Other instances might be cited, in support of the immediate principle for which we are now contending,—namely, that many areas have (from local circumstances) a natural superiority over others for the increase of the animal tribes, even apart from the direct action of heat and cold:—but space will only permit me to glance at a very few of them. We may detect evidences of this fact, in Ireland; which, in spite of the narrowness of the straits which separate it from our own country, and of its independent commerce with all parts of the civilized world, has an insect fauna curiously limited. From what cause this may arise,—whether from some obscure physical influences peculiar to the soil, or (as Professor E. Forbes has suggested) from the sudden impediment which the establishment of St. George's Channel presented to the westward progress of the various species from the Germanic plains,—it is difficult to speculate: yet the fact of its poverty remains, and we must explain it as best we are able. There can be no question, that, from more frequent communication with England, its entomological fauna has of late years been considerably increased; and it is equally easy to detect, through an examination of its less inhabited provinces, that at a period geologically recent its insect population must have been singularly scanty. I know of few regions (not even excepting the uplands of Madeira) which are more deficient in insect life than the mountains of Kerry. Although abounding, throughout extensive districts, with wood and water, and presenting every apparent requisite for its full development; the naturalist will often be disappointed by finding that a hard day's work has not ensured him the same amount of success as he would have reaped in less than half an hour in many an English meadow. Do we ask, why this is so?—it is impossible to reply, except on the supposition that there are real physical agents, independently of heat and cold, which are unfavourable in Ireland to the existence of these lower creatures. We may perhaps be told, by the advocates of Professor Forbes's theory, that it is the result of isolation,—the quondam land of passage having been broken up before the proper complement of species had reached this large portion of their western destination. But even this, although I believe it to contain much presumptive truth, will not altogether suffice to account for the phænomena which we see; for Ireland is not only remarkable for the paucity of its species, but also for the paucity of its individuals,—and the latter fact cannot be explained by any stretch of the migration-hypothesis. We are compelled therefore to conclude, that Ireland, like the other countries to which we have already alluded, presents conditions (altogether irrespective of latitude) which must be regarded as adverse to the general prosperity of the insect races.
And so it is with localities (no less than with larger countries),—many of which are eminently unproductive, when compared with others situated at but a short distance from them. Thus, the south-western corner of England is by far the most unprofitable portion of our island, unless indeed I am much mistaken, for insect ascendency. I have made some remarks on this subject in the 'Zoologist,'—from which I extract the following: "Unlike the easy collecting to which we are accustomed in the more favoured East, miles of unprofitable country have often to be gone over, be it swampy moorland or iron-bound coast, where scarcely an insect is to be seen; or, at any rate, where the few which exist are so ordinary, and so sparingly dispersed, as to be scarcely worth the labour of obtaining them,—more especially since the identical species are many of them to be met with in the utmost profusion in more central, or eastern districts. Whether it be the moisture of the climate, or the violence of the south-west winds, which (continually sweeping, as they do, over the high central mass of Devonshire and the bleak, barren downs of Cornwall) present as great an obstacle to the development of animal, as they clearly do of vegetable life, I will not venture to suggest; yet certain it is, from observation, that insects not only become fewer in number in proportion as they are exposed to these external agencies of wind and water; but likewise, in many instances, diminish so considerably in stature as to be scarcely reconcileable with their normal types[20]."
There can be no doubt that islands are, for the most part, more unproductive (even in proportion) than continents; and that, the smaller the area, the less favourable will it be for the development of insect life. Mr. Darwin has noticed this fact in the Galapagos (which he remarks are only equalled by Tierra del Fuego, in barrenness), on Keeling Island (in the Indian Ocean), where he succeeded in detecting but thirteen species, in St. Helena, and at Ascension; and I have added fresh evidence to the same in the various portions of the Madeiran Group[21]. It is however to geological causes that we must mainly look for the explanation of this phænomenon; and, therefore, since I propose to examine that branch of our subject in a future chapter of this treatise, we will not discuss it now. It will also be better perhaps to defer for the present the general question of self-diffusion, which, at the opening of this section, we proposed to consider, along with that of insect productiveness (as dependent on other local influences, besides climatal ones),—it being scarcely possible to render the problem of dispersion in any degree intelligible without calling in geology to our aid.
Having then disposed of this preliminary appendage to our inquiry, by expressing our belief (which I am satisfied that observation will tend more and more to corroborate) that certain countries and spots are by constitution more favourable than others for the increase (apart from the after dissemination) of the insect tribes,—and that too through local influences amongst which mere heat and cold are but secondary in importance; let us proceed to consider, how far the nature of the several districts may assist us in accounting for some of those numerous aberrations from the typical state which various insects are accustomed to display, and on which it has too often happened that "species" (so called) have been attempted to be established. I may premise however, that, whilst (as already urged) I would regard climate per se as subsidiary to many other agents, I would not wish to ignore its action altogether even under the present section, since in combination with peculiar circumstances and conditions it may have (and probably has) considerable controlling power: nevertheless I would desire it to be looked upon here as, at any rate, an inferior element, and as working in conjunction with physical influences of greater significance than itself. If therefore under the preceding heads it has been treated (so far at least as the exceptions would permit) as a great geographical principle, possessing a certain modifying quality on a large scale, let us now merely recognize it to the extent in which we are actually compelled to do, when dealing with areas of smaller magnitude,—namely as a topographical one.
