On the Variation of Species, with Especial Reference to the Insecta Part 6

Finding that it was possible for numerous species, whose structural characteristics were less conspicuously p.r.o.nounced 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 a.s.semblages_ which that scheme would, if it were practicable, discountenance. Order and symmetry, however (which involve cla.s.sification, 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 a.s.sume 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 p.r.o.nounce 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 _cl.u.s.ters_, or material combinations, of objects (in which case it is absolute), but even between the objects themselves,--under which circ.u.mstances, 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 a.n.a.lysis 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 h.o.m.ogeneous,--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 a.n.a.logous 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 a.s.semblages 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 ill.u.s.tration) 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, p.r.o.nounced. 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 circ.u.mferences,--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 a.s.semblages that those foci are usually to be discerned;--and where the groups are large, it is not often difficult to p.r.o.nounce 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 a.s.sociates_; 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 a.s.semblage 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 a.s.sociates, 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 a.s.semblages, 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 ma.s.s), that, unless we are prepared to accept them as they are, we must needs repudiate them altogether: under which circ.u.mstances, our difficulties, both in determination and nomenclature, would be increased tenfold. We should also recollect, that cl.u.s.ters 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 a.s.semblages 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 cl.u.s.ters of them possess to a.s.sume 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 ma.s.ses 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 ma.s.s, 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 h.o.m.ogeneous 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 pa.s.sing 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 a.s.semblages 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 cl.u.s.ters, 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 s.p.a.ce 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 p.r.o.nounced_. 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 cl.u.s.ter 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 a.s.semblages. 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 a.s.sortments, 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 a.n.a.logy 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 const.i.tuta 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 ill.u.s.trate 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 Ess.e.x, 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 a.n.a.logy, 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 circ.u.mstances 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 princ.i.p.al 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 ma.s.s, 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 a.n.a.lysed, 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 circ.u.mstances 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 a.s.semblages,--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 ill.u.s.tration 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 circ.u.mstances and conditions in regulating, within definite limits, the outward aspect of the Articulate tribes.

I do not, however, a.s.sert 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 const.i.tuted, 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 trans.m.u.tation 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 a.s.sume 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 compa.s.s 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 circ.u.mstances, 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 const.i.tutions, 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 circ.u.mscribed, need scarcely be appealed to, in exposing the absurdity of the trans.m.u.tation 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 a.s.semblage, 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 princ.i.p.ally 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 trans.m.u.tation: yet they are no exceptions to the ordinary rule,--occupying an a.n.a.logous position to the members of every other endemic group.

But I will not occupy more s.p.a.ce on the trans.m.u.tation theory: suffice it to have shown that, in thus conceding a legitimate power of self-adaptation, in accordance with external circ.u.mstances, 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 a.s.suredly pa.s.s 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 a.n.a.lysis? 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 antic.i.p.ated. 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 const.i.tution more tractile (and possess, therefore, more decided powers for aberration) than the rest. Whether traces of design may be recognized in this circ.u.mstance, 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 circ.u.mferences 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 sh.o.r.e, races may ebb and flow; but they have their boundaries, in either direction, beyond which they can never pa.s.s. 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.