a href="#Footnote_73_73" class="fnanchor">[73]</a>," +says Sir Charles Lyell, "they can readily spread themselves +<span class="pagenum"><a id="Page_149">[Pg 149]</a></span>wherever their progress is not opposed by uncongenial +climates, or by seas, mountains, and other +physical impediments; and <i>these</i> 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<a id="FNanchor_74_74"></a><a href="#Footnote_74_74" class="fnanchor">[74]</a>." 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 <i>Hydradephaga</i> are +thus occasionally transported; for Darwin mentions +having captured a <i>Colymbetes</i> off Cape S<sup>ta</sup> 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, +<span class="pagenum"><a id="Page_150">[Pg 150]</a></span>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 <i>Colymbetes</i>, <i>Hydroporus</i>, <i>Hydrobius</i>, <i>Notaphus</i>, +<i>Cynucus</i>, <i>Adimonia</i>, and <i>Scarabæus</i>. 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<a id="FNanchor_75_75"></a><a href="#Footnote_75_75" class="fnanchor">[75]</a>."</p> + +<p>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 +<span class="pagenum"><a id="Page_151">[Pg 151]</a></span>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.</p> + +<p>But if the sea and mountain ranges, when of a sufficient +age <i>in situ</i>, 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) <i>united</i>, 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 <i>be</i> +so joined? Undoubtedly it is: and hence we arrive at +the conclusion, that a <i>mountain island</i> should afford us +the <i>minimum of size, as regards the areas its species +have overspread</i>, which any country is able to furnish.</p> + +<p>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 <i>since</i> +that epoch: so that, whilst <i>one</i> of the natural barriers +against dispersion which it involves (namely, mountain +ridges) may be considered as primary; the <i>other</i> (to wit, +the sea, as it now obtains) has played, as an agent of<span class="pagenum"><a id="Page_152">[Pg 152]</a></span> +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 +phænomena of <i>areal limitation</i> upon which we have +been just commenting.</p> + +<p>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<span class="pagenum"><a id="Page_153">[Pg 153]</a></span> +island of floods; therefore, how much more must it +have been so when its primæval 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 <i>rejectamenta</i> 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<a id="FNanchor_76_76"></a><a href="#Footnote_76_76" class="fnanchor">[76]</a>.</p><p><span class="pagenum"><a id="Page_154">[Pg 154]</a></span></p> + +<p>But, after making due allowance for these +powerful means of dissemination (which, in the common +order of things, must necessarily obtain in <i>mountain +islands</i>, as it were, <i>par excellence</i>), 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 <i>will</i> act) as impassable barriers +to vast numbers of the insect tribes.</p> + +<p>With this single example (by way of illustration), +which the Madeiras have supplied, I will take my leave +of the question of <i>natural barriers, as tending to regulate +the topographical diffusion of the Annulosa</i>,—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 +<span class="pagenum"><a id="Page_155">[Pg 155]</a></span>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 <i>à priori</i> bound to do), we must at least act +consistently with ourselves, and not anticipate phænomena +where we have neither reason nor right to look +for them.</p> + +<p>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 <i>principle</i> which is <i>true</i>,<span class="pagenum"><a id="Page_156">[Pg 156]</a></span> +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 <i>may</i> 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 phænomena 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.<a id="FNanchor_77_77"></a><a href="#Footnote_77_77" class="fnanchor">[77]</a>"</p> + +<div class="footnotes"><h3>FOOTNOTES:</h3> + +<div class="footnote"><p><a id="Footnote_60_60"></a><a href="#FNanchor_60_60"><span class="label">[60]</span></a> "When we consider indeed the apterous nature of <i>Deucalion</i>, +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."</p></div> + +<div class="footnote"><p><a id="Footnote_61_61"></a><a href="#FNanchor_61_61"><span class="label">[61]</span></a> 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.</p></div> + +<div class="footnote"><p><a id="Footnote_62_62"></a><a href="#FNanchor_62_62"><span class="label">[62]</span></a> Insecta Maderensia, p. 435.</p></div> + +<div class="footnote"><p><a id="Footnote_63_63"></a><a href="#FNanchor_63_63"><span class="label">[63]</span></a> 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, <i>generic areas +of radiation</i> were part of the elementary design. Thus, Professor +E. Forbes records his belief that most, if not indeed <i>all</i>, 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.</p></div> + +<div class="footnote"><p><a id="Footnote_64_64"></a><a href="#FNanchor_64_64"><span class="label">[64]</span></a> Assuming the <i>Helix Lowei</i> and <i>Bowdichiana</i> to be gigantic +phases of the <i>H. Portosanctana</i> and <i>punctulata</i>, respectively; four +only, namely <i>H. fluctuosa</i> and <i>lapicida</i>, <i>Achatina Eulina</i>, and <i>Cyclostoma +lucidum</i> (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 Caniçal +beds show the <i>H. tiarella</i> 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 <i>H. coronata</i>, 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 +<i>H. coronula</i> 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.</p></div> + +<div class="footnote"><p><a id="Footnote_65_65"></a><a href="#FNanchor_65_65"><span class="label">[65]</span></a> Origin of the Fauna and Flora of the British Isles (in Mem. of +the Geol. Survey of Great Britain, vol. i. p. 336, <span class="smcap">a.d.</span> 1846).</p></div> + +<div class="footnote"><p><a id="Footnote_66_66"></a><a href="#FNanchor_66_66"><span class="label">[66]</span></a> "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."</p></div> + +<div class="footnote"><p><a id="Footnote_67_67"></a><a href="#FNanchor_67_67"><span class="label">[67]</span></a> 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 <i>quondam</i> existence) should have recently referred to +it seriously as the possible "Atlantis <i>of the ancients</i>!" Considering +that there is good reason to believe that all these islands <i>were +islands in a miocene sea</i>, 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 <i>Insulæ Fortunatæ</i> of Juba are +almost universally identified with the present Canarian Group (as +indeed the accurate description of Pliny well nigh demonstrates), +and the <i>Purpurariæ</i> with the Madeiras, ought at once, apart from +geological evidence, to point out the absurdity of the hypothesis, +that an Atlantic continent, <i>in the very position which those islands +occupy</i>, could have been acknowledged to have any existence by the +literature of either Rome or Greece.</p></div> + +<div class="footnote"><p><a id="Footnote_68_68"></a><a href="#FNanchor_68_68"><span class="label">[68]</span></a> Insecta Maderensia, p. 214.</p></div> + +<div class="footnote"><p><a id="Footnote_69_69"></a><a href="#FNanchor_69_69"><span class="label">[69]</span></a> Journal of Researches, pp. 326, 327.</p></div> + +<div class="footnote"><p><a id="Footnote_70_70"></a><a href="#FNanchor_70_70"><span class="label">[70]</span></a> Many of the <i>Calosomata</i> 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 <i>Calosoma</i> 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 <i>C. Syncophanta</i> 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.</p></div> + +<div class="footnote"><p><a id="Footnote_71_71"></a><a href="#FNanchor_71_71"><span class="label">[71]</span></a> Introduction to Entomology, ii. p. 13.</p></div> + +<div class="footnote"><p><a id="Footnote_72_72"></a><a href="#FNanchor_72_72"><span class="label">[72]</span></a> Principles of Geology, 9th ed. p. 657.</p></div> + +<div class="footnote"><p><a id="Footnote_73_73"></a><a href="#FNanchor_73_73"><span class="label">[73]</span></a> 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.</p></div> + +<div class="footnote"><p><a id="Footnote_74_74"></a><a href="#FNanchor_74_74"><span class="label">[74]</span></a> Principles of Geology, p. 656.</p></div> + +<div class="footnote"><p><a id="Footnote_75_75"></a><a href="#FNanchor_75_75"><span class="label">[75]</span></a> Journal of Researches, p. 159.</p></div> + +<div class="footnote"><p><a id="Footnote_76_76"></a><a href="#FNanchor_76_76"><span class="label">[76]</span></a> Insecta Maderensia, p. 81.</p></div> + +<div class="footnote"><p><a id="Footnote_77_77"></a><a href="#FNanchor_77_77"><span class="label">[77]</span></a> Religion of Nature Delineated, pp. 73, 74.</p></div> +</div> + +<hr class="chap" /> + +<p><span class="pagenum"><a id="Page_157">[Pg 157]</a></span></p> + +<h2>CHAPTER VI.<br /> +<br /> +<span class="m2">THE GENERIC THEORY.</span></h2> + +<p>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 <i>certain</i> 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.</p> + +<p>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<span class="pagenum"><a id="Page_158">[Pg 158]</a></span> +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 phænomena (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<span class="pagenum"><a id="Page_159">[Pg 159]</a></span> +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 <i>reasoning</i> [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 <i>particulars</i>, +in their turn easily point out and mark off new +particulars; and so render the sciences active<a id="FNanchor_78_78"></a><a href="#Footnote_78_78" class="fnanchor">[78]</a>." 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.</p> + +<p>With this preliminary stricture on the express duty +which devolves upon the naturalist (with whom the +phænomena of the organic world principally rest, for +interpretation) to make facts, rather than reason and +<span class="pagenum"><a id="Page_160">[Pg 160]</a></span>argument, the basis of his various doctrines,—at any +rate of those in which the critical subject of <i>arrangement</i> +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 +<i>genera</i>.</p> + +<p>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 <i>specific</i> +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 <i>dilatations</i> (as it were) along a<span class="pagenum"><a id="Page_161">[Pg 161]</a></span> +chain <i>which is itself composed of separate</i>, though differently +shaped, <i>links</i>? 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, <i>by degrees</i>, 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, <i>on the whole</i>, 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 <i>principle</i> is in every instance +the same; the difference being merely relative, and not +absolute.