From amongst the many results which I have been long accustomed to associate (whether rightly so, or not, I leave it for others to decide) with certain special situations, I would draw attention to the singular inconstancy which numerous insects are liable to when existing on the coast,—and which frequently causes them to assume an aspect so permanently different from their inland types, that, without local knowledge to guide us, they might be supposed at first sight to be specifically distinct. Ten years ago I offered a few comments on this fact in the pages of the 'Zoologist'; which, as I have seen no reason subsequently to modify them, I will transcribe at length:—
"The extraordinary changes which many insects are subject to when occurring near the sea, is a fact worthy of notice, and one which I do not remember to have seen recorded. The strictly maritime species must be left out of the question; for although many of them are exceedingly variable both in size and colour, still we have no means of ascertaining whether that variation is referable to the locality in which they are placed,—for, never being found inland, nobody can have an opportunity of asserting that the same changes would not take place, were they to occur in positions far removed from the influence of the sea. When we find, however, the same insects in profusion both inland and on the coast, and observe also numerous and marked deviations from the typical forms peculiar to the latter situation; then, à priori, we have strong presumptive evidence that the changes in question are the result of local circumstances, and not referable to chance. The alteration in size I have almost always observed to be from large to small, and scarcely ever the reverse; whereas in colour the change takes place very nearly as much from light to dark as it does from dark to light: nevertheless the majority of instances I possess come under the latter department. It has been remarked that all the specimens of Mesites Tardii, which I captured in Devonshire, were much smaller than the original series taken by Mr. Tardy at Powerscourt Waterfall, in the county of Wicklow; and so decided was the difference, that many of my friends, at first sight, concluded the two to be distinct species. This, however, I consider entirely owing to their locality, for my specimens were found only on the coast, and Mr. Tardy's at a considerable distance inland. And, inasmuch as neither of these instances rested on mere individual examples, but on long and conspicuous series, the certainty of the change from large to small was the more apparent. Mr. Holme of Oxford mentions having taken Olisthopus rotundatus in the Scilly Islands, in great profusion, none of the specimens of which exceeded two lines and a half in length. At Whitsand Bay in Cornwall I have captured Gymnaëtron Campanulæ, none of which exceeded three-quarters of a line,—the usual length being from a line to a line and three-quarters. Anthonomus ater, the average length of which is two lines, I have taken a series of in Lundy Island, none of which exceeded one. In the same locality, also, the common Ceutorhynchus contractus scarcely ever reaches its natural size; and is, moreover, so variable in colour, that I was long before I could persuade myself that the species was not distinct. Instead of the bluish-black elytra which I had always considered invariable, they all possess a yellowish or brassy tinge; and the legs, instead of being black, are in most instances entirely of a light yellow,—and in all, more or less inclined to that colour. I have received from Mr. Hardy, of Gateshead, specimens of Haltica rufipes[22], captured by him on the coast, in which the entire insect is of a uniform brownish-red hue. Of the rare Mantura Chrysanthemi I have taken beautiful varieties at Mount Edgcumbe and in Lundy Island,—many of which inclined to a rich metallic-yellow, instead of the brassy-brown of the ordinary specimens: also, in the latter locality, particularly dark specimens of Telephorus testaceus. In like manner, I might enumerate other species equally remarkable; but I trust that those already mentioned are sufficient to verify my observations, of the extreme liability to change which, more or less, most insects possess when placed within the immediate influence of the sea. How to account for it, I know not. I mention it as a mere fact, and leave it for others to assign a reason for its existence[23]."
Apparently dependent, in a large measure, on the same circumstance (namely proximity to the coast), the Bembidium saxatile, Gyll., so common at the edges of the mountain streams in the north of England, in Scotland, and throughout a portion of Ireland, presents itself along our southern shores in the form of a permanent variety; being, as the Rev. J. F. Dawson remarks, "more depressed, never narrower in front (the sides therefore more parallel), whilst the colour is always much paler and the spots larger,—that before the apex being round and very conspicuous, and the anterior one occasionally expanding over the surface very considerably[24]." I have taken it in profusion on the coasts of the Isle of Wight, Dorsetshire, and Devon. And so with the Cistela sulphurea, Linn., which in certain maritime localities (as I have particularly noticed on the sand-hills at Deal) is liable to become so dark in colouring, that, without the intermediate shades to judge from (which however may usually be obtained in situ), it might stand a fair chance, occasionally, of being mistaken for a separate species. A Psylliodes in Lundy Island, allied to (if not identical with) the chrysocephala, Linn., found in abundance on a Brassica along the ascent from the eastern landing-place, varies "in every consecutive shade between the limits of light yellow and dark metallic-green[25]," the former of which states (the normal one on that rock) might have been fairly set down as specifically distinct from the latter, did not observation on the spot decide the question for us without doubt.