Читать книгу On the Variation of Species, with Especial Reference to the Insecta - Thomas Vernon Wollaston - Страница 6

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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.