</p> + +<p>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<span class="pagenum"><a id="Page_162">[Pg 162]</a></span> +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 <i>precisely</i> alike (for +every species, however it may resemble its next ally, has +<i>some</i> 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 <i>dramatis personæ</i>, of independent +existence, which are nevertheless compelled to occupy +the situations we have described,—thus <i>causing</i> the divisions +to be mapped out) are here typified, as everywhere, +by the several beads themselves.</p> + +<p>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<span class="pagenum"><a id="Page_163">[Pg 163]</a></span> +forth, equally, the compartments of <i>primary</i> significance. +Nor would I wish to imply, by the above similes, that I +regard a <i>lineal</i> 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 <i>principle</i> +is concerned.</p> + +<p>It will consequently be seen, from what has been said, +that the terms "genus" and "species" not only differ +very considerably in <i>importance</i>, but in signification also. +Whilst the former is merely suggestive of a particular +<i>position</i> which a creature occupies in a systematic scale +(a position, however, which depends upon the various +structural peculiarities which it possesses <i>in common with +other beings</i>,—which thus more or less resemble it); the +latter expresses the actual creature itself: so that while +one applies to <i>several</i> animals (of distinct natures and +origins, though bound together by a certain bond of +imitation), the other belongs to <i>a single race alone</i>, 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 <i>animal itself</i>? 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 <i>convenient</i>, both for simplification and +analysis. We should assuredly be surprised were a man<span class="pagenum"><a id="Page_164">[Pg 164]</a></span> +to object to his surname, as unnecessary, because he has +a christian (or specific<a id="FNanchor_79_79"></a><a href="#Footnote_79_79" class="fnanchor">[79]</a>) one which is the exponent of +him <i>alone</i>. 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 +<i>individual</i> 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 +<i>natural groups</i>. The 'Méthode Mononomique' has indeed +been attempted<a id="FNanchor_80_80"></a><a href="#Footnote_80_80" class="fnanchor">[80]</a>; 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.</p> + +<p>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, +<span class="pagenum"><a id="Page_165">[Pg 165]</a></span>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 <i>either</i> +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 <i>not</i> 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 +'Méthode Mononomique' (who, paralysed by the fact that +genera are seldom <i>clearly defined at their extremes</i>, would +seem to repudiate them <i>in toto</i>) 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<span class="pagenum"><a id="Page_166">[Pg 166]</a></span> +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.</p> + +<p>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 <i>existence +of natural assemblages</i> 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 +<i>groups themselves</i> must still remain (however equivocal +it may be where they exactly commence or terminate), +and cannot be wiped out. To suppose <i>à priori</i> +that the allied divisions of the animate creation are perfectly +disconnected <i>inter se</i>, 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<span class="pagenum"><a id="Page_167">[Pg 167]</a></span> +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, <i>without injuring its individual reality</i>) 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 <i>general +principle</i> of fusion (either absolute or apparent) is most +obvious. From first to last, traces of it are everywhere +to be detected; not only between <i>clusters</i>, 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<span class="pagenum"><a id="Page_168">[Pg 168]</a></span> +they are specifically dissimilar, it can only arise from +their <i>near resemblance</i> 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 <i>therefore</i> should be recognized; we may perhaps +be told by the believers in the 'Méthode Mononomique,' +that they do not intend to ignore the <i>arrangement</i> which +Nature has so broadly laid down, but that, on the +contrary, they tacitly endorse it,—their device having +reference to the <i>names</i> 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 +<i>acknowledge</i> 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 <i>do</i> consider them of primary importance; it is +more in unison with our notions of what ought to be; +more suggestive of what actually <i>is</i>; more honest and +generous to those who have laboured (as describers), with +such care and diligence, before us.</p> + +<p>It will be perceived, from the above remarks, that, +although professedly criticizing the 'Méthode Mononomique,' +into the analysis of which my subject has +unintentionally drawn me, it is the absurdity of objecting +to genera <i>because they are not rigidly defined +throughout</i>, that I have been mainly striving to condemn.<span class="pagenum"><a id="Page_169">[Pg 169]</a></span> +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, <i>cannot</i> be homogeneous,—seeing +that they are necessarily composed of detached species, +no two of which are <i>precisely</i> similar, even in the few +structural details which may have been accidentally +chosen for generic diagnostics. How is it possible, +therefore, that mere <i>groups</i>, 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?</p> + +<p>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 <i>dilatations</i> (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<span class="pagenum"><a id="Page_170">[Pg 170]</a></span> +"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 <i>species</i>, 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 +<i>concentric bulb</i>, which becomes denser, as it were, in its +successive component layers, and more typical, as it +approaches its core.</p> + +<p>If, then, the theory of genera be such as I have endeavoured +to expound, it results from what has been said, +<i>that every generic type is to be looked for in, or about, +the centre of its peculiar group</i>,—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,<span class="pagenum"><a id="Page_171">[Pg 171]</a></span> +through using the metaphors <i>which I selected for the +elucidation of a principle</i>, 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 <i>equally</i> from their normal foci: so that it is in the +direction of the <i>higher</i> (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.</p> + +<p>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, <i>the diagnostic characters</i>, although (in +combination) carried out to the full, <i>are more evenly +balanced in a generic type than in any of its associates</i>; +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 <i>à priori</i> regarded as in all probability<span class="pagenum"><a id="Page_172">[Pg 172]</a></span> +a transition form, leading us onwards into some +neighbouring group<a id="FNanchor_81_81"></a><a href="#Footnote_81_81" class="fnanchor">[81]</a>.</p> + +<p>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 <i>cannot</i> 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.</p> + +<p>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 +<span class="pagenum"><a id="Page_173">[Pg 173]</a></span>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, <i>with equal +reason</i>, 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 <i>convenience</i> may be +omitted from our speculations <i>in toto</i>,—seeing that <i>all</i> +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<span class="pagenum"><a id="Page_174">[Pg 174]</a></span> +to the judgment of the particular naturalist who has to +deal with them.</p> + +<p>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 +<i>Carabidæ</i>, of the Coleoptera, is eminently a +case in point. In the details of their oral organs the +<i>whole</i> of that family display (as I have elsewhere<a id="FNanchor_82_82"></a><a href="#Footnote_82_82" class="fnanchor">[82]</a> remarked) +so great a similarity <i>inter se</i>, or rather shade +off into each other by such imperceptible gradations, +that the <i>tendency</i> 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 <i>generic</i>, 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 <i>either</i> to +accept the Linnæan 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 +<span class="pagenum"><a id="Page_175">[Pg 175]</a></span>under the former contingency the <i>determination of species</i> +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 <i>Pterostichus</i>, which has nearly 200 representatives +in Europe alone: true it is that its several sections +(<i>Pœcilus</i>, <i>Argutor</i>, <i>Omaseus</i>, <i>Corax</i>, <i>Steropus</i>, <i>Platysma</i>, +<i>Cophosus</i>, <i>Pterostichus</i> proper, <i>Abax</i>, <i>Percus</i>, and <i>Molops</i>), +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 <i>Pterostichi</i> (the +Argutors, for instance) approach so closely, in their +trophi, to <i>Calathus</i>, as to be hardly discernible from it; +which latter genus is scarcely distinguishable (structurally) +from <i>Pristonychus</i>,—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, <i>Calosoma</i> and <i>Carabus</i>, were not +thoroughly detached <i>inter se</i>? yet what naturalist <i>now</i> +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: <i>that if genera must be rejected because +they are not homogeneous and isolated throughout, the<span class="pagenum"><a id="Page_176">[Pg 176]</a></span> +only ones that will remain are those which have become +abrupt from causes which are merely accidental</i>.</p> + +<p>Having now, however, examined the question in its +broadest phasis, that is to say, on the supposition that +Nature is <i>complete</i> 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, <i>from accident</i>. 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 <i>definition</i> from examples +which are the exception, and not the rule,—and, <i>more</i> +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 <i>anterior</i> +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 <i>generic +provinces</i> of radiation (no less than specific centres) be<span class="pagenum"><a id="Page_177">[Pg 177]</a></span> +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, <i>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</i>.</p> + +<p>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: <i>that it is in +islands that we should mainly look for genera which +are to be rigidly pronounced</i>. 