Chapter V: Geological Reflections (2)

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) pass 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 shores, and, falling into the water, had been brought back by the tide; or whether they had succeeded in the attempt to pass 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 shore, 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 altitudes 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 masses,--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 shore. 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 shore; 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 circumstance 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 altitude of more than 6000 feet. Yet it is only partially a case in point; for, although it was a mountain mass, 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 passage 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 anticipate 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 buttresses 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, homogeneous 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 altitudes 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 shore, this gradual process of deportation is particularly evident,--a circumstance 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 circumscribed. 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 impassable barriers to vast numbers of the insect tribes.

With this single example (by way of illustration), 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 space 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 principally 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 anticipate 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 assume (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 circumstance 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 analysis, 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 assuredly 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 assisting 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 assume that those plains were one of the primary areas of diffusion for a large mass 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] Assuming 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 passed away. And so, reasoning from analogy, I think it far from improbable that the third representative of this little geographical assemblage,--the _H. coronula_ of the Bugio (which has hitherto 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 latitude, 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 title of "Atlantis;" yet it seems incredible that certain writers (assuming 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 passage was broken up long anterior to the appearance of man upon the earth, "the ancients" must have assuredly 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 assistance 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 shore. 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.