On the Variation of Species, with Especial Reference to the Insecta ; Followed by an Inquiry into the Nature of Genera.
by Thomas Vernon Wollaston.
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 compa.s.s 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.
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 a.s.suredly 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 p.r.o.nounce 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 _circ.u.mstances_ 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) n.o.body 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 a.s.signing 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 ident.i.ty 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 acc.u.mulating 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, a.s.sumed to ill.u.s.trate 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 a.s.serted 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 ill.u.s.tration, 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 circ.u.mstances 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 pa.s.s, 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 p.r.o.neness 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, alt.i.tude, and a combination of other circ.u.mstances 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 a.s.serts 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 a.s.suredly 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_ antic.i.p.ated between the hard, st.u.r.dy 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 circ.u.mstances 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 circ.u.mstances 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 a.n.a.logous 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 a.n.a.logy, 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 a.s.sertion (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 a.n.a.logies_, 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 circ.u.mstances 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 circ.u.mstances seemingly coincident, our first object should be (_not_ to regard the phaenomena as indicative, that no constant result can be antic.i.p.ated from causes which are similar, but), to inquire whether the circ.u.mstances 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 a.n.a.logy, 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 a.s.sume 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 ident.i.ty, 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 a.n.a.logies, 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 a.n.a.logies 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 a.n.a.logies, 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 a.n.a.logies 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 mult.i.tudes 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_ a.n.a.logy 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 a.n.a.logy (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 a.n.a.logy teaches us to antic.i.p.ate it, and observation tends more and more, as our knowledge advances, to corroborate the fact, I shall be pardoned for venturing a pa.s.sing thought upon a question even thus difficult of demonstration.
Perhaps we are too p.r.o.ne 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 n.o.bler 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 a.n.a.logy, and which our understanding, as it becomes gradually brighter, no less a.s.suredly 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 a.n.a.logy 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 ma.s.s 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 ma.s.s, 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 a.n.a.loga, tam in integralibus, quam partibus: illae enim sunt, quae naturam uniunt, et const.i.tuere scientias incipiunt."--_Novum Organum_, lib.
ii. 27.
CHAPTER III.
CAUSES OF VARIATION.