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, <i>that the greater proportion of +those groups which are more especially isolated in their +character</i> (I do not say, necessarily, the most anomalous; +though this in some measure follows from the fact of +their detachment) <i>are peculiar to countries which are +insular</i>.</p> + +<p>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<span class="pagenum"><a id="Page_178">[Pg 178]</a></span> +medium of which genera may become well-defined. We +should recollect that the removal of a <i>very few</i> 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<a id="FNanchor_83_83"></a><a href="#Footnote_83_83" class="fnanchor">[83]</a>. 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 <i>now</i> be consequently looked +<span class="pagenum"><a id="Page_179">[Pg 179]</a></span>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 primæval 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.</p> + +<p>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 <i>they</i> 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 <i>must</i> +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 <i>unity</i> at which I<span class="pagenum"><a id="Page_180">[Pg 180]</a></span> +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.</p> + +<div class="footnotes"><h3>FOOTNOTES:</h3> + +<div class="footnote"><p><a id="Footnote_78_78"></a><a href="#FNanchor_78_78"><span class="label">[78]</span></a> "Nullo modo fieri potest, ut axiomata per argumentationem +constituta ad inventionem novorum operum valeant; quia subtilitas +naturæ subtilitatem argumentandi multis partibus superat. Sed +axiomata a particularibus rite et ordine abstracta, nova particularia +rursus facile indicant et designant; itaque scientias reddunt activas."—<i>Novum +Organum</i>, Aphoris. xxiv.</p></div> + +<div class="footnote"><p><a id="Footnote_79_79"></a><a href="#FNanchor_79_79"><span class="label">[79]</span></a> In selecting this simple method to illustrate the <i>principle</i> 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 <i>specifically +distinct</i> from his neighbour!</p></div> + +<div class="footnote"><p><a id="Footnote_80_80"></a><a href="#FNanchor_80_80"><span class="label">[80]</span></a> Considérations sur un Nouveau Système de Nomenclature, par +C. J. B. Amyot (<i>Rev. Zool.</i>, p. 133, <span class="smcap">a.d.</span> 1838).</p></div> + +<div class="footnote"><p><a id="Footnote_81_81"></a><a href="#FNanchor_81_81"><span class="label">[81]</span></a> 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.</p></div> + +<div class="footnote"><p><a id="Footnote_82_82"></a><a href="#FNanchor_82_82"><span class="label">[82]</span></a> Annals of Nat. Hist. (2nd series), xiv., p. 199.</p></div> + +<div class="footnote"><p><a id="Footnote_83_83"></a><a href="#FNanchor_83_83"><span class="label">[83]</span></a> 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 (<i>Mus rattus</i>) 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 <i>quite</i> exterminated: +it has been recorded (<i>vide</i> '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.</p></div> +</div> + +<hr class="chap" /> + +<p><span class="pagenum"><a id="Page_181">[Pg 181]</a></span></p> + +<h2>CHAPTER VII.<br /> +<br /> +<span class="m2">CONCLUSION</span></h2> + +<p class="h4">Depositâ sarcinâ, levior volabo ad cœlum.—<i>S. Jerome.</i></p> + +<p>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.</p> + +<p>Commencing with the thesis, that specific variation, +whether as a matter of experience or as probable from +analogy, does <i>ipso facto</i> 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<span class="pagenum"><a id="Page_182">[Pg 182]</a></span> +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 <i>absolute</i> +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 <i>must</i> be reasonable to direct our +steps by probability, when we have nothing clearer to +walk by".<a id="FNanchor_84_84"></a><a href="#Footnote_84_84" class="fnanchor">[84]</a></p> + +<p>What my chief aim in the present treatise has been, +will be easily perceived,—namely, to substantiate, as +such, those <i>elements of disturbance</i> (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 +<span class="pagenum"><a id="Page_183">[Pg 183]</a></span>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 <i>primâ-facie</i> 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 <i>representative species</i> 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 <i>representation</i>, 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 <i>actual</i> 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 <i>nearly</i> 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 primæval haunts, and have become +gradually altered by the circumstances amongst which<span class="pagenum"><a id="Page_184">[Pg 184]</a></span> +they have been placed, than that the respective phases +were produced <i>in situ</i> on patterns almost coincident.</p> + +<p>I have before announced my conviction, that <i>generic +areas</i> 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 "<i>presumptive</i> 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 <i>entire</i> +interval since the first appearance of the present animals +and plants upon our earth, there is at any rate an +<i>à priori</i> probability that they are no <i>species</i> at all,—but +permanent geographical states, which have been slowly +matured since their casual introduction beyond their +legitimate bounds.</p> + +<p>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<span class="pagenum"><a id="Page_185">[Pg 185]</a></span> +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 primæval 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 <i>Helices</i> may be cited (which I have already +done<a id="FNanchor_85_85"></a><a href="#Footnote_85_85" class="fnanchor">[85]</a>) 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.</p> + +<p>But, apart from the fact that I would not wish to +resign <i>in toto</i> 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, +<i>á fortiori</i>, 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.</p> + +<p><span class="pagenum"><a id="Page_186">[Pg 186]</a></span></p><p>I do not, however, assert that <i>every</i> species is liable to +be interfered with <i>ab extra</i>; 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 <i>all</i>-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 <i>within fixed specific bounds</i> 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.</p> + +<p>It does indeed appear strange that naturalists, who +have combined great synthetic qualities with a profound +knowledge of minutiæ and detail, should ever have +upheld so monstrous a doctrine as that of the transmission +of one species into another,—a doctrine, however,<span class="pagenum"><a id="Page_187">[Pg 187]</a></span> +which arises almost spontaneously,—if we are to assume +that there exists in every race the tendency to <i>an unlimited +progressive improvement</i>. 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 phænomenon, 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<span class="pagenum"><a id="Page_188">[Pg 188]</a></span> +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<a id="FNanchor_86_86"></a><a href="#Footnote_86_86" class="fnanchor">[86]</a>."</p> + +<p>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 <i>Tarphii</i> (to which +I have already alluded<a id="FNanchor_87_87"></a><a href="#Footnote_87_87" class="fnanchor">[87]</a>) 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 +<span class="pagenum"><a id="Page_189">[Pg 189]</a></span>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 <i>début</i> 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.</p> + +<p>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<span class="pagenum"><a id="Page_190">[Pg 190]</a></span> +have often, because permanent, been at once regarded +as specific, may not be <i>sometimes</i> 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.</p> + +<p>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<span class="pagenum"><a id="Page_191">[Pg 191]</a></span> +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 <i>modus operandi</i> 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 <i>much</i>. +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<span class="pagenum"><a id="Page_192">[Pg 192]</a></span> +sweet, full and harmonious; but never reaches a close: +no cadence is heard with which the intellectual ear can +feel satisfied<a id="FNanchor_88_88"></a><a href="#Footnote_88_88" class="fnanchor">[88]</a>."</p> + +<p>As regards that most obscure of questions, <i>what the +limits of species really are</i>, 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 <i>facts</i>, 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 +<span class="pagenum"><a id="Page_193">[Pg 193]</a></span>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 <i>on account</i> of their pliability, it is not +for me to conjecture; nevertheless, the first of these inferences +is the one which I should, myself, be <i>à priori</i> +inclined to subscribe to.</p> + +<p>In examining, however, this enigma, <i>of the limits +within which variation is</i> (as such) <i>to be recognized</i>; 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, <i>inter se</i>, 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.</p> + +<p>But, whatever be the several ranges within which the +members of the organic creation are free to vary; we +are positively certain that, <i>unless the definition of a species, +as involving relationship, be more than a delusion or romance</i>, +their circumferences are of necessity real, and +must be indicated <i>somewhere</i>,—as strictly, moreover, and +rigidly, as it is possible for anything in Nature to be<span class="pagenum"><a id="Page_194">[Pg 194]</a></span> +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,—</p> + +<div class="inset22"> +<p> +"Still changing, yet unchanged; still doom'd to feel<br /> +<i>Endless mutation, in perpetual rest</i>."<br /> +</p> +</div> + +<div class="footnotes"><p>FOOTNOTES:</p> + +<div class="footnote"><p><a id="Footnote_84_84"></a><a href="#FNanchor_84_84"><span class="label">[84]</span></a> Religion of Nature Delineated, p. 103.</p></div> + +<div class="footnote"><p><a id="Footnote_85_85"></a><a href="#FNanchor_85_85"><span class="label">[85]</span></a> Vide <i>supra</i>, p. 128.</p></div> + +<div class="footnote"><p><a id="Footnote_86_86"></a><a href="#FNanchor_86_86"><span class="label">[86]</span></a> Principles of Geology, 9th edition, pp. 583, 584.</p></div> + +<div class="footnote"><p><a id="Footnote_87_87"></a><a href="#FNanchor_87_87"><span class="label">[87]</span></a> Vide <i>supra</i>, p. 121.</p></div> + +<div class="footnote"><p><a id="Footnote_88_88"></a><a href="#FNanchor_88_88"><span class="label">[88]</span></a> Indications of the Creator (London, 1845), p. 163.</p></div> +</div> + +<hr class="chap" /> + +<p><span class="pagenum"><a id="Page_195">[Pg 195]</a></span></p> + +<h2>INDEX.</h2> + +<ul class="index"><li class="ifrst">Aberration, perhaps indicated universally, <a href="#Page_16">16</a>, <a href="#Page_17">17</a>, <a href="#Page_18">18</a>.