On the Variation of Species, with Especial Reference to the Insecta Part 5

We must not however forget, that it is only to the _aborigines_ of this _quondam_ land that the above speculations apply. a.s.suming the region not to have been insular, that is to say, to have been connected, on its outer limits, with a European, or Mediterranean, continent; it would necessarily follow, that a certain number of colonists must have found their way over its area, and moreover _in an opposite direction_ to the living stream (if we may so call it) which had been long flowing in a north-easterly course across its surface.

Whatever be the length of the periods, however, during which these counter migrations were going on, I think it sufficient to state that I would refer them to epochs altogether different,--so that, accompanied as they may have been by special geological phaenomena, which, if known, would in all probability become at once explanatory, we should be the less inclined to regard as absurd what might appear at first sight difficult to understand. In the case of the British Isles indeed, no less than five of these distinct migratory eras have been a.s.sumed, and specified[65], by Professor Edward Forbes; therefore (whatever value be attached to his able and interesting theory) I do not consider it necessary to apologize for requiring _at least two_ in behalf of this ancient Atlantic province. Not to insist upon those of his faunas and floras which are of a less evident, or more questionable, character, he has at any rate proved, I think, almost to a demonstration, the _westward progress_ of the great ma.s.s of our British animals and plants, over a then unbroken land (the upheaved bed of the glacial sea), from the central Germanic plains; whilst the accurate calculations of the late Mr. Thompson of Belfast, concerning the reptile statistics of Ireland, England, and Belgium, respectively, have succeeded in showing, with much presumptive reason, how the formation of St. George's Channel, _before_ that of the German Ocean, interrupted the march of these wanderers to the far West, and debarred an immense proportion of them from an entry into Ireland,--which would otherwise have colonized that country equally with our own.

As regards Professor Forbes's views of the creation of a vast continent (reaching far into the Atlantic[66]) at the close of the miocene epoch, through the upheaved bed of a shallow miocene sea,--a region moreover of such an extent as to have connected the various island groups between the Fucus bank and the sh.o.r.es of the Old World, not only with each other, but with a Mediterranean province, Asturias, and even the south-west of Ireland,--I must be content to pa.s.s them by, hazarding only a few crude and desultory remarks. So large a question, indeed, cannot be safely handled without a corresponding amount of data, in all departments of natural science, to reason from,--which I do not possess: still, if a speculation from entomological premises, _per se_, be not altogether worthless, I would point to the conclusions (lately adverted to) which my Madeiran researches have forced upon me, concerning the _direction_ of the former insect migrations,--inferences which are, from first to last, of necessity erroneous, if the requisite medium for transit (into South-European lat.i.tudes, at all events) be a mere conjecture or romance. Such a notion, however, I would not for a moment entertain,--for there is too much direct evidence in support of distinct epochs of diffusion, to allow of any hypothesis, when endeavouring to account for the phaenomena which we now behold, to supersede the a.s.sumption of a once continuous tract. No matter if we be compelled to suppose, whilst attempting to interpret what we see, that the disseminating current has flowed in exactly opposite courses, at different and remote periods, over the surface of that ancient land,--seeing that the _fact_ (if such in reality it be) remains untouched, that _the land itself is_ at any rate _there_. I am not, however, prepared to a.s.sert that the opinion at which I had independently arrived, from the insect statistics, does positively require a northerly prolongation of that area beyond the line of the central Mediterranean districts; yet, after making every possible allowance for accidental introductions since the subsidences have taken place, there is still left a large residuum which I am convinced can never be explained (unless the doctrine of specific centres be a myth) except through the means of ordinary and regular migration over an unbroken continent. Nevertheless, though I would not presume, from insufficient material, to insist upon an extension of this Atlantic region into higher lat.i.tudes than those which I have just referred to, I must express my individual belief that, the more the subject is examined, with reference to the distribution of the Annulosa, the less will Professor Forbes's idea suffer from the inquiry. In the 'Insecta Maderensia,' I have already thrown out a few scattered hints which bear on this immediate consideration; and, since no subsequent reason has induced me either to withdraw or modify them (but rather the reverse), I will select the following,--extracted from my preface to that work.

"Taking a cursory view of the Coleoptera here described, the fauna may perhaps be p.r.o.nounced as having a greater affinity with that of Sicily than of any other country which has been hitherto properly investigated. Apart from the large number of our genera (and even species) which are diffused over more or less of the entire Mediterranean basin, this is especially evinced in some of the most characteristic forms,--such as _Apotomus_, _Xenostrongylus_, _Tarphius_, _Cholovocera_, _Holoparamecus_, _Berginus_, _Litargus_, _Thorictus_, and _Boromorphus_. There is, moreover, strange though it may appear to be, some slight (though decided) collective a.s.similation with what we observe in the south-western extremity of our own country and of Ireland,--nearly all the species which are common to Madeira and the British Isles being found in those particular regions; whilst one point of coincidence at any rate, and of a very remarkable nature, has been fully discussed under _Mesites_. Whether or not this partial parallelism may be employed to further Professor E. Forbes's theory of the _quondam_ approximation, by means of a continuous land, of the Kerry and Gallician hills, and of a huge miocene continent extending beyond the Azores, and including all these Atlantic cl.u.s.ters within its embrace, I will not venture to suggest: nevertheless, it is impossible to deny that, so far as the Madeiras betoken, everything would go to favour this grand and comprehensive idea. Partaking in the main of a Mediterranean fauna, the _northern tendency_ of which is in the evident direction of the south-western portions of England and Ireland, and with a profusion of endemic modifications of its own (bearing witness to the engorgement of this ancient tract with centres of radiation created expressly for itself), whilst geology proclaims the fact that _subsidences_ on a stupendous scale have taken place, by which means the ocean's groups were const.i.tuted; we seem to trace out on every side records of the past, and to catch the glimpses, as it were, of a _veritable_ Atlantis from beneath the waves of time[67]."

