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 primæval 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 à priori bound to do), we must at least act consistently with ourselves, and not anticipate phænomena 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 phænomena 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.