</li> + +<li class="indx">Aborigines, insect, unimportant for climatal modifications, <a href="#Page_25">25</a>, <a href="#Page_26">26</a>, <a href="#Page_27">27</a>.</li> + +<li class="indx"><i>Acalles</i>, the Canarian type of, apparent on the Salvages and Dezertas, <a href="#Page_124">124</a>.</li> + +<li class="indx"><i>—— Neptunus</i>, Woll., perhaps a state of <i>A. argillosus</i>, <a href="#Page_124">124</a>.</li> + +<li class="indx"><i>Achatina Eulima</i>, Lowe, its extinction in Porto Santo, <a href="#Page_131">131</a>.</li> + +<li class="indx"><i>Achenium Hartungii</i>, Heer, a form of <i>A. depressum</i>, <a href="#Page_65">65</a>.</li> + +<li class="indx"><i>Acherontia Atropos</i>, Linn., its introduction into Madeira perhaps recent, <a href="#Page_74">74</a>.</li> + +<li class="indx"><i>Adimonia</i>, the capture of, out at sea, <a href="#Page_150">150</a>.</li> + +<li class="indx"><i>Aëpus marinus</i>, Ström., pallid hue of, <a href="#Page_64">64</a>.</li> + +<li class="indx"><i>—— Robinii</i>, Lab., pallid hue of, <a href="#Page_64">64</a>.</li> + +<li class="indx"><i>Agabus bipustulatus</i>, Linn., unaffected by climate, <a href="#Page_31">31</a>.</li> + +<li class="indx">Alligators, their peculiarity to S. America, <a href="#Page_143">143</a>.</li> + +<li class="indx">Alpine species, some peculiarly so, <a href="#Page_40">40</a>.</li> + +<li class="indx">Altitude and latitude, sometimes reciprocal, <a href="#Page_35">35</a>, <a href="#Page_114">114</a>.</li> + +<li class="indx"><i>Amyeterus</i>, its concentration in Australia, <a href="#Page_143">143</a>.</li> + +<li class="indx">Amyot, M., his 'Méthode Mononomique,' <a href="#Page_164">164</a>.</li> + +<li class="indx">Analogies, Lord Bacon on the importance of, <a href="#Page_13">13</a>;</li> +<li class="isub1">why necessary to be studied, <a href="#Page_14">14</a>.</li> + +<li class="indx">Analogy, argument from, <a href="#Page_10">10</a>, <a href="#Page_11">11</a>, <a href="#Page_12">12</a>.</li> + +<li class="indx"><i>Anchomenus marginatus</i>, Linn., slightly modified in Madeira, <a href="#Page_38">38</a>.</li> + +<li class="indx">Andes, dissimilarity of the fauna on the opposite sides of the, <a href="#Page_146">146</a>.</li> + +<li class="indx"><i>Anobium striatum</i>, Oliv., unaffected by climate, <a href="#Page_31">31</a>.</li> + +<li class="indx">Antennæ, joints of, said occasionally to vary, <a href="#Page_96">96</a>.</li> + +<li class="indx"><i>Anthicus bimaculatus</i>, Illig., variability of, near the sea, <a href="#Page_63">63</a>.</li> + +<li class="indx"><i>—— fenestratus</i>, Schmidt, slightly modified in Madeira, <a href="#Page_38">38</a>.</li> + +<li class="indx"><i>—— humilis</i>, Germ., variability of in salt places, <a href="#Page_63">63</a>.</li> + +<li class="indx"><i>—— instabilis</i>, Hoffm., pallid hue of, <a href="#Page_64">64</a>.</li> + +<li class="indx"><i>Anthonomus ater</i>, Mshm, very small in Lundy Island, <a href="#Page_58">58</a>, <a href="#Page_73">73</a>.</li> + +<li class="indx"><i>Aphelocheirus æstivalis</i>, Fabr., the hemelytra of, sometimes fully developed, <a href="#Page_100">100</a>.</li> + +<li class="indx"><i>Aphodius nitidulus</i>, Fabr., paler in Madeira than in Europe generally, <a href="#Page_65">65</a>.<span class="pagenum"><a id="Page_196">[Pg 196]</a></span></li> + +<li class="indx"><i>Aphodius plagiatus</i>, Linn., usually black in England, <a href="#Page_61">61</a>;</li> +<li class="isub1">two distinct states of, indicated, <a href="#Page_105">105</a>.</li> + +<li class="indx"><i>Apocyrtus</i>, its concentration in the Philippine Islands, <a href="#Page_143">143</a>.</li> + +<li class="indx"><i>Apotomus</i>, common to Madeira and Sicily, <a href="#Page_139">139</a>.</li> + +<li class="indx"><i>Argutor</i>, always apterous in Madeira, <a href="#Page_82">82</a>;</li> +<li class="isub1">trophi of, almost identical with those of <i>Calathus</i>, <a href="#Page_175">175</a>.</li> + +<li class="indx">Armadillos, their peculiarity to S. America, <a href="#Page_143">143</a>.</li> + +<li class="indx">Armitage, Mr., on <i>Cicindela fasciatopunctata</i> from Mount Olympus, <a href="#Page_41">41</a>.</li> + +<li class="indx">Arrangement, a lineal one is not indicated in Nature, <a href="#Page_163">163</a>.</li> + +<li class="indx">Atlantic continent, Prof. E. Forbes on the former existence of, <a href="#Page_137">137</a>.</li> + +<li class="indx">Atlantis of the ancients, the impossibility of its being identified with a former Atlantic region, <a href="#Page_140">140</a>;</li> +<li class="isub1">perhaps the New World, <a href="#Page_141">141</a>.</li> + +<li class="indx"><i>Atlantis</i>, the genus, a modification of <i>Laparocerus</i>, <a href="#Page_143">143</a>.</li> + +<li class="indx">Azores, the colonization of, by two Madeiran <i>Helices</i>, <a href="#Page_133">133</a>.</li> + +<li class="ifrst">Bacon, Lord, on the importance of analogies, <a href="#Page_13">13</a>;</li> +<li class="isub1">on the Atlantis of the ancients, <a href="#Page_141">141</a>;</li> +<li class="isub1">on the necessity of observation for forming science, <a href="#Page_159">159</a>.</li> + +<li class="indx">Banksias, their concentration in Australia, <a href="#Page_142">142</a>.</li> + +<li class="indx">Barriers, natural, the difference between primary and recent, <a href="#Page_145">145</a>;</li> +<li class="isub1">their hindrance to insect diffusion, <a href="#Page_145">145</a>.</li> + +<li class="indx"><i>Bembidium Atlanticum</i>, Woll., paler in Porto Santo than in Madeira, <a href="#Page_66">66</a>;</li> +<li class="isub1">the variations to which it is subject, <a href="#Page_107">107</a>, <a href="#Page_108">108</a>.</li> + +<li class="indx"><i>—— bistriatum</i>, Dufts., paler in saline districts, <a href="#Page_62">62</a>.</li> + +<li class="indx"><i>—— ephippium</i>, Mshm, pallid hue of, <a href="#Page_64">64</a>.</li> + +<li class="indx"><i>—— obtusum</i>, Sturm, varies in southern latitudes, <a href="#Page_33">33</a>.</li> + +<li class="indx"><i>—— pallidipenne</i>, Illig., pallid hue of, <a href="#Page_64">64</a>.</li> + +<li class="indx"><i>—— saxatile</i>, Gyll., variety of, on the south coast of England, <a href="#Page_60">60</a>.</li> + +<li class="indx"><i>—— Schmidtii</i>, Woll., perhaps a state of <i>B. callosum</i>, <a href="#Page_66">66</a>.</li> + +<li class="indx"><i>—— scutellare</i>, Germ., pallid hue of, <a href="#Page_64">64</a>.</li> + +<li class="indx"><i>—— tabellatum</i>, Woll., perhaps a state of <i>B. tibiale</i>, <a href="#Page_66">66</a>.</li> + +<li class="indx"><i>Berginus</i>, common to Madeira and Sicily, <a href="#Page_139">139</a>.</li> + +<li class="indx">Black Rat, nearly exterminated in England, <a href="#Page_178">178</a>.</li> + +<li class="indx"><i>Blemus areolatus</i>, Creutz., paler in brackish places, <a href="#Page_62">62</a>.</li> + +<li class="indx"><i>Bolitochara assimilis</i>, Kby, smallness of, in the Scilly Islands, <a href="#Page_73">73</a>.</li> + +<li class="indx"><i>Boromorphus</i>, common to Madeira and Sicily, <a href="#Page_139">139</a>.</li> + +<li class="indx"><i>Brachinus crepitans</i>, Linn., two distinct sizes of, frequently indicated, +<a href="#Page_105">105</a>.</li> + +<li class="indx"><i>Bradycellus fulvus</i>, Mshm, apterous in Madeira, <a href="#Page_85">85</a>.</li> + +<li class="indx">Bread-fruit Trees, their peculiarity to the South Sea Islands, <a href="#Page_142">142</a>.</li> + +<li class="ifrst"><i>Calathus</i>, apterous in Madeira, <a href="#Page_82">82</a>; its trophi almost identical with those of <i>Pristonychus</i>, <a href="#Page_175">175</a>.</li> + +<li class="indx"><i>—— complanatus</i>, Koll., varies from altitude, <a href="#Page_39">39</a>;</li> +<li class="isub1">variety of, on one of the Madeira Islands, <a href="#Page_88">88</a>.</li> + +<li class="indx"><span class="pagenum"><a id="Page_197">[Pg 197]</a></span><i>—— fuscus</i>, Fabr., slightly modified in Madeira, <a href="#Page_38">38</a>, <a href="#Page_85">85</a>.</li> + +<li class="indx"><i>Calathus melanocephalus</i>, Linn., smallness of, in the Scilly Islands, <a href="#Page_73">73</a>.</li> + +<li class="indx"><i>—— mollis</i>, Mshm, variable in its wings, <a href="#Page_43">43</a>;</li> +<li class="isub1">lurid colour of, <a href="#Page_64">64</a>.</li> + +<li class="indx">Calcareous soils, effect of, on the aspect of insects, <a href="#Page_66">66</a>.</li> + +<li class="indx">Calceolarias, their concentration on the Andes, <a href="#Page_142">142</a>.</li> + +<li class="indx"><i>Calosoma</i>, a species of, ten miles from shore, <a href="#Page_147">147</a>;</li> +<li class="isub1">the genus, mergescgradually into <i>Carabus</i>, <a href="#Page_175">175</a>.</li> + +<li class="indx"><i>—— Syncophanta</i>, Linn., its power of crossing the sea, <a href="#Page_147">147</a>.</li> + +<li class="indx">Canary Islands, migratory direction of their insect population, <a href="#Page_119">119</a>.</li> + +<li class="indx"><i>Carabidæ</i>, inconstant in their organs of flight, <a href="#Page_43">43</a>;</li> +<li class="isub1">family of, nearly similar throughout in its oral organs, <a href="#Page_174">174</a>.</li> + +<li class="indx"><i>Carpophilus hemipterus</i>, Linn., unaffected by climate, <a href="#Page_31">31</a>.</li> + +<li class="indx"><i>Caulotrupis conicollis</i>, Woll., large size of, on one of the Madeira Islands, <a href="#Page_88">88</a>, <a href="#Page_109">109</a>.</li> + +<li class="indx"><i>—— lucifugus</i>, Woll., varies from isolation, <a href="#Page_90">90</a>, <a href="#Page_109">109</a>.</li> + +<li class="indx">Causes, never final ones which we investigate, <a href="#Page_191">191</a>.</li> + +<li class="indx"><i>Centrinus</i>, its concentration in S. America, <a href="#Page_143">143</a>.</li> + +<li class="indx"><i>Ceutorhynchus contractus</i>, Mshm, smallness of, in Lundy Island, <a href="#Page_59">59</a>, <a href="#Page_73">73</a>.</li> + +<li class="indx"><i>Cholovocera</i>, common to Madeira and Sicily, <a href="#Page_139">139</a>.</li> + +<li class="indx"><i>Choreius ineptus</i>, Westw., on a winged state of, <a href="#Page_44">44</a>.</li> + +<li class="indx"><i>Chorosoma miriforme</i>, the development of the wings of, <a href="#Page_100">100</a>.</li> + +<li class="indx"><i>Chrysomela</i>, apterous in Madeira, <a href="#Page_82">82</a>.</li> + +<li class="indx"><i>Chrysomelæ</i>, vary from altitude, <a href="#Page_41">41</a>.</li> + +<li class="indx"><i>Chrysomelidæ</i>, almost absent in Tierra del Fuego, <a href="#Page_47">47</a>.</li> + +<li class="indx"><i>Cicindela fasciatopunctata</i>, Germ., a state of <i>C. sylvatica</i> <a href="#Page_41">41</a>.</li> + +<li class="indx"><i>Cicindelidæ</i>, often variable, <a href="#Page_41">41</a>.</li> + +<li class="indx"><i>Cillenum laterale</i>, Sam., lurid hue of, <a href="#Page_64">64</a>.</li> + +<li class="indx"><i>Cimex apterus</i>, Linn., the development of the wings of, <a href="#Page_100">100</a>.