Читать книгу On the Variation of Species, with Especial Reference to the Insecta - Thomas Vernon Wollaston - Страница 5

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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.