Chapter III · CAUSES OF VARIATION. (continued 2)
Reading edition
In the Madeiras, this diminution and enlargement of stature, accompanied for the most part respectively by the retention and annihilation of the powers of flight, is singularly traceable on the selfsame rocks, particularly the smaller ones of the group. Thus, on the Flat Deserta, or Ilheo Chão, the Scarites abbreviatus, Koll., Laparocerus morio, Schön., and the Helops Vulcanus, Woll., attain a gigantic size; yet it is on that very island that the Ptinus albopictus, Woll., finds its minimum of development,—scarcely exceeding in dimensions some of the larger members of the Trichopterygia. The Deserta Grande has some special modifying capability of its own,—the Eurygnathus Latreillei, Lap., Notiophilus geminatus, Dej., Zargus pellucidus, Woll., Calathus complanatus, Koll., Olisthopus Maderensis, Woll., Caulotrupis conicollis, Woll., Laparocerus morio, Schön., Omias Waterhousei, Woll., Helops Vulcanus, Woll., and the Ellipsodes glabratus, Fab., being also larger on that rock than is typical: all of them, however, with the exception of Notiophilus geminatus, are there, as elsewhere, apterous.
Other qualifying results, from isolation, are equally apparent. Take colour, for instance; and we shall perceive that in the Dromius sigma, Rossi, it is sensibly affected. The normal state of that insect "does not occur at all in Madeira proper, but only in Porto Santo. True it is that the modifications in the several islands present but slight differences inter se; nevertheless, being constant, I would lay particular stress upon them, since they go very materially to prove that the effects of isolation on external insect form are even more important, if possible, than those of latitude. That this is the case in the present instance, appears clear from facts so minute as these. For, out of the many specimens which have come under my observation from various countries of Europe, if there is one point more constant than another in this otherwise variable species, it is, I believe, to all circumstances, its immaculate prothorax. Now, whilst this (we may almost say essential) character obtains in Porto Santo, in Madeira it does not hold good: the prothorax there is invariably infuscate in the centre; and on a small adjacent rock (the Ilheo de Fora) it is entirely dark. Nor let anyone suppose that details apparently so trivial are beneath our notice, or the mere result of chance, since it is by the observation of such-like points, and by marking their development according to the circumstances of the several localities in which they obtain, that we are alone able to appreciate their importance, and so to form, in a wider and geographical sense, a correct estimate of their value[40]." The Olisthopus Maderensis, Woll., is much paler, larger, and more opake, on the Dezerta Grande than it is in Madeira proper. So great indeed is the change which it has undergone through a long isolation on that rock, "that, had the case been a solitary one, I should not have hesitated in regarding the specimens obtained from thence as specifically distinct; nevertheless, with the knowledge both of the modifying effects of isolation, and also of the kind of modification essentially peculiar to that island, I am perfectly satisfied that it is a mere local state, although a very remarkable one, and has no claim whatsoever to be otherwise considered[41]." The Pecteropus Maderensis, Woll., is of a greenish-brassy tinge in Porto Santo, and much acuminated in front; whereas on the Dezerta Grande it is almost invariably coppery, and less narrowed anteriorly. The Caulotrupis lucifugus, Woll., although ranging through no very opposite phases, either of outline or sculpture, "appears to possess a slight modification for every island of the Madeiran Group: and hence small shades of difference, which might otherwise be regarded as trifling, become directly important, and cannot be ignored in a local fauna,—even though a general collector may deem it unnecessary to recognize them. In real fact, however, such distinctions, when viewed geographically, are of the greatest interest, as serving to illustrate what we have so often had occasion to comment upon, namely the influence of isolation and other circumstances on external insect form[42]." The Psylliodes vehemens, Woll., is permanently paler in Porto Santo than it is in Madeira proper, being almost entirely testaceous. "That the species is identical, however, with the Madeiran one I have not the slightest doubt,—the sculpture and colour, as I conceive, having merely undergone a change since the remote period of its isolation on a comparatively calcareous soil[43]." The Scarites abbreviatus, Koll., occupies the loftiest peaks of nearly all the Madeiran islands, and was probably once abundant over the entire ancient continent, whatsoever its limits may have been, of which the present group forms but an isolated part. "There are traces of it in the Canaries, from whence occasional specimens have been brought, and which, from the want of local data and of sufficient numbers to reason upon, have in their turn been severally regarded as