</li> + +<li class="indx"><i>—— lectularius</i>, Linn., on the development of the wings of, <a href="#Page_45">45</a>.</li> + +<li class="indx"><i>Cistela sulphurea</i>, Linn., its variability near the sea, <a href="#Page_60">60</a>.</li> + +<li class="indx"><i>Clausilia deltostoma</i>, Lowe, a Porto-Santan form of, <a href="#Page_134">134</a>.</li> + +<li class="indx">Climatal modifications significant, although small, <a href="#Page_42">42</a>.</li> + +<li class="indx">Climate, not important as a disturbing cause, <a href="#Page_23">23</a>, <a href="#Page_24">24</a>, <a href="#Page_31">31</a>, <a href="#Page_32">32</a>, <a href="#Page_42">42</a>.</li> + +<li class="indx">Clouded-yellow Butterfly, unaffected by climate, <a href="#Page_31">31</a>.</li> + +<li class="indx"><i>Clypeaster pusillus</i>, Gyll., differs slightly in Madeira, <a href="#Page_65">65</a>.</li> + +<li class="indx">Coast, inconstancy of insects in the vicinity of the, <a href="#Page_57">57</a>.</li> + +<li class="indx"><i>Coccinella 7-punctata</i>, Linn., unaffected by climate, <a href="#Page_31">31</a>.</li> + +<li class="indx"><i>Colias Edusa</i>, Fabr., unaffected by climate, <a href="#Page_31">31</a>.</li> + +<li class="indx">Colour, its inconstancy in insects found near the sea, <a href="#Page_57">57</a>, <a href="#Page_58">58</a>.</li> + +<li class="indx">—— of insects, affected by isolation, <a href="#Page_88">88</a>.</li> + +<li class="indx"><i>Colymbetes</i>, a species of, captured forty-five miles from shore, <a href="#Page_149">149</a>, <a href="#Page_150">150</a>.</li> + +<li class="indx">Compensation, generally apparent when an insect is deprived of an organ or sense, <a href="#Page_81">81</a>.</li> + +<li class="indx"><i>Coranus subapterus</i>, Curt., the development of the wings of, <a href="#Page_101">101</a>.</li> + +<li class="indx">Cordillera, Mr. Darwin on the fauna of the, <a href="#Page_145">145</a>. +<span class="pagenum"><a id="Page_198">[Pg 198]</a></span></li> +<li class="indx"><i>Corylophus</i>, apterous in Madeira, <a href="#Page_82">82</a>.</li> + +<li class="indx"><i>Criomorphus</i>, Curtis, referable to the genus <i>Delphax</i>, <a href="#Page_45">45</a>.</li> + +<li class="indx"><i>Cyclostoma lucidum</i>, Lowe, its extinction in Porto Santo, <a href="#Page_131">131</a>.</li> + +<li class="indx"><i>Cynthia Cardui</i>, Linn., unaffected by climate, <a href="#Page_32">32</a>.</li> + +<li class="indx"><i>Cynucus</i>, a species of, seventeen miles from shore, <a href="#Page_150">150</a>.</li> + +<li class="indx"><i>Cyrtonota</i>, its concentration in S. America, <a href="#Page_143">143</a>.</li> + +<li class="ifrst">Darwin, Mr., on the fauna of the Galapagos, <a href="#Page_23">23</a>;</li> +<li class="isub1">relative proportions of the insect tribes in the tropics, <a href="#Page_28">28</a>, <a href="#Page_29">29</a>;</li> +<li class="isub1">on the insects of Tierra del Fuego, <a href="#Page_47">47</a>;</li> +<li class="isub1">on the natural features of Tierra del Fuego, <a href="#Page_50">50</a>;</li> +<li class="isub1">on the insects of Keeling Island, <a href="#Page_55">55</a>;</li> +<li class="isub1">on the insects of St. Helena, <a href="#Page_55">55</a>;</li> +<li class="isub1">on the insects of Ascension, <a href="#Page_55">55</a>;</li> +<li class="isub1">on the apterous condition of insular species, <a href="#Page_86">86</a>;</li> +<li class="isub1">on the fauna of the Cordillera, <a href="#Page_145">145</a>;</li> +<li class="isub1">on a <i>Calosoma</i> captured at sea, <a href="#Page_147">147</a>;</li> +<li class="isub1">on insects captured in the sea, <a href="#Page_149">149</a>, <a href="#Page_150">150</a>;</li> +<li class="isub1">on the disappearance of animals before more powerful ones than themselves, <a href="#Page_178">178</a>.</li> + +<li class="indx">Dawson, Rev. J. F., on a variety of <i>Bembidium saxatile</i>, <a href="#Page_60">60</a>.</li> + +<li class="indx">Definition of the term 'species,' <a href="#Page_4">4</a>;</li> +<li class="isub1">of the term 'variety,' <a href="#Page_4">4</a>.</li> + +<li class="indx"><i>Delphax</i>, on the development of the wings of, <a href="#Page_45">45</a>.</li> + +<li class="indx"><i>Dermestes vulpinus</i>, Fabr., unaffected by climate, <a href="#Page_31">31</a>.</li> + +<li class="indx"><i>Deucalion</i>, its occurrence on the Salvages and Dezertas, <a href="#Page_125">125</a>.</li> + +<li class="indx"><i>—— Desertarum</i>, Woll., its sedentary nature, <a href="#Page_125">125</a>, <a href="#Page_126">126</a>, <a href="#Page_127">127</a>.</li> + +<li class="indx"><i>Dichelus</i>, its concentration in S. Africa, <a href="#Page_143">143</a>.</li> + +<li class="indx">Differences, when to be regarded as specific, <a href="#Page_6">6</a>;</li> +<li class="isub1">too exclusively studied, <a href="#Page_12">12</a>.</li> + +<li class="indx">Diffusion, various means of, which operate on the insect tribes, <a href="#Page_148">148</a>.</li> + +<li class="indx">Disturbing agents, Prof. Henfrey on, <a href="#Page_8">8</a>.</li> + +<li class="indx"><i>Ditylus</i>, the same type of, indicated in the Canaries and Salvages, <a href="#Page_124">124</a>.</li> + +<li class="indx">Domesticated animals, pliable nature of, <a href="#Page_187">187</a>, <a href="#Page_192">192</a>.</li> + +<li class="indx"><i>Dromius arenicola</i>, Woll., representative of <i>D. obscuroguttatus</i>, <a href="#Page_66">66</a>.</li> + +<li class="indx"><i>—— fasciatus</i>, Gyll., its paleness near the sea, <a href="#Page_63">63</a>.</li> + +<li class="indx"><i>—— negrita</i>, Woll., perhaps an ultimate state of <i>D. glabratus</i>, <a href="#Page_85">85</a>.</li> + +<li class="indx"><i>—— obscuroguttatus</i>, Dufts., its changes in Madeira, <a href="#Page_36">36</a>, <a href="#Page_37">37</a>, <a href="#Page_38">38</a>;</li> +<li class="isub1">apterous in Madeira, <a href="#Page_84">84</a>.</li> + +<li class="indx"><i>—— sigma</i>, Rossi, its colour affected by isolation, <a href="#Page_88">88</a>, <a href="#Page_89">89</a>.</li> + +<li class="ifrst">Elevation, sometimes corresponds with latitude, <a href="#Page_35">35</a>, <a href="#Page_114">114</a>.</li> + +<li class="indx"><i>Ellipsodes glabratus</i>, Fabr., singular variety of, on one of the Madeira Islands, <a href="#Page_88">88</a>, <a href="#Page_109">109</a>.</li> + +<li class="indx">Elytra, connateness of, a variable character, <a href="#Page_96">96</a>.</li> + +<li class="indx">'Endemic,' to what species the term is applicable, <a href="#Page_118">118</a>.</li> + +<li class="indx">Entomology, the study of, does not necessarily cramp the mind, <a href="#Page_111">111</a>.</li> + +<li class="indx"><i>Ephistemus</i>, apterous in Madeira, <a href="#Page_82">82</a>.</li> + +<li class="indx"><i>Eucalypti</i>, their concentration in Australia, <a href="#Page_142">142</a>.</li> + +<li class="indx"><i>Eunectes sticticus</i>, Linn., unaffected by climate, <a href="#Page_31">31</a>.</li> + +<li class="indx">Euphorbias, their concentration in Southern Africa, <a href="#Page_142">142</a>. +<span class="pagenum"><a id="Page_199">[Pg 199]</a></span></li> +<li class="indx"><i>Eurygnathus Latreillei</i>, Lap., variety of, on one of the Madeira Islands, <a href="#Page_88">88</a>, <a href="#Page_109">109</a>.</li> + +<li class="indx">Exceptions, not be allowed to negative a law, <a href="#Page_72">72</a>, <a href="#Page_73">73</a>.</li> + +<li class="indx">Extinction of species, as indicated in the Madeiran <i>Helices</i>, <a href="#Page_131">131</a>;</li> +<li class="isub1">the only cause by which genera may be abruptly defined, <a href="#Page_176">176</a>.</li> + +<li class="ifrst">Forbes, Prof. E., on the origin of the British animals and plants, <a href="#Page_130">130</a>;</li> +<li class="isub1">his epochs of migration of the British animals and plants, <a href="#Page_136">136</a>;</li> +<li class="isub1">on the existence of a former Atlantic continent, <a href="#Page_137">137</a>.</li> + +<li class="indx">Forests, the hindrance which they offer to insect-diffusion, <a href="#Page_154">154</a>.</li> + +<li class="indx">"Fortunate Islands" of the ancients, probably the Canarian group, <a href="#Page_141">141</a>.</li> + +<li class="ifrst">Galapagos, fauna of, <a href="#Page_23">23</a>.</li> + +<li class="indx">Genera, the nature of, often misunderstood, <a href="#Page_160">160</a>;</li> +<li class="isub1">a familiar explanation of, <a href="#Page_160">160</a>, <a href="#Page_161">161</a>, <a href="#Page_162">162</a>;</li> +<li class="isub1">cannot be abrupt except from accident, <a href="#Page_169">169</a>;</li> +<li class="isub1">how to be defined, <a href="#Page_169">169</a>;</li> +<li class="isub1">the types of, usually situated towards the centres of the several groups, <a href="#Page_170">170</a>;</li> +<li class="isub1">the types of, usually evenly balanced in their structural characters, <a href="#Page_171">171</a>, <a href="#Page_172">172</a>;</li> +<li class="isub1">may be abruptly defined from accidental causes, <a href="#Page_176">176</a>, <a href="#Page_177">177</a>.</li> + +<li class="indx">Generic areas, an important feature throughout Nature, <a href="#Page_130">130</a>, <a href="#Page_141">141</a>, <a href="#Page_184">184</a>.</li> + +<li class="indx">Geology, a necessary item in the study of insect-diffusion, <a href="#Page_113">113</a>.</li> + +<li class="indx">Germanic plains, the, probably a primary area of diffusion, <a href="#Page_130">130</a>.</li> + +<li class="indx"><i>Gerris</i>, on the development of the wings of, <a href="#Page_100">100</a>.</li> + +<li class="indx">Gould, Mr., on the Swallows of Malta, <a href="#Page_102">102</a>.</li> + +<li class="indx"><i>Gymnaëtron</i>, blood-red dashes characteristic of, <a href="#Page_62">62</a>.</li> + +<li class="indx"><i>—— Campanulæ</i>, Linn., its smallness on the Cornish coast, <a href="#Page_58">58</a>.</li> + +<li class="indx"><i>—— Veronicæ</i>, Germ., a variety of <i>G. niger</i>, <a href="#Page_62">62</a>.</li> + +<li class="ifrst"><i>Hadrus illotus</i>, Woll., perhaps a form of <i>H. cinerascens</i>, <a href="#Page_66">66</a>.</li> + +<li class="indx"><i>Haliplus obliquus</i>, Gyll., dark state of, in Ireland, <a href="#Page_67">67</a>.</li> + +<li class="indx"><i>Haltica exoleta</i>, Fabr., its variability on the coast, <a href="#Page_59">59</a>.</li> + +<li class="indx">Harcourt, Mr., on the discovery of Madeira, <a href="#Page_49">49</a>, <a href="#Page_50">50</a>.</li> + +<li class="indx"><i>Harpalus vividus</i>, Dej., changes to which it is subject, <a href="#Page_67">67</a>, <a href="#Page_68">68</a>, <a href="#Page_69">69</a>;</li> +<li class="isub1">variable in the connateness of its elytra, <a href="#Page_96">96</a>, <a href="#Page_97">97</a>.</li> + +<li class="indx"><i>Hegeter</i>, its maximum attained in the Canaries, <a href="#Page_120">120</a>.</li> + +<li class="indx"><i>—— elongatus</i>, Oliv., its migration from the Canaries, <a href="#Page_120">120</a>;</li> +<li class="isub1">of a more adaptive nature than its allies, <a href="#Page_121">121</a>.