The _Mesites Maderensis_, Woll., to which I alluded in the above quotation, is undoubtedly a strong case in point. Although specifically dissimilar from the _M. Tardii_, its Irish counterpart, it nevertheless approaches it so closely, that it might be literally mistaken, _prima facie_, for that insect; and we know that it is one of the plans on which Nature commonly proceeds, that species which are not merely representative of (or a.n.a.logous to) each other, but which are actual h.o.m.ologues, or allies, should usually emanate at first from foci not far removed _inter se_; or, at all events, if distant, connected by an intervening land:--in other words, that _generic areas_, no less than specific centres, of radiation, form a substantial item of the comprehensive scheme on which the system of created things was originally planned. We detect traces of this primary law in each division, or cla.s.s, of the organic world; nor is its reality _as a law_ interfered with, through the occasional exceptions which are liable, as in every other instance, to present themselves. Such deviations are often easily to be accounted for, whether by natural or artificial means; and do not affect the subject, as a whole. Sometimes indeed they become at once intelligible from the historical records connected with them, proving that human agencies have been at work acting as transporting media, within a period comparatively recent; whilst at others, the fact of the creature having been endowed with self-diffusive powers to an extravagant degree may succeed equally in rendering the phaenomena explicable. But, even where neither of these solutions would seem to suffice, we should still recollect that it is only in the ma.s.s that such questions can be p.r.o.nounced upon; and that, consequently, where we are able to discover a rule which is _for the most part_ adhered to, it is more philosophical to conclude that the departures from it are the result of special disturbing causes (whatsoever they may have been), than to permit them to undermine our faith in what would be otherwise universally true. Thus, the botanist tells us of Ixias, Stapelias, Mesembrianthemums, Pelargoniums, and Euphorbias, as concentrated in Southern Africa; of Magnolias in Central America; of Calceolarias on the Andes; of Myrtles, Banksias, Mimosas, and _Eucalypti_, in Australia; and of the Bread-fruit Trees in the South Sea Islands: the ornithologist points, _inter alia_, to the Toucans and Humming-Birds from South America and the West Indies; whilst the student of the higher animals informs us of the Kangaroos (indeed of the whole of the subcla.s.s _Marsupialia_, except the genus _Didelphys_) as peculiar to Australia and a few islands to the north of it; of _Lemur_ _proper_ to Madagascar; of the Sloths, Armadillos, Tree Porcupines, and of Alligators, and of the _Platyrrhini_ (amongst the Monkeys), to South America; and of the Ourangs to the islands of the Indian Archipelago.

And so it is with the Insecta; many of the larger groups of which (as _Amycterus_ and _Paropsis_, in Australia; _Pachyrhynchus_ and _Apocyrtus_, in the Philippine Islands; _Hipporhinus_, _Monochelus_, _Dichelus_, and _Moluris_, in Southern Africa; _Macronota_, in Java; and _Naupactus_, _Hypsonotus_, _Centrinus_, _Platyomus_, and _Cyrtonota_, in South America) are confined to countries of proportionate magnitude, whilst the smaller ones are more commonly (as it were) shaped out for special provinces or regions, according as local circ.u.mstances may require primary adaptations to harmonize with them. Thus, whilst we frequently find an extensive genus diffused over the greater portion of the known world, we perceive that even its _structural_ characteristics are not uniform throughout, but afford fixed geographical modifications (_not_, in this case, however, the effect of development),--which have often, in their turn, obtained the name of 'genera,' and have been described as such. Whether genera, however, or not, they are undeniably small topographical a.s.semblages, satellites around their central types; and they may therefore be safely regarded as genera, if we choose to view them in that light. Of such a nature I have already pointed out[68] is _Saprinus_, as compared with _Hister_; _Atlantis_ with _Laparocerus_; and _Oxyomus_ with _Aphodius_; and, I might also add, _Mesites_ with _Cossonus_. I believe indeed that _Mesites_ will be found to attain its maximum on the Pyrenees (I already possess two or three species, in abundance, from that region); and, if such should be the case, we shall be able to appreciate the significance of two representatives so closely allied as the _M. Tardii_ and _Maderensis_,--one of which has been given off in the direction of Ireland, and the other of the Madeiran Archipelago.