distinct. The fact however is, that the species in question is an extremely variable one, assuming differences of size according to the altitude at which it lives, and differences of sculpture according to the circumstances of the spot on which it is isolated. That such is actually the case, a careful observation of the many minute changes which the insect has undergone in the various islands and altitudes of the Madeiran group will, I think, prove to a demonstration. For it is impossible to suppose that every rock contains its own species, that is to say, has had a separate creation expressly for itself,—a conclusion at which we must assuredly arrive, if small and even constant differences are of necessity specific. Rejecting therefore this hypothesis as utterly untenable, and as contrary to all experience, we are driven to acknowledge that isolation does, in nearly every instance, in the course of time, affect, more or less sensibly, external insect form;—which being admitted, we have at once an intelligible principle whereby to account for modifications innumerable, each of which, when viewed simply as a difference, independently of the circumstances producing it, might have been regarded as sufficient to erect a 'species' upon, had the desire for multiplying them overbalanced the love of truth[44]."
Such are a few of the circumstances, influences, and conditions, by which the outward aspect of the insect tribes is liable, within definite limits, to be more or less regulated: and it is impossible to view them with an unbiassed mind and not arrive at the conclusion, that physical agents generally have a very decided control over the external contour of these lower creatures. In selecting the examples which we have lately discussed, I have avoided as much as possible those startling instances of variation which distant quarters of the globe will readily supply, because there are vast numbers of our naturalists who will not acknowledge the validity of any evidence which would tend to amalgamate, in a broad sense, the species of the Old and New Worlds. I have therefore contented myself with such data as must fall within our common experience, feeling satisfied that if the principle be allowed in the one case, it cannot long be objected to in the other. There are few entomologists who would not recognize, in the abstract, a legitimate capacity for adaptation in every insect with which they have to do; yet I believe there are not many, who, if modifications were to be shown them as the fixed result of disturbances from without, would be prepared at once practically to accept them as such. The collectors of the present day are so prone to regard every permanent difference as a specific one, that a large proportion of them do not sufficiently realize, that well-marked races, or states, are no longer matters of hypothesis, but of fact; and that, therefore, a sensible amount of aberration should not only be conceded to the action of certain physical combinations and elements, but even anticipated and looked for. Such however ought not to be; and earnestly therefore would I advocate a greater latitude for geographical influences than has been hitherto admitted by many of us. Especially would I urge the necessity for a more careful study of insular phænomena, for I am convinced that a due allowance is seldom, if ever, made for the qualifying power of isolation, per se,—the most significant perhaps of all the conditions which we have attempted in the preceding pages to examine.
"Felix qui potuit rerum cognoscere causas" is a motto which the student of Nature should keep constantly in view; for it is undoubtedly a more honourable task to discover the reasons for what we see, than the mere appearances themselves. He who has dived deeply into the everyday circumstances around him will be reluctant to ascribe so much as a single item of all that comes within his ken, to chance; for to him the whole system of created things is, from first to last, replete with design. Natura nil agit sine causâ is as true now as it ever was, and it will be so to the end. Let us not therefore be discouraged at the apparent smallness of the data from which many of our conclusions have to be drawn, for nothing is in reality trivial which is the effect of a wisely appointed law; and, even were such the case, it would not be thereby proved that the investigation of the law itself (however liable it may be to exceptions) is unimportant. Nor ought we, on the other hand, to be discouraged if we cannot always reconcile conflicting phænomena, and detect in each a primary controlling cause. We should rather bear in mind, that the elements with which we have to deal are obscure, and subject to permutations from which various results must of necessity arise; and that it is only, therefore, on a broad scale that we can look for uniformity of action, even from conditions which may appear to be identical. "Nature is not irregular, or without method, because there are some seeming deviations from the common rule. These are generally the effects of that influence which free agents, and various circumstances, have upon natural productions[45]."