</li> + +<li class="indx"><i>—— latebricola</i>, Woll., its occurrence in the Salvages, <a href="#Page_120">120</a>.</li> + +<li class="indx"><i>Helices</i>, have often two distinct states, <a href="#Page_106">106</a>;</li> +<li class="isub1">many of them representative in the Madeira Islands, <a href="#Page_128">128</a>, <a href="#Page_129">129</a>;</li> +<li class="isub1">those in the Madeiras chiefly of slow migratory powers, <a href="#Page_130">130</a>, <a href="#Page_131">131</a>.</li> + +<li class="indx"><i>Helix attrita</i>, Lowe, its local character, <a href="#Page_132">132</a>.</li> + +<li class="indx"><i>—— Bowdichiana</i>, Fér., perhaps a gigantic state of <i>H. punctulata</i>, <a href="#Page_106">106</a>.</li> + +<li class="indx"><span class="pagenum"><a id="Page_200">[Pg 200]</a></span><i>—— calculus</i>, Lowe, sedentary nature of, <a href="#Page_132">132</a>.</li> + +<li class="indx"><i>Helix commixta</i>, Lowe, sedentary nature of, <a href="#Page_132">132</a>.</li> + +<li class="indx"><i>—— coronata</i>, Desh., its peculiarity to Porto Santo, <a href="#Page_128">128</a>;</li> +<li class="isub1">its occurrence beneath the surface of the ground, <a href="#Page_131">131</a>.</li> + +<li class="indx"><i>—— coronula</i>, Lowe, its peculiarity to the Southern Dezerta, <a href="#Page_128">128</a>.</li> + +<li class="indx"><i>—— Delphinula</i>, Lowe, the Madeiran representative of <i>H. tectiformis</i> in Porto Santo, <a href="#Page_129">129</a>.</li> + +<li class="indx"><i>—— discina</i>, Lowe, a form of <i>H. polymorpha</i>, <a href="#Page_133">133</a>.</li> + +<li class="indx"><i>—— erubescens</i>, Lowe, its powers of diffusion greater than those of its allies, <a href="#Page_133">133</a>;</li> +<li class="isub1">sensitive to external influences, <a href="#Page_134">134</a>.</li> + +<li class="indx"><i>—— fluctuosa</i>, Lowe, its extinction in Porto Santo, <a href="#Page_131">131</a>.</li> + +<li class="indx"><i>—— hirsuta</i>, Say, two distinct states of, <a href="#Page_106">106</a>.</li> + +<li class="indx"><i>—— lapicida</i>, Linn., its extinction in Porto Santo, <a href="#Page_131">131</a>.</li> + +<li class="indx"><i>—— latens</i>, Lowe, the Madeiran representative of <i>H. obtecta</i> in Porto Santo, <a href="#Page_129">129</a>.</li> + +<li class="indx"><i>—— lincta</i>, Lowe, the common Madeiran form of <i>H. polymorpha</i>, <a href="#Page_134">134</a>.</li> + +<li class="indx"><i>—— Lowei</i>, Pfr., perhaps a gigantic state of <i>H. Portosanctana</i>, <a href="#Page_106">106</a>.</li> + +<li class="indx"><i>—— papilio</i>, Lowe, a form of <i>H. polymorpha</i>, <a href="#Page_133">133</a>.</li> + +<li class="indx"><i>—— paupercula</i>, Lowe, its powers of diffusion greater than those of its allies, <a href="#Page_133">133</a>.</li> + +<li class="indx"><i>—— polymorpha</i>, Lowe, sensitive to external influences, and of great diffusive powers, <a href="#Page_133">133</a>.</li> + +<li class="indx"><i>—— Portosanctana</i>, Sow., its peculiarity to Porto Santo, <a href="#Page_129">129</a>.</li> + +<li class="indx"><i>—— pulvinata</i>, Lowe, a form of <i>H. polymorpha</i>, <a href="#Page_133">133</a>.</li> + +<li class="indx"><i>—— saccharata</i>, Lowe, a local state of <i>H. polymorpha</i>, <a href="#Page_134">134</a>.</li> + +<li class="indx"><i>—— senilis</i>, Lowe, the Dezertan form of <i>H. polymorpha</i>, <a href="#Page_134">134</a>.</li> + +<li class="indx"><i>—— squalida</i>, Lowe, the Madeiran representative of <i>H. depauperata</i> in Porto Santo, <a href="#Page_129">129</a>.</li> + +<li class="indx"><i>—— tiarella</i>, Webb, its sedentary nature, <a href="#Page_128">128</a>.</li> + +<li class="indx"><i>—— undata</i>, Lowe, its peculiarity to Madeira proper, <a href="#Page_129">129</a>.</li> + +<li class="indx"><i>—— Vulcania</i>, Lowe, its peculiarity to the Dezertas, <a href="#Page_129">129</a>.</li> + +<li class="indx"><i>—— Wollastoni</i>, Lowe, sedentary nature of, <a href="#Page_132">132</a>.</li> + +<li class="indx"><i>Helobia nivalis</i>, Payk., perhaps a state of <i>H. brevicollis</i>, <a href="#Page_40">40</a>.</li> + +<li class="indx"><i>Helops</i>, always apterous in Madeira, <a href="#Page_82">82</a>.</li> + +<li class="indx"><i>—— confertus</i>, Woll., varies from altitude, <a href="#Page_39">39</a>.</li> + +<li class="indx"><i>—— futilis</i>, Woll., varies from isolation, <a href="#Page_109">109</a>.</li> + +<li class="indx"><i>—— testaceus</i>, Küst., pallid hue of, <a href="#Page_64">64</a>.</li> + +<li class="indx"><i>—— Vulcanus</i>, Woll., large size of, on one of the Madeira Islands, <a href="#Page_88">88</a>.</li> + +<li class="indx">Henfrey, Prof., on disturbing agents, <a href="#Page_8">8</a>.</li> + +<li class="indx">Herschel, Sir John, on the requisites for an observer, <a href="#Page_12">12</a>.</li> + +<li class="indx"><i>Hipparchia Semele</i>, Linn., has a distinct aspect in Madeira, <a href="#Page_34">34</a>.</li> + +<li class="indx"><i>Hipporhinus</i>, its concentration in S. Africa, <a href="#Page_143">143</a>.</li> + +<li class="indx">Holme, Mr., on <i>Olisthopus rotundatus</i> in the Scilly Islands, <a href="#Page_58">58</a>, <a href="#Page_102">102</a>;</li> +<li class="isub1">on a winged state of <i>Phosphuga atrata</i>, <a href="#Page_102">102</a>.</li> + +<li class="indx"><i>Holoparamecus</i>, common to Madeira and Sicily, <a href="#Page_139">139</a>.</li> + +<li class="indx"><span class="pagenum"><a id="Page_201">[Pg 201]</a></span><i>—— Niger</i>, Aubé, different in Madeira and Sicily, <a href="#Page_33">33</a>.</li> + +<li class="indx">Hooker, Dr., on the insects of Kerguelen's Land, <a href="#Page_86">86</a>.</li> + +<li class="indx">Humboldt, his notice of Sphinxes and flies high up on the Andes, <a href="#Page_149">149</a>.</li> + +<li class="indx">Humming-Birds, their peculiarity to S. America and the W. Indies, <a href="#Page_142">142</a>.</li> + +<li class="indx"><i>Hydrobius</i>, apterous in Madeira, <a href="#Page_82">82</a>;</li> +<li class="isub1">the capture of, out at sea, <a href="#Page_150">150</a>.</li> + +<li class="indx"><i>Hydrometridæ</i>, on the development of the wings of, <a href="#Page_100">100</a>.</li> + +<li class="indx"><i>Hydroporus</i>, the capture of, out at sea, <a href="#Page_150">150</a>.</li> + +<li class="indx"><i>—— confluens</i>, Fabr., unaffected by climate, <a href="#Page_31">31</a>.</li> + +<li class="indx"><i>Hypsonotus</i>, its concentration in S. America, <a href="#Page_143">143</a>.</li> + +<li class="ifrst">Influence of climate not important, <a href="#Page_23">23</a>.</li> + +<li class="indx">Insect-aberration, perhaps a universal fact, <a href="#Page_16">16</a>, <a href="#Page_17">17</a>, <a href="#Page_18">18</a>.</li> + +<li class="indx"><i>Insulæ Fortunatæ</i> of Juba, probably the Canarian Group, <a href="#Page_141">141</a>.</li> + +<li class="indx">Ireland, poverty of the fauna of, <a href="#Page_52">52</a>, <a href="#Page_53">53</a>;</li> +<li class="isub1">the south-west of, has something in common with Madeira, <a href="#Page_139">139</a>.</li> + +<li class="indx">Islands, faunas of, often too greatly magnified, <a href="#Page_70">70</a>;</li> +<li class="isub1">the species of, generally more isolated in their structure than those of continents, <a href="#Page_177">177</a>.</li> + +<li class="indx">Isolation, effects of, on insect-stature, <a href="#Page_71">71</a>.</li> + +<li class="indx">Ixias, their concentration in Southern Africa, <a href="#Page_142">142</a>.</li> + +<li class="ifrst">Kangaroos, their concentration in Australia, <a href="#Page_142">142</a>.</li> + +<li class="indx">Kerguelen's Land, insects of, <a href="#Page_86">86</a>.</li> + +<li class="indx">Kirby, Rev. W., on insects washed up on the Suffolk coast, <a href="#Page_147">147</a>.</li> + +<li class="ifrst"><i>Læmophlœus pusillus</i>, Schönh., unaffected by climate, <a href="#Page_31">31</a>.</li> + +<li class="indx"><i>Lamprias chlorocephalus</i>, Ent. H., two distinct sizes of, frequently indicated, <a href="#Page_105">105</a>.</li> + +<li class="indx"><i>Laparocerus morio</i>, Schönh., large size of, on one of the Madeira Islands, <a href="#Page_88">88</a>.</li> + +<li class="indx">Latitude and altitude, sometimes reciprocal, <a href="#Page_35">35</a>.</li> + +<li class="indx"><i>Leistus montanus</i>, Steph., has been supposed to be equal to <i>L. fulvibarbis</i>, <a href="#Page_40">40</a>.</li> + +<li class="indx"><i>Lemur</i>, its peculiarity to Madagascar, <a href="#Page_143">143</a>.</li> + +<li class="indx"><i>Litargus</i>, common to Madeira and Sicily, <a href="#Page_139">139</a>.</li> + +<li class="indx"><i>Lixus angustatus</i>, Fabr., unaffected by climate, <a href="#Page_31">31</a>.</li> + +<li class="indx">Localities, some naturally more productive than others, <a href="#Page_53">53</a>, <a href="#Page_54">54</a>.</li> + +<li class="indx"><i>Longitarsus</i>, the native species of, apterous in Madeira, <a href="#Page_82">82</a>.</li> + +<li class="indx"><i>Loricera</i>, apterous in Madeira, <a href="#Page_82">82</a>.</li> + +<li class="indx">Lowe, Rev. R. T., his capture of the <i>Deucalion Desertarum</i>, <a href="#Page_127">127</a>.</li> + +<li class="indx">Lundy Island, smallness of the insects in, <a href="#Page_58">58</a>, <a href="#Page_59">59</a>;</li> +<li class="isub1">occurrence of the Black Rat in, <a href="#Page_178">178</a>.</li> + +<li class="indx"><i>Lycæna Phlœas</i>, Linn., darker in Madeira than in England, <a href="#Page_34">34</a>.</li> + +<li class="indx">Lyell, Sir Charles, on <i>Helix hirsuta</i>, <a href="#Page_106">106</a>;</li> +<li class="isub1">on the fossil period of the Madeiran <i>Helices</i>, <a href="#Page_129">129</a>;</li> +<li class="isub1">on insects washed up on the shore, <a href="#Page_148">148</a>;</li> +<li class="isub1">on the effect of gales in the transportation of insects, <a href="#Page_148">148</a>; +<span class="pagenum"><a id="Page_202">[Pg 202]</a></span></li> +<li class="isub1">on the effects of a volcanic eruption in destroying species, <a href="#Page_179">179</a>;</li> +<li class="isub1">on the flexible nature of certain animals and plants, <a href="#Page_187">187</a>;</li> +<li class="isub1">on the greater differences which varieties often present than do species, <a href="#Page_193">193</a>.</li> + +<li class="indx"><i>Lygæus brevipennis</i>, Latr., on the development of the wings of, <a href="#Page_101">101</a>.</li> + +<li class="ifrst"><i>Macronota</i>, its peculiarity to Java, <a href="#Page_143">143</a>.</li> + +<li class="indx">Madeira, has some features in common with Tierra del Fuego, <a href="#Page_48">48</a>, <a href="#Page_49">49</a>, <a href="#Page_50">50</a>, <a href="#Page_51">51</a>;</li> +<li class="isub1">former state of, <a href="#Page_48">48</a>, <a href="#Page_49">49</a>;</li> +<li class="isub1">great fire on the southern side of, <a href="#Page_49">49</a>;</li> +<li class="isub1">origin of the name of, <a href="#Page_50">50</a>; the insects of, <a href="#Page_55">55</a>;</li> +<li class="isub1">the tendency of its insects to become apterous, <a href="#Page_82">82</a>;</li> +<li class="isub1">the migratory direction of its insect population, <a href="#Page_119">119</a>;</li> +<li class="isub1">the local nature of its various species, <a href="#Page_152">152</a>, <a href="#Page_153">153</a>.