But I will not digress further on the subject of this Atlantic province; since, however much I may individually regard it as a reality of the past (which the Coleopterous statistics have compelled me to do), it must of necessity remain, as heretofore, a matter of much controversy and doubt. I should indeed apologize for having trespa.s.sed on the reader's attention, in wandering this far from the immediate results of _subsidences_,--which I proposed, at the outset of this chapter, to examine, with reference to the impeded diffusion of the Annulose races. Nevertheless, concluding that a practical ill.u.s.tration of the effects of one of those great downward movements to which geology so repeatedly bears witness would not be irrelevant to the _a.s.sumed consequences_ which I had previously ventured to define, I have acted on that judgment; and, having finished my task, will now proceed to notice, briefly, a few other considerations which should not be omitted, when inquiring into insect distribution as influenced by geological phaenomena.

Next in importance, perhaps, to the elevations and sinkings (traces of one or the other of which are more or less manifest in almost every region of the world), _natural barriers_ may be cited,--as presenting, not unfrequently, insurmountable obstacles to the self-dissemination of the insect tribes. By natural barriers, however, I would be understood to imply natural _primary_ barriers,--or, in other words, such as have continued as barriers ever since the present animals and plants came into existence upon the earth. For, the _ocean_ (by way of ill.u.s.tration) is a natural barrier; and yet it is not necessarily a primary one, as may be readily gathered from the above remarks, in which the results of _subsidences_ are discussed,--subsidences which have had the effect of letting it in over portions of an _already tenanted_, and unbroken, continent. Mountain-chains, also, are barriers; but it may happen that they have not been so from the beginning,--as in instances, for example, where they have been gradually upraised during periods geologically recent. But both sea and alpine ranges are barriers, when (as usually happens) they have remained as such since the creation of the several species which now inhabit our globe. Mr. Darwin has acknowledged this distinction, whilst commenting upon the marked divergence of the faunas on the eastern and western slopes of the Cordillera. "This fact," says he, "is in perfect accordance with the geological history of the Andes; for these mountains have existed as a great barrier since the present races of animals have appeared; and therefore, unless we suppose the same species to have been created in two different places, we ought not to expect any closer similarity between the organic beings on the opposite sides of the Andes, than on the opposite sh.o.r.es of the ocean.

In both cases, we must leave out of the question those kinds which have been able to cross the barrier,--whether of solid rock or salt-water[69]."

Conceding, therefore, this distinction between barriers of a primaeval and more recent character, it is not difficult to understand why the opposite sides of an alpine chain, as well as countries separated by the sea, should display different phaenomena from each other. On the contrary indeed, if we could feel satisfied that no means of accidental transportation had operated to take them there, and that the animals themselves were incapable of enduring great diversities of temperature, and other contingencies; we should be startled to discover creatures specifically identical in such regions,--so long at least as the doctrine of unique centres of radiation formed part of our zoological creed. We must not, however, be too hasty in questioning (if I may be pardoned for the completion of a metaphor of which I thoroughly disapprove) this article of our faith, through the occurrence of similar beings in areas between which there exist barriers, both primary and well-defined; for the methods of diffusion are so complicated and numerous, that, even where human agency (that most important of elements) is not concerned, what at first sight may frequently appear to be impossible becomes clear enough when more critically inquired into. Some species, we know, are gifted with greater powers for horizontal and vertical progression than their comrades, and can (though they are doubtless exceptions to the general rule) pa.s.s through extremes of atmosphere sufficient to render even lofty mountain summits no obstacles to _them_. Others, as the _Calosoma Syncophanta_ of Europe, have been stated to traverse the ocean unhurt[70]; and I believe that many do at times accidentally arrive, in a half-drowned state, especially after boisterous weather, across channels of considerable breadth. Mr. Kirby, on examining the marine _rejectamenta_, during one of these apparent occurrences, along the Suffolk coast, writes as follows: "Whether the insects I observed upon the beach, wetted by the waves, had flown from our own sh.o.r.es, and, falling into the water, had been brought back by the tide; or whether they had succeeded in the attempt to pa.s.s from the continent to us, by flying as far as they could, and then falling had been brought by the waves, cannot certainly be ascertained; but Kalm's observation inclines me to the latter opinion[71]." And Sir Charles Lyell remarks:--"Exotic beetles are sometimes thrown on our sh.o.r.e, which revive after being drenched in salt water[72]." Nor should we forget that chance agencies of every description, which we are too apt to overlook, are daily at work (and have been so since, at any rate, the last creative epoch) to transport these variously organized beings beyond their original spheres. Sometimes they are carried on, or within, the bodies of larger animals, which is especially the case with the parasitic tribes; at others on floating trunks of trees, and casual substances of divers kinds, which are able to resist for a definite period the destructive action of an element saturated with salt. Unwilling victims, again, are ever and anon hurried to comparatively distant lands by the very winds that blow; and not only to distant lands, but over alt.i.tudes in which the severity of the cold would quickly annihilate them, were they (as perhaps usually happens) to be deposited there on their headlong and compulsory course. "As almost all insects are winged[73]," says Sir Charles Lyell, "they can readily spread themselves wherever their progress is not opposed by uncongenial climates, or by seas, mountains, and other physical impediments; and _these_ barriers they can sometimes surmount by abandoning themselves to violent gales, which may in a few hours carry them to very considerable distances. On the Andes some sphinxes and flies have been observed by Humboldt, at the height of 19,180 feet above the sea, and which appeared to him to have been involuntarily carried into those regions by ascending currents of air[74]." With respect to the accidental conveyance of numerous species across the sea, it is not to the winds alone that we must look for an explanation. Large and rapid rivers are liable to inundate their banks and bring down insects in prodigious ma.s.ses,--which are disgorged into the ocean, and carried to a distance from the coast, in proportion to the violence of the ejecting stream. When the body of water is considerable, the sea becomes diluted to an unusual extent; and creatures which must have otherwise perished, from the action of the salt, are able to survive for a time, and may be deposited, by means of rapid currents into which they are borne, on neighbouring islands and continents. Even the _Hydradephaga_ are thus occasionally transported; for Darwin mentions having captured a _Colymbetes_ off Cape S^{ta} Maria (to the north of the Rio de la Plata), when forty-five miles from the sh.o.r.e. And, in his 'Journal of Researches,'

he records the following remarkable facts, which bear upon this immediate question. "On another occasion, when seventeen miles off Cape Corrientes, I had a net overboard to catch pelagic animals. Upon drawing it up, to my surprise I found a considerable number of beetles in it, and, although in the open sea, they did not appear much injured by the salt water. I lost some of the specimens; but those which I preserved belonged to the genera _Colymbetes_, _Hydroporus_, _Hydrobius_, _Notaphus_, _Cynucus_, _Adimonia_, and _Scarabaeus_. At first I thought that these insects had been blown from the sh.o.r.e; but upon reflecting that, out of the eight species, four were aquatic (and two partly so) in their habits, it appeared to me most probable that they were floated into the sea by a small stream which drains a lake near Cape Corrientes. On any supposition, it is an interesting circ.u.mstance to find live insects swimming in the open ocean seventeen miles from the nearest point of land[75]."

Accidental means of dissemination, such as those to which I have just alluded, and others to which we might appeal, will generally account, and with much presumptive truth, for the many exceptional cases which present themselves, during our investigation into the effects of natural barriers, as visible in the distribution of the Annulose races, on the earth's surface. I say "exceptional cases," because any one who has laboured practically in mountain tracts cannot have failed to recognize the marked difference which is often displayed by the insect population on opposite sides of some alpine chain; whilst he whose lot has been cast amidst island groups, will have become even more conscious than the former of the permanency of those impediments which have been placed (in this instance by the broad arms of the mighty ocean) as checks upon a too rapid system of diffusion.

But if the sea and mountain ranges, when of a sufficient age _in situ_, are amongst the most effectual of Nature's barriers against the self-dispersion of the animate tribes; it follows that, if the two could be (as it were) _united_, we should have found the greatest obstacle which physical conditions can ordinarily present against the wandering capabilities of the latter. The question therefore arises,--Is it possible for them to _be_ so joined? Undoubtedly it is: and hence we arrive at the conclusion, that a _mountain island_ should afford us the _minimum of size, as regards the areas its species have overspread_, which any country is able to furnish.

Madeira is a mountain island,--its highest peaks rising, although resting on so small a base, to an alt.i.tude of more than 6000 feet. Yet it is only partially a case in point; for, although it was a mountain ma.s.s, and perhaps a very elevated one, when its endemic beings made their first appearance upon its surface, we have already intimated that it has become isolated _since_ that epoch: so that, whilst _one_ of the natural barriers against dispersion which it involves (namely, mountain ridges) may be considered as primary; the _other_ (to wit, the sea, as it now obtains) has played, as an agent of obstruction, but a secondary part. Still, there is good reason to believe that the ancient tract of which it is a portion was broken up at a comparatively early date after the creation of those peculiar organic forms which found their birthplace within its bounds; and that, consequently, the latter could not have wandered far (if we except those species on which unusual powers of diffusion were bestowed) when the land of pa.s.sage began to give way. Hence, even the sea, in this particular instance, partakes almost of the character (no less than the mountain heights) of an original impediment; and Madeira therefore may be safely quoted as an example in which two barriers, of a primary nature, are united; and where, consequently, we may antic.i.p.ate those ultra phaenomena of _areal limitation_ upon which we have been just commenting.

But let us now inquire, whether the hypothesis at which we have arrived will stand the test of experience; for unless it will do so, we might have been spared the labour of propounding it. Madeira is a country composed of narrow mountain ridges, which radiate from central crests, and form the lateral boundaries of deep and precipitous ravines. Modifications of this structural type are of course traceable everywhere; the upland tracts are often undulating and broad, and the b.u.t.tresses which slope towards the sea are sometimes expansive and irregular: yet upon the whole the above description is correct, and we may accept it in a generic sense. Now we may premise that, even to this day, it is an island of floods; therefore, how much more must it have been so when its primaeval forests, in all their splendour, caused an amount of exhalation and moisture of which at present we can have but a remote conception! Hence, it is hardly to be imagined, that (however limited may have been the naturally acquired areas of those of its inmates which are most sluggish and sedentary) a fusion would not have taken place, in the course of ages, so as to render its modern fauna, in a large measure, h.o.m.ogeneous throughout. Yet, in spite of this esoteric tendency, it is surprising how little amalgamation has been effected amongst the tenants of its several districts. Scarcely a gorge or woodland serra exists within its bounds which does not harbour some species essentially its own; and in many instances the ranges of these creatures are so local or confined, that they might be easily overlooked even in their respective neighbourhoods. It is certain, however, that the floods (which happen periodically) have done considerable work in naturalizing many of the subalpine forms, which could adapt themselves to the climatal change, in alt.i.tudes below their normal ones: and, in the north of the island, where the temperature is cooler than on the opposite side, and where the lofty defiles terminate, even at their lowest outlets, in abrupt precipices along the coast, so that the _rejectamenta_ during the annual rains are brought into direct contact with the sh.o.r.e, this gradual process of deportation is particularly evident,--a circ.u.mstance to which I have already alluded elsewhere[76].