</li> + +<li class="indx">Magnolias, their concentration in Central America, <a href="#Page_142">142</a>.</li> + +<li class="indx">Malta, Mr. Gould on the birds of, <a href="#Page_102">102</a>.</li> + +<li class="indx"><i>Malthodes Kiesenwetteri</i>, Woll., perhaps a state of <i>M. brevicollis</i>, <a href="#Page_66">66</a>.</li> + +<li class="indx">Man, agency of, in the destruction of species, <a href="#Page_179">179</a>.</li> + +<li class="indx"><i>Mantura Chrysanthemi</i>, Ent. H., variability of, in Lundy Island, <a href="#Page_59">59</a>.</li> + +<li class="indx"><i>Marsupialia</i>, their concentration in Australia, <a href="#Page_142">142</a>.</li> + +<li class="indx">Mesembryanthemums, their concentration in Southern Africa, <a href="#Page_142">142</a>.</li> + +<li class="indx"><i>Mesites</i>, a modification of <i>Cossonus</i>, <a href="#Page_144">144</a>.</li> + +<li class="indx"><i>—— Maderensis</i>, Woll., its near relationship to the <i>M. Tardii</i>, <a href="#Page_141">141</a>.</li> + +<li class="indx"><i>—— Tardii</i>, Curtis, its variability near the coast, <a href="#Page_58">58</a>.</li> + +<li class="indx">'Méthode Mononomique,' the unsoundness of, <a href="#Page_164">164-168</a>.</li> + +<li class="indx">Migratory powers, slowness of, in the Madeiran <i>Helices</i>, <a href="#Page_130">130-132</a>.</li> + +<li class="indx">—— progress, direction of, in the Madeiran animals, <a href="#Page_120">120</a>, <a href="#Page_135">135</a>.</li> + +<li class="indx">Mimosas, their concentration in Australia, <a href="#Page_142">142</a>.</li> + +<li class="indx">Mollusca, Terrestrial, often present two distinct states, <a href="#Page_106">106</a>.</li> + +<li class="indx"><i>Moluris</i>, its concentration in S. Africa, <a href="#Page_143">143</a>.</li> + +<li class="indx"><i>Monochelus</i>, its concentration in S. Africa, <a href="#Page_143">143</a>.</li> + +<li class="indx">Mountain-chains, their hindrance to insect-diffusion, <a href="#Page_145">145</a>.</li> + +<li class="indx">Mountain-tops, either very prolific in insect life, or else barren, <a href="#Page_115">115</a>.</li> + +<li class="indx"><i>Mus Rattus</i>, almost exterminated in England, <a href="#Page_178">178</a>.</li> + +<li class="indx"><i>Mycetoporus pronus</i>, Erichs., two distinct states of, indicated, <a href="#Page_106">106</a>.</li> + +<li class="indx">Myrtles, their concentration in Australia, <a href="#Page_142">142</a>.</li> + +<li class="ifrst">Naturalist, the, what his province to investigate, <a href="#Page_158">158</a>.</li> + +<li class="indx">Nature, not irregular because presenting occasional anomalies, <a href="#Page_94">94</a>.</li> + +<li class="indx"><i>Naupactus</i>, its concentration in S. America, <a href="#Page_143">143</a>.</li> + +<li class="indx"><i>Nebria complanata</i>, Linn., unusually pale near Bordeaux, <a href="#Page_33">33</a>;</li> +<li class="isub1">pallid hue of, <a href="#Page_64">64</a>.</li> + +<li class="indx">New World, some of its insects perhaps but states of those of the Old, <a href="#Page_37">37</a>.</li> + +<li class="indx">Nomenclature, a binomial system the only true one, <a href="#Page_164">164</a>, <a href="#Page_168">168</a>.</li> + +<li class="indx"><i>Notaphus</i>, the capture of, out at sea, <a href="#Page_150">150</a>.</li> + +<li class="indx"><i>Notiophili</i>, extremely variable, <a href="#Page_40">40</a>. +<span class="pagenum"><a id="Page_203">[Pg 203]</a></span></li> +<li class="indx"><i>Notiophilus geminatus</i>, Dej., large size of, on one of the Madeira Islands, <a href="#Page_88">88</a>.</li> + +<li class="ifrst">Observation, indispensable in natural science, <a href="#Page_20">20</a>, <a href="#Page_159">159</a>, <a href="#Page_192">192</a>.</li> + +<li class="indx">Ocean, the, its hindrance to insect-diffusion, <a href="#Page_145">145</a>.</li> + +<li class="indx"><i>Ochthebius marinus</i>, Payk., lurid hue of, <a href="#Page_64">64</a>.</li> + +<li class="indx"><i>Olisthopus</i>, apterous in Madeira, <a href="#Page_82">82</a>.</li> + +<li class="indx"><i>—— Maderensis</i>, Woll., large state of, on one of the Madeira Islands, <a href="#Page_88">88</a>, <a href="#Page_89">89</a>.</li> + +<li class="indx"><i>—— rotundatus</i>, Payk., very small in the Scilly Islands, <a href="#Page_58">58</a>, <a href="#Page_73">73</a>;</li> +<li class="isub1">subapterous in the Scilly Islands, <a href="#Page_102">102</a>.</li> + +<li class="indx"><i>Omaseus nigerrimus</i>, Dej., a form of <i>O. aterrimus</i>, <a href="#Page_33">33</a>.</li> + +<li class="indx"><i>Omias Waterhousei</i>, Woll., large state of, on one of the Madeira Islands, <a href="#Page_88">88</a>, <a href="#Page_109">109</a>.</li> + +<li class="indx"><i>Oncocephalus griseus</i>, development of the wings of, <a href="#Page_101">101</a>.</li> + +<li class="indx"><i>Othius</i>, apterous in Madeira, <a href="#Page_82">82</a>.</li> + +<li class="indx">Ourangs, their peculiarity to the Indian Islands, <a href="#Page_143">143</a>.</li> + +<li class="indx"><i>Oxyomus</i>, a modification of <i>Aphodius</i>, <a href="#Page_144">144</a>.</li> + +<li class="ifrst"><i>Pachymerus brevipennis</i>, the development of the wings of, <a href="#Page_100">100</a>.</li> + +<li class="indx"><i>Pachyrhynchus</i>, its concentration in the Philippine islands, <a href="#Page_143">143</a>.</li> + +<li class="indx">Painted-Lady Butterfly, unaffected by climate, <a href="#Page_32">32</a>.</li> + +<li class="indx"><i>Papilio Machaon</i>, Linn., unaffected by climate, <a href="#Page_31">31</a>.</li> + +<li class="indx"><i>Paropsis</i>, its concentration in Australia, <a href="#Page_143">143</a>.</li> + +<li class="indx">Patagonia, insects of, distinct from those of Tierra del Fuego, <a href="#Page_47">47</a>, <a href="#Page_48">48</a>.</li> + +<li class="indx"><i>Patrobus septentrionis</i>, Dej., has been supposed to be a state of <i>P. excavatus</i>, <a href="#Page_40">40</a>.</li> + +<li class="indx"><i>Pecteropus</i>, its maximum attained in the Canaries, <a href="#Page_124">124</a>.</li> + +<li class="indx"><i>—— Maderensis</i>, Woll., varies from altitude, <a href="#Page_39">39</a>.</li> + +<li class="indx"><i>—— rostratus</i>, Woll., varies from isolation, <a href="#Page_90">90</a>.</li> + +<li class="indx">Pelargoniums, their concentration in Southern Africa, <a href="#Page_142">142</a>.</li> + +<li class="indx"><i>Pelophila borealis</i>, Payk., larger in Ireland than in the Orkneys, <a href="#Page_33">33</a>.</li> + +<li class="indx"><i>Phaleria cadaverina</i>, Fabr., pallid hue of, <a href="#Page_64">64</a>.</li> + +<li class="indx"><i>Philhydrus melanocephalus</i>, Oliv., two states of, frequently indicated, <a href="#Page_105">105</a>.</li> + +<li class="indx"><i>Phlæophagus</i>, apterous in Madeira, <a href="#Page_82">82</a>.</li> + +<li class="indx"><i>Phosphuga atrata</i>, Linn., taken with the wings developed, <a href="#Page_102">102</a>.</li> + +<li class="indx"><i>—— subrotundata</i>, Leach, the Irish form of the <i>P. atrata</i>, <a href="#Page_33">33</a>.</li> + +<li class="indx"><i>Phytophaga</i>, preponderance of, in the tropics, <a href="#Page_28">28</a>, <a href="#Page_29">29</a>.</li> + +<li class="indx"><i>Pieris Brassicæ</i>, Linn., varies in Nepaul and Japan, <a href="#Page_34">34</a>.</li> + +<li class="indx"><i>Pissodes notatus</i>, Fabr., unaffected by climate, <a href="#Page_30">30</a>.</li> + +<li class="indx"><i>Platyomus</i>, its concentration in S. America, <a href="#Page_143">143</a>.</li> + +<li class="indx"><i>Platyrrhini</i>, their peculiarity to S. America, <a href="#Page_143">143</a>.</li> + +<li class="indx"><i>Pogonus luridipennis</i>, Germ., lurid hue of, <a href="#Page_64">64</a>.</li> + +<li class="indx"><i>Pontia Brassicæ</i>, Linn., its introduction into Madeira probably recent, <a href="#Page_74">74</a>. +<span class="pagenum"><a id="Page_204">[Pg 204]</a></span></li> +<li class="indx">Porto Santo, origin of the name of, <a href="#Page_49">49</a>;</li> +<li class="isub1">a generic area of radiation for certain <i>Helices</i>, <a href="#Page_130">130</a>.</li> + +<li class="indx">Predacious insects, less numerous in the tropics, <a href="#Page_28">28</a>, <a href="#Page_29">29</a>.</li> + +<li class="indx"><i>Prostemma guttula</i>, Fabr., the development of the wings of, <a href="#Page_100">100</a>, <a href="#Page_101">101</a>.</li> + +<li class="indx"><i>Psylliodes</i>, a variable species of, in Lundy Island, <a href="#Page_60">60</a>.</li> + +<li class="indx"><i>—— erythrocephala</i>, Linn., two distinct states of, frequently indicated, <a href="#Page_105">105</a>.</li> + +<li class="indx"><i>—— marcida</i>, Illig., pallid hue of, <a href="#Page_64">64</a>.</li> + +<li class="indx"><i>—— nigricollis</i>, Mshm, a pale state of the <i>P. erythrocephala</i>, <a href="#Page_105">105</a>.</li> + +<li class="indx"><i>—— vehemens</i>, Woll., varies from isolation, <a href="#Page_90">90</a>.</li> + +<li class="indx"><i>Pterostichus</i>, its various divisions are natural ones, <a href="#Page_175">175</a>.</li> + +<li class="indx"><i>Ptini</i>, their stature affected by isolation, <a href="#Page_74">74</a>;</li> +<li class="isub1">which characters of, are the most constant, <a href="#Page_104">104</a>.</li> + +<li class="indx"><i>Ptinus albopictus</i>, Woll., its changes on the islands of the Madeiran Group, <a href="#Page_75">75-77</a>.</li> + +<li class="indx"><i>Pupa</i>, often two distinct states of, <a href="#Page_106">106</a>.</li> + +<li class="indx"><i>Purpurariæ</i> of the ancients, probably the Madeiran Group, <a href="#Page_141">141</a>.</li> + +<li class="indx">Pyrenean region, the, perhaps a primary area of diffusion, <a href="#Page_130">130</a>.</li> + +<li class="ifrst">Reasoning, not sufficient of itself for the formation of science, <a href="#Page_159">159</a>.</li> + +<li class="indx">Red-Admiral Butterfly, its introduction into Madeira perhaps recent, <a href="#Page_74">74</a>.</li> + +<li class="indx"><i>Reduviadæ</i>, on the development of the wings of a representative of the, <a href="#Page_101">101</a>.</li> + +<li class="indx">Representative species, exemplified by the Madeiran <i>Helices</i>, <a href="#Page_128">128</a>, <a href="#Page_129">129</a>, <a href="#Page_185">185</a>;</li> +<li class="isub1">where frequently to be recognized, <a href="#Page_183">183</a>.</li> + +<li class="indx"><i>Rhyzopertha pusilla</i>, Fabr., unaffected by climate, <a href="#Page_31">31</a>.</li> + +<li class="indx">Rivers, their power of transporting insects along their course, <a href="#Page_149">149</a>.</li> + +<li class="ifrst">Saline spots, variation of insects in, <a href="#Page_57">57</a>.</li> + +<li class="indx">Salvages, occurrence of a Canarian form on the, <a href="#Page_120">120</a>, <a href="#Page_124">124</a>.