But, after making due allowance for these powerful means of dissemination (which, in the common order of things, must necessarily obtain in _mountain islands_, as it were, _par excellence_), the fact still remains, that in the Madeiran Group the acquired areas, even up to the present date, of a vast proportion of the insect inhabitants, are wonderfully circ.u.mscribed. The real state of the case, however, would appear to be simply this: that the floods, although they may have tended to diffuse the members of a comparatively uniform alpine fauna in the various clefts or gorges beneath, can have had no power to combine the aborigines of the several gorges themselves; and, since a large proportion of the endemic species of those islands are (as I have previously stated) apterous, the perpendicular edges of the ravines, which in many instances rise to an elevation of 2000 feet, have acted (and ever _will_ act) as impa.s.sable barriers to vast numbers of the insect tribes.

With this single example (by way of ill.u.s.tration), which the Madeiras have supplied, I will take my leave of the question of _natural barriers, as tending to regulate the topographical diffusion of the Annulosa_,--feeling that I have already devoted too much time and s.p.a.ce to this portion of the subject (if such indeed it be) which I had proposed in the present treatise to discuss. Other barriers might have been adverted to,--such as large rivers, extensive deserts, and thickly set forests (especially of pine-trees, which frequently offer a very decided impediment to insect progress),--but they are of secondary importance, when compared with marine and alpine ones; and their consequences may be, to a certain extent, deduced from the considerations which I have just entered into. My main object has been to draw attention to the fact, that the great obstacles which Nature has placed against the too rapid dispersion of animal life should be more strictly taken into account (as a matter of positive reality) than it is, during our investigations into entomological geography. To be aware that these barriers exist, and yet to feel surprised, especially in a country where the species are princ.i.p.ally wingless, that we do not discover indications of a general uniformity in its fauna, involves an absurdity,--unless the doctrine of specific centres of creation be a mere coinage of the brain. But, if we believe in that theory (which, until it can be shown to be impossible, I hold that we are _a priori_ bound to do), we must at least act consistently with ourselves, and not antic.i.p.ate phaenomena where we have neither reason nor right to look for them.

We are too apt to draw a line of imaginary demarcation between the sciences, as though each had its own propositions to establish, and nothing more: indeed, some of us would appear to a.s.sume (though perhaps tacitly), that what is proved to be true in one department may be, at least, rendered inconsistent (if not actually negatived) in another. But surely this requires no argument to refute,--since a _principle_ which is _true_, is true under every circ.u.mstance and condition; for otherwise, it could be both true and false. We need not therefore be afraid of comparing truth with truth, under whatever shape it may arrive, as though it were possible that either of its phases could ever suffer from the ordeal of a close contact; since, if they be really true, and free from deception, they must needs go hand in hand, and _may_ become (however opposite they be in their subjects) directly explanatory of each other. The astronomer who is not intimately acquainted with pure mathematical a.n.a.lysis, in its various aspects and bearings, is in fact no astronomer at all. The geologist who would interpret the grand phaenomena of the earth's crust apart from statical and dynamical knowledge, and without the help which the chemist, mineralogist, anatomist, zoologist, and botanist can afford him, stands a fair chance of leaving his problems unsolved; whilst the students of zoology and botany who would endeavour to understand, and account for, what they see in the animal and vegetable worlds around them, without calling in geology to their aid, must a.s.suredly be prepared to fail signally in their attempts. All indeed must work in concert, if the whole is to be advanced,--and not only in concert, but as mutually a.s.sisting each other. "By the help of truths already known, more may be discovered; for those inferences which arise from the application of general truths to the particular things and cases contained under them, must be just.[77]"

FOOTNOTES:

[60] "When we consider indeed the apterous nature of _Deucalion_, its subconnate elytra, and its attachment (at any rate in the larva state) to the interior of the stems of particular, local plants, or its retiring propensities within the crevices of rocks; we are at once struck with the conviction, that, during the enormous interval of time which has elapsed since the mighty convulsions which rent asunder these regions terminated, it has probably never removed many yards from the weather-beaten ledges which it now inhabits."

[61] Since the above was published, I have succeeded in detecting one more example,--namely (in June 1855) on the summit of the Ilheo Bugio, or Southern Dezerta, within a few yards of the self-same spot where it was found by the Rev. R. T. Lowe in May 1850. Although I searched diligently on the Dezerta Grande, during my late campaign in the Madeira Islands, I was not able (so great is its rarity) to discover farther traces of it on that rock.

[62] Insecta Maderensia, p. 435.

[63] It would seem, when viewed on a broad scale, as if particular districts throughout the world had been made as it were the special fields for the exercise of the creative force,--or that, _generic areas of radiation_ were part of the elementary design. Thus, Professor E. Forbes records his belief that most, if not indeed _all_, of the terrestrial animals and plants now inhabiting Britain are members of specific centres beyond bounds,--they having migrated to it over a continuous land, before, during, or after the glacial epoch.

Hence, since the greater number of them are supposed to have come from the central Germanic plains, we may a.s.sume that those plains were one of the primary areas of diffusion for a large ma.s.s of created beings.

There is good cause for suspecting that the Pyrenean region may have been another; and certainly all evidence would tend to prove that this vast Atlantic province was, also, well stocked with aboriginal forms.

[64] a.s.suming the _Helix Lowei_ and _Bowdichiana_ to be gigantic phases of the _H. Portosanctana_ and _punctulata_, respectively; four only, namely _H. fluctuosa_ and _lapicida_, _Achatina Eulina_, and _Cyclostoma lucidum_ (the first three of which are extinct throughout the entire group), seem to have altogether disappeared. Nevertheless, the gradual dying-out, as it were, of species, both here and in Madeira proper, is singularly evident. Thus, in the latter, the Canical beds show the _H. tiarella_ to have been once most abundant (it literally teems in those calcareous formations). Yet so rare is it in a recent state, that, until the summer of 1855, when it was detected by myself and the Rev. R. T. Lowe in two remote spots along the perpendicular cliffs of the northern coast, it was supposed to have been lost for ages. And the same may be said of its counterpart, the _H. coronata_, in Porto Santo,--which, likewise, swarms in every fossil-bed of that island; but which was, also, until I met with it, on the 15th of December 1848, adhering to slabs of stone at a considerable depth beneath the ground, on the extreme eastern peak (opposite to the Ilheo de Cima), imagined to have long pa.s.sed away.

And so, reasoning from a.n.a.logy, I think it far from improbable that the third representative of this little geographical a.s.semblage,--the _H. coronula_ of the Bugio (which has. .h.i.therto only occurred in the mud deposits on the summit of that rock),--may be still alive, though perhaps in very small numbers, on some of the inaccessible ridges of those dangerous heights.

[65] Origin of the Fauna and Flora of the British Isles (in Mem. of the Geol. Survey of Great Britain, vol. i. p. 336, A.D. 1846).

[66] "My own belief," says Professor Forbes, "is, that the great belt of gulf-weed, ranging between the 15th and 45th degrees of north lat.i.tude, and constant in its place, marks the position of the coast-line of that ancient land."

[67] Although, for want of a better name, it may be admissible, when speaking either figuratively or poetically, to allude to this former region (as I have done in the above quotation) under the t.i.tle of "Atlantis;" yet it seems incredible that certain writers (a.s.suming its _quondam_ existence) should have recently referred to it seriously as the possible "Atlantis _of the ancients_!" Considering that there is good reason to believe that all these islands _were islands in a miocene sea_, and that, if (through a general elevation) they were subsequently connected, the land of pa.s.sage was broken up long anterior to the appearance of man upon the earth, "the ancients" must have a.s.suredly merited their appellation, if they could have thrown any light on a problem which belongs to an epoch thus remote. Whether the "Atlantis" had any being at all except in the imagination of the Latin poets, or whether (as Lord Bacon has suggested) it was the New World, will probably never now be known; yet the fact that the _Insulae Fortunatae_ of Juba are almost universally identified with the present Canarian Group (as indeed the accurate description of Pliny well nigh demonstrates), and the _Purpurariae_ with the Madeiras, ought at once, apart from geological evidence, to point out the absurdity of the hypothesis, that an Atlantic continent, _in the very position which those islands occupy_, could have been acknowledged to have any existence by the literature of either Rome or Greece.

[68] Insecta Maderensia, p. 214.

[69] Journal of Researches, pp. 326, 327.

[70] Many of the _Calosomata_ would appear to possess this power of crossing, either by flight or by abandoning themselves to the waves (though more probably by the a.s.sistance of both), even marine barriers with impunity. Numerous instances are on record to this effect; and I am informed by Mr. Darwin that a _Calosoma_ flew on board the 'Beagle,' off the Bay of San Blas, in South America, whilst they were ten miles from sh.o.r.e. It seems likely, therefore, that the occasional occurrence of the _C. Syncophanta_ in our own country, along the southern and eastern coasts, is due to this generic capability,--and consequently (as indeed it is usually acknowledged to be), the result of accident.

[71] Introduction to Entomology, ii. p. 13.

[72] Principles of Geology, 9th ed. p. 657.

[73] Although this is true on a broad scale, a reference to my observations in a preceding chapter will show, that in some countries, especially islands, the reverse will frequently be found to obtain.

[74] Principles of Geology, p. 656.

[75] Journal of Researches, p. 159.

[76] Insecta Maderensia, p. 81.

[77] Religion of Nature Delineated, pp. 73, 74.

CHAPTER VI.