</li> + +<li class="indx"><i>Saprinus</i>, a modification of <i>Hister</i> proper, <a href="#Page_143">143</a>.</li> + +<li class="indx"><i>—— nitidulus</i>, Fabr., two distinct states of, indicated, <a href="#Page_106">106</a>.</li> + +<li class="indx"><i>Scarabæus</i>, the capture of, out at sea, <a href="#Page_150">150</a>.</li> + +<li class="indx"><i>Scarites abbreviatus</i>, Koll., large size of, on one of the Madeira Islands, <a href="#Page_88">88</a>;</li> +<li class="isub1">varies both from isolation and altitude, <a href="#Page_91">91</a>.</li> + +<li class="indx">Sciences, the, should assist rather than oppose each other, <a href="#Page_155">155</a>, <a href="#Page_156">156</a>.</li> + +<li class="indx"><i>Scydmænus Helferi</i>, Schaum, smaller in Madeira than in Sicily, <a href="#Page_65">65</a>.</li> + +<li class="indx"><i>Scymnus</i>, an apterous species of, in Porto Santo, <a href="#Page_82">82</a>.</li> + +<li class="indx">Sea, inconstancy of insects in the vicinity of the, <a href="#Page_57">57</a>.</li> + +<li class="indx">Sicily, the fauna of, has much in common with that of Madeira, <a href="#Page_139">139</a>.</li> + +<li class="indx"><i>Silpha atrata</i>, Linn., presents a distinct state in Ireland, <a href="#Page_33">33</a>.</li> + +<li class="indx"><i>Silybum Marianum</i>, Grtn., its stalks the food of a <i>Ptinus</i>, <a href="#Page_76">76</a>.</li> + +<li class="indx">Similitudes, Lord Bacon on the importance of, <a href="#Page_13">13</a>.</li> + +<li class="indx"><i>Sitonia gressoria</i>, Illig., perhaps a form of the <i>S. grisea</i>, <a href="#Page_33">33</a>.</li> + +<li class="indx"><i>Sitophilus granarius</i>, Linn., unaffected by climate, <a href="#Page_31">31</a>. +<span class="pagenum"><a id="Page_205">[Pg 205]</a></span></li> +<li class="indx"><i>Sitophilus oryzæ</i>, Linn., unaffected by climate, <a href="#Page_31">31</a>.</li> + +<li class="indx">Sloths, their peculiarity to S. America, <a href="#Page_143">143</a>.</li> + +<li class="indx">Species, definition of the term, <a href="#Page_4">4</a>;</li> +<li class="isub1">familiar explanation concerning the nature of, <a href="#Page_161">161</a>, <a href="#Page_162">162</a>;</li> +<li class="isub1">limitation of, how to be attempted, <a href="#Page_192">192</a>;</li> +<li class="isub1">limits of, real, though often difficult to trace out, <a href="#Page_193">193</a>;</li> +<li class="isub1">in a certain sense both unstable and permanent, <a href="#Page_194">194</a>.</li> + +<li class="indx">Specific centres of creation, <a href="#Page_5">5</a>.</li> + +<li class="indx"><i>Sphinx Convolvuli</i>, Linn., its introduction into Madeira probably recent, <a href="#Page_74">74</a>.</li> + +<li class="indx">Spinola, on one of the <i>Reduviadæ</i>, <a href="#Page_101">101</a>;</li> +<li class="isub1">on <i>Oncocephalus griseus</i>, <a href="#Page_101">101</a>.</li> + +<li class="indx">Stapelias, their concentration in Southern Africa, <a href="#Page_142">142</a>.</li> + +<li class="indx">States, large and small ones indicated in some insects, <a href="#Page_105">105</a>.</li> + +<li class="indx">Stature of insects, smaller in islands than on continents, <a href="#Page_70">70</a>.</li> + +<li class="indx"><i>Stenolophus Skrimshiranus</i>, Steph., perhaps a state of <i>S. Teutonus</i>, <a href="#Page_63">63</a>.</li> + +<li class="indx"><i>Stenus Heeri</i>, Woll., two distinct states of, indicated, <a href="#Page_106">106</a>.</li> + +<li class="indx">Structural characters, seldom variable in the Insecta, <a href="#Page_95">95</a>.</li> + +<li class="indx">Subsidences, the effect of, on insect life, <a href="#Page_114">114</a>.</li> + +<li class="indx">Swallow-Tail Butterfly, unaffected by climate, <a href="#Page_31">31</a>.</li> + +<li class="indx"><i>Syncalypta</i>, apterous in Madeira, <a href="#Page_82">82</a>.</li> + +<li class="ifrst"><i>Tachyporus nitidicollis</i>, Steph., perhaps a state of <i>T. obtusus</i>, <a href="#Page_33">33</a>.</li> + +<li class="indx"><i>Tarphii</i>, their economy in the Madeira Group, <a href="#Page_121">121</a>.</li> + +<li class="indx"><i>Tarphius</i>, its maximum attained in Madeira proper, <a href="#Page_121">121</a>;</li> +<li class="isub1">common to Madeira and Sicily, <a href="#Page_139">139</a>.</li> + +<li class="indx"><i>—— gibbulus</i>, Germ., the Sicilian exponent of the genus, <a href="#Page_123">123</a>.</li> + +<li class="indx"><i>—— Lowei</i>, Woll., of a more adaptive nature than its allies, <a href="#Page_122">122</a>.</li> + +<li class="indx"><i>Tarus</i>, always apterous in Madeira, <a href="#Page_82">82</a>.</li> + +<li class="indx"><i>—— lineatus</i>, Schönh., assumes a distinct state in Madeira, <a href="#Page_65">65</a>.</li> + +<li class="indx"><i>Telephorus testaceus</i>, Linn., its variability in Lundy Island, <a href="#Page_59">59</a>.</li> + +<li class="indx">Thompson, Mr., on the reptiles of Ireland, England, and Belgium, <a href="#Page_136">136</a>.</li> + +<li class="indx"><i>Thorictus</i>, common to Madeira and Sicily, <a href="#Page_139">139</a>.</li> + +<li class="indx">Tierra del Fuego, insects of, <a href="#Page_47">47</a>;</li> +<li class="isub1">has many characters in common with Madeira, <a href="#Page_48">48-51</a>.</li> + +<li class="indx">Time, an important item in the question of modifications, <a href="#Page_77">77</a>.</li> + +<li class="indx">Toucans, their peculiarity to S. America and the W. Indies, <a href="#Page_142">142</a>.</li> + +<li class="indx">Transmutation-theory, unsoundness of the, <a href="#Page_186">186-189</a>;</li> +<li class="isub1">how it took its rise, <a href="#Page_190">190</a>.</li> + +<li class="indx"><i>Trechus</i>, always apterous in Madeira, <a href="#Page_82">82</a>.</li> + +<li class="indx"><i>—— alticola</i>, Woll., perhaps a state of <i>T. custos</i>, <a href="#Page_39">39</a>.</li> + +<li class="indx"><i>—— lapidosus</i>, Daws., pallid hue of, <a href="#Page_64">64</a>.</li> + +<li class="indx">Tree-Porcupines, their peculiarity to S. America, <a href="#Page_143">143</a>.</li> + +<li class="indx"><i>Tribolium ferrugineum</i>, Fabr., unaffected by climate, <a href="#Page_31">31</a>.</li> + +<li class="indx"><i>Trogosita mauritanica</i>, Linn., unaffected by climate, <a href="#Page_31">31</a>. +<span class="pagenum"><a id="Page_206">[Pg 206]</a></span></li> +<li class="indx">Tropics, exuberance of the, <a href="#Page_27">27</a>, <a href="#Page_28">28</a>;></li> +<li class="isub1">relative proportions of the insect tribes within the, <a href="#Page_28">28</a>, <a href="#Page_29">29</a>.</li> + +<li class="indx"><i>Tychius</i>, always apterous in Madeira, <a href="#Page_82">82</a>.</li> + +<li class="ifrst">Unity, indicated in the organic creation, <a href="#Page_179">179</a>, <a href="#Page_180">180</a>.</li> + +<li class="ifrst"><i>Vanessa Atalanta</i>, Linn., has a different aspect in N. America, <a href="#Page_34">34</a>;</li> +<li class="isub1">perhaps a recent introduction into Madeira, <a href="#Page_74">74</a>.</li> + +<li class="indx"><i>—— Callirhoë</i>, Fabr., smaller in Porto Santo than in Madeira, <a href="#Page_73">73</a>.</li> + +<li class="indx">Variation in the Insecta, a matter of experience, <a href="#Page_7">7</a>, <a href="#Page_8">8</a>, <a href="#Page_15">15</a>;</li> +<li class="isub1">probable from analogy, <a href="#Page_15">15</a>;</li> +<li class="isub1">perhaps indicated in every individual, <a href="#Page_16">16</a>, <a href="#Page_17">17</a>, <a href="#Page_18">18</a>;</li> +<li class="isub1">restricted, <a href="#Page_35">35</a>.</li> + +<li class="indx">Variety, definition of the term, <a href="#Page_4">4</a>.</li> + +<li class="indx"><i>Velia</i>, on the development of the wings of, <a href="#Page_100">100</a>.</li> + +<li class="ifrst">Waterhouse, Mr., his opinion concerning generic types, <a href="#Page_172">172</a>.</li> + +<li class="indx">Westwood, Mr., on <i>Papilio Machaon</i> from the Himalayas, <a href="#Page_32">32</a>;</li> +<li class="isub1">on American specimens of <i>Lycæna Phlœas</i>, <a href="#Page_34">34</a>;</li> +<li class="isub1">on the effect of heat in developing the wings of insects, <a href="#Page_44">44</a>;</li> +<li class="isub1">on a winged state of <i>Choreius ineptus</i>, <a href="#Page_44">44</a>;</li> +<li class="isub1">on the development of the wings in <i>Delphax</i>, <a href="#Page_45">45</a>;</li> +<li class="isub1">on a winged state of <i>Cimex lectularius</i>, <a href="#Page_45">45</a>;</li> +<li class="isub1">on <i>Aphelocheirus æstivalis</i>, <a href="#Page_100">100</a>;</li> +<li class="isub1">on the development of the wings of the <i>Hydrometridæ</i>, <a href="#Page_100">100</a>;</li> +<li class="isub1">on <i>Cimex apterus</i>, <a href="#Page_100">100</a>;</li> +<li class="isub1">on <i>Prostemma guttala</i> and <i>Coranus subapteras</i>, <a href="#Page_101">101</a>;</li> +<li class="isub1">on the development of the wings of <i>Lygæus brevipennis</i>, <a href="#Page_101">101</a>.</li> + +<li class="indx">Whewell, Dr., on the natural causes which science has to investigate, <a href="#Page_191">191</a>.</li> + +<li class="indx">White-Cabbage Butterfly, varies in Nepaul and Japan, <a href="#Page_34">34</a>.</li> + +<li class="indx">Winds, the effects of, in the diffusion of insects, <a href="#Page_148">148</a>.</li> + +<li class="indx">Wings of insects, subject to undue development in hot seasons, <a href="#Page_43">43</a>;</li> +<li class="isub1">liable to become gradually obsolete in islands, <a href="#Page_81">81</a>;</li> +<li class="isub1">more variable than other organs, <a href="#Page_97">97</a>.</li> + +<li class="ifrst"><i>Xenostrongylus</i>, its geographical distribution, <a href="#Page_124">124</a>;</li> +<li class="isub1">common to Madeira and Sicily, <a href="#Page_139">139</a>.</li> + +<li class="ifrst"><i>Zargus pellucidus</i>, Woll., variety of, on one of the Madeira Islands, <a href="#Page_88">88</a>.</li> +</ul> + +<p class="h3">FINIS.</p> + +<p class="spacer"> </p> + +<p class="h5">Printed by Taylor and Francis, Red Lion Court, Fleet Street.</p> + +<hr class="chap" /> + +<p class="h5">Lately published, by the same Author, +in large 4to (with Thirteen Coloured Plates), price £2 2<i>s.</i>,</p> + +<p class="h3"><br />INSECTA MADERENSIA;</p> + +<p class="h5">BEING</p> + +<p class="h4">AN ACCOUNT OF THE INSECTS</p> + +<p class="h5">OF</p> + +<p class="h4">THE ISLANDS</p> + +<p class="h5">OF</p> + +<p class="h4">THE MADEIRAN GROUP.</p> + +<p class="h6">London: <span class="smcap">John Van Voorst</span>, 1, Paternoster Row.</p> + +<hr class="chap" /> + +<div class="trnote"> +<h2><a id="Transcribers_Notes"></a>Transcriber's Notes:</h2> + +<p>Inconsistent/archaic spelling and punctuation retained from original.</p> +</div> + + + + + + + + +<pre> + + + + + +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-h.htm or 38584-h.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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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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