THE GENERIC THEORY.

How glorious to the observant eye is the great system of the organic world, how perfect in each separate part, how complete and harmonious the whole! The unity of the comprehensive plan, amidst the infinite modifications which it includes, has ever been a theme of admiration and delight; for the mind, which has once caught a glimpse, even in physics, of what it is not possible to disprove, instinctively clings to it, as to a grand material truth. The discovery, at all times, of what we feel to be actually _certain_ is in itself so fascinating, that the very data which it gives us are scarcely more prized than the mere knowledge that we have gained a single additional light to guide us on our forward way: for, since in the inductive sciences we can but climb from step to step, at a slow and even pace, we hail with inward satisfaction whatsoever may tend to lighten our task, and to lead us more quickly onwards (gradually though we must of necessity advance) towards its final accomplishment.

But how, it may be asked, is this general harmony of the organic creation to be insisted upon, when beings so extravagant and dissimilar are everywhere to be met with? Is it possible to recognize anything like a unity of type amongst creatures so differently constructed, and so widely removed from each other in their habits, aspects, functions, and attributes? Such questions as these, however, though they may occasionally perplex the tyro, or amateur, are not likely to be raised by anyone who has mastered the merest alphabet of zoology,--and who is aware that the integrity of Nature is something real and positive, as experience indeed is ever tending more and more to corroborate, and by no means the day-dream of an enthusiastic, or fertile, imagination. To trace out the progressive development of animal life, from its humblest phases; and to mark, as they become visible in the intermediate grades, the first rudiments of organs and instincts which are destined to attain their maximum in the higher ones, embody but a small portion of what it is the naturalist's mission to investigate. To him belongs the special privilege of inquiring dogmatically into this structural advancement; and of suggesting methods of cla.s.sification which shall accord, in their several component divisions, so far at least as is practicable, with the const.i.tutional change. We should recollect, however, that this system, being based upon truth, must, if it would be consonant throughout, adapt itself to all the various phaenomena (in their respective positions, in the scale), from the consideration of which it should be exclusively deduced, or built. To draw broad conclusions of any kind, or to attempt the establishment of propositions and principles, from simple dialectics, without a previous training in the practical bearings of the subject, would be absurd, and almost certain to beget error. "It cannot be that axioms established by means of _reasoning_ [alone] should be of any value for the discovery of new results; because the subtilty of Nature far exceeds the subtilty of reasoning. But axioms duly and orderly abstracted from _particulars_, in their turn easily point out and mark off new particulars; and so render the sciences active[78]." Such were the words of the greatest philosopher which this country has ever produced; and it would be well, whilst examining the causes of what we see, and endeavouring to obtain some faint and distant notion of the vast scheme of Nature as originally designed, to keep them constantly in view,--lest, by trusting to theory only, apart from observation and facts; or by venturing to pervert the latter (instead of being led by them), so as to tally with our preconceived ideas of what ought to be, we miss our road, and become lost in the mazy labyrinth of our own fanciful inventions.

With this preliminary stricture on the express duty which devolves upon the naturalist (with whom the phaenomena of the organic world princ.i.p.ally rest, for interpretation) to make facts, rather than reason and argument, the basis of his various doctrines,--at any rate of those in which the critical subject of _arrangement_ is concerned; I shall perhaps be pardoned, after having been drawn, in the preceding chapters (however involuntarily), into the question of 'species,' as rigidly defined, if I now offer a few pa.s.sing remarks on the theory of _genera_.

There can be no doubt that amongst a large cla.s.s of ordinary observers a clear perception of the generic system, in an abstract sense, does not by any means prevail. What the nature of a genus really is, would appear to have been very commonly overlooked, or perhaps misunderstood, by people of this stamp; and the consequence has been, that the wildest notions have frequently arisen, even from men of sound _specific_ attainments, as to the claims (for annihilation or retention, as 'genera') of certain subsidiary zoological a.s.semblages.

The terms 'genus' and 'species' have been conjointly so long a.s.sociated in our minds with the selfsame things (whatsoever they may be), that they have become almost part and parcel of the objects themselves; so that the student who does not sufficiently reflect on their true signification, is apt to regard them as of equal importance,--or, rather, more often perhaps than otherwise, to make the latter subservient (or inferior) to the former! This however is, in reality, the very reverse of what should be the case, as a moment's consideration will indeed at once convince us: for what are genera, after all, but _dilatations_ (as it were) along a chain _which is itself composed of separate_, though differently shaped, _links_? The links (or the actual, independent bodies which const.i.tute the chain) are the species; but the k.n.o.bs, or swellings, which their several forms may tend, _by degrees_, to establish along its course (through the slight disparity which each of them presents from that which is next in succession to it; and therefore through the gradual manner in which the bulbs, or nodules, may be said, _on the whole_, to be produced), are the groups into which those species naturally fall. It matters not a straw whether these a.s.semblages be primary, secondary, tertiary, &c.,--in other words, whether they be departments, families, or genera, as usually understood,--the _principle_ is in every instance the same; the difference being merely relative, and not absolute.

Or, if we choose to vary the simile, we may compare the whole system to a cord, upon which beads, of innumerable sizes, patterns, and colours, have been densely strung. Now, if there were no such things as natural divisions in the organic world, these beads (which represent the separate species) might have been disposed of anyhow,--their positions, with respect to each other, would under those circ.u.mstances have been of no importance. But such is not the case: there is an order and method throughout Nature, which shows that every individual portion of it has been adjusted by the Master's hand, and that nothing has been left to chance. Those beads (to follow up the metaphor) of countless magnitudes and hues, have had their proper places allotted to them,--and moreover with such care and regularity, that a complete plan, or scheme, of distribution is at once conspicuous. Although there are not even two, amongst that enormous mult.i.tude, which are _precisely_ alike (for every species, however it may resemble its next ally, has _some_ distinctive feature of its own), we immediately perceive that those beads which have most in common, are, as it were, attracted to each other,--so as, by their close approximation, or contact, to create excrescences and stripes, of divers kinds, along the entire length of the cord. If we a.s.sume now that the red beads have been collected together, to the length (for instance) of a yard, and that within that s.p.a.ce a dozen protuberances, of discordant aspects and dimensions, have (by the union of those beads which more nearly simulate each other) been brought about; we shall have a very fair idea of the ordinary grouping of the animate tribes. The red beads, taken in the ma.s.s, may be likened to a perfect "family;" the differing gibbosities to twelve well-marked "genera,"

which that family includes; whilst the "species" (the real _dramatis personae_, of independent existence, which are nevertheless compelled to occupy the situations we have described,--thus _causing_ the divisions to be mapped out) are here typified, as everywhere, by the several beads themselves.

I have not thought it necessary to pursue this reasoning into higher divisions than "families;" but of course it may be extended to any amount,--so as to shadow forth, equally, the compartments of _primary_ significance. Nor would I wish to imply, by the above similes, that I regard a _lineal_ method of arrangement as the correct one. Every zoologist is aware, that in Nature such does not exist: but the mode of ill.u.s.tration which I have selected is applicable to all systems alike, so far as the _principle_ is concerned.

It will consequently be seen, from what has been said, that the terms "genus" and "species" not only differ very considerably in _importance_, but in signification also. Whilst the former is merely suggestive of a particular _position_ which a creature occupies in a systematic scale (a position, however, which depends upon the various structural peculiarities which it possesses _in common with other beings_,--which thus more or less resemble it); the latter expresses the actual creature itself: so that while one applies to _several_ animals (of distinct natures and origins, though bound together by a certain bond of imitation), the other belongs to _a single race alone_, which it therefore exclusively indicates. But if such be the case, it will perhaps be asked,--Why then insist upon a generic name at all, if the specific one be sufficient to denote all that is required, namely, the _animal itself_? To which, however, we may reply, that the binomial nomenclature is demanded for two elementary reasons,--first, because it is founded upon a natural truth, which (to say the least) it would be unwise to violate; and, secondly, because it is _convenient_, both for simplification and a.n.a.lysis. We should a.s.suredly be surprised were a man to object to his surname, as unnecessary, because he has a christian (or specific[79]) one which is the exponent of him _alone_. True it is that his family (or generic) t.i.tle applies to the rest of his kin also; but, since there are other people (of other families) who may have the same _individual_ appellation as himself, it is clearly desirable, even as a matter of expediency alone, that patronymic and christian name should be alike retained. We need not, however, plead expediency, in favour of this acceptance of what has been so long tested, and shown to be correct; we appeal to a higher tribunal,--that of experience,--in proof that it draws its origin from Nature itself, and is implied by the very existence, or reality, of _natural groups_. The 'Methode Mononomique'

has indeed been attempted[80]; and it has failed,--or at any rate it has shown itself to be inferior, both ideally and in practice, to the plan commonly in use: and if I might be pardoned a pa.s.sing conjecture on its ultimate success, I should be inclined, since it is contrary to the canon of the organic world, to regard its case as utterly hopeless.

Let us not be unfair, however, towards those who have sought to establish a nomenclature which they conceived would be less open to objections than that which we have been hitherto accustomed to endorse. The notion did, at any rate, arise out of an apparent defect in the binomial process,--for the inconveniences which they complained of are real ones; and, having felt them practically, they aspired to sweep them away by remodelling the whole system afresh. But, had it not been for an evident misconception of the generic theory, in the abstract, the trial would in all probability have never been made; and we should have been spared the downfall of a contrivance which has had but little to recommend it beyond the ingenuity of its machinery and detail. If we a.n.a.lyse the motives for this experiment, we shall find that it originated from a belief, that genera are _either_ purely imaginary, or else that they must (like species) have a definite and isolated existence. Now both of these conclusions appear to be equally gratuitous and untenable; and such as a lack of observation could alone beget. Genera are _not_ mere phantoms of the brain (as most naturalists will readily admit); but they are, likewise, by no means abrupt, or well-marked, on their outer limits (except indeed by accident,--of which hereafter), but merge into each other by gradations, more or less slow and perceptible. Such being the case, we can easily understand why it is that the followers of the 'Methode Mononomique' (who, paralysed by the fact that genera are seldom _clearly defined at their extremes_, would seem to repudiate them _in toto_) have rashly regarded the binomial system as intolerable.