Russel Wallace : Alfred Russell Wallace (sic)
(S96 & S98: 1864)
[[p. 14]] The President exhibited several new Australian Longicornia, presented to him by Mr. F. G. Waterhouse, of Adelaide, some of which were taken by that gentleman during the recent exploring expedition under Stuart; among the more interesting was a new genus, in some respects connecting those remarkable and isolated forms Bimia and Hesthesis. Also a considerable number of new species from Gawler, near Adelaide, part of a large collection for which he was indebted to Mrs. Kruesler and Mr. Odewahn; one of these had the appearance of a Malacoderm, for which, at the first glance, it might very readily be taken. It might, in fact, be regarded as another case of "mimetic resemblance"; but if so, there was this peculiarity about it, that the stronger insect was here imitating the weaker, which seemed not quite consistent with the theory that the imitated form was copied with a view to the protection of the imitating.
Mr. A. R. Wallace said that the case was quite consistent with the theory of mimetic resemblances, and that that theory did not depend upon the relative strength or weakness of the imitated or imitating forms; an insect might be very weak in structure, and yet be a proper subject for mimicry; many insects of weak structure were extremely abundant, were, in fact, dominant species; such species no doubt possessed some protection against their enemies with which we were unacquainted, [[p. 15]] and of which other species of stronger structure were deprived; and having that protection, whatever it might be, they were on that account fit and likely subjects for imitation. So far from conflicting with the theory, he felt no surprise whatever at finding a structurally-stronger insect mimicking a Malacoderm.
The President also exhibited diagrams of the under surface of the abdomen of the females of Obrium cantharinum and other Longicornia, showing the presence of large lumps or tufts of hair on particular segments; he was anxious to ascertain whether such a structure was invariably present in the female; he had reason to believe that in some Australian genera (Penthea and Symphyletes) it was not invariably present, but further observation was required. If it should prove that it was not invariably present in the female, this would furnish another instance of "dimorphism," by which he (Mr. Pascoe) meant a case where there was an addition to or alteration of a part or organ; the term, in his opinion, ought to be confined to such a phenomenon, and was not applicable to simple variation, such, e.g., as variation in colour.
Mr. A. R. Wallace exhibited specimens of Papilio Memnon, P. Pammon, P. Theseus, P. Ormenus, P. Erectheus and P. Tydeus, for the purpose of illustrating his views on "polymorphism." Mr. Wallace remarked that he did not apply the term "dimorphism" to simple variation; on the contrary, he regarded "polymorphism," of which "dimorphism" was but the first and least complicated stage, as a totally different phenomenon from "variation," and one which required a separate name. Under the common term "variety" many distinct phenomena were confounded; he proposed to confine that term to those cases in which there was indefiniteness and irregularity in the variation. "Variation," then, was an indefinite and inconstant phenomenon, the instances of which passed from one to the other by irregular, often by insensible, gradations; "polymorphism," on the other hand, consisted in the existence of several distinct forms of the same insect which do not graduate into each other. Thus, in Papilio Memnon, the male was in each locality constant; it had rounded hind wings, and was always nearly black, with a few ashy rays; the female, however, existed under two distinct forms; the first had the wings shaped like those of the male, but had a very different colouration, being more or less olive-coloured, and often banded on the hind wings with whitish yellow, and with marginal black spots; the second form of female differed remarkably from the first, the hind wings being produced into a large spatulate tail, and marked with white patches radiating from the base. Both these forms exhibited varieties in the same locality, but there were no connecting links between them. The males paired with both forms of female, and in each case the resulting brood assumed the distinct forms above described. This was a case of dimorphism. Papilio Pammon was a parallel case; there was a form of female resembling the male, and there was as a second aberrant form of female, viz. the form which had been called P. Polytes: he believed that P. Romulus would prove to be a third form of the female of P. Pammon, though of this he had not evidence at present; but the male of P. Romulus had never been found, whilst the females occurred along with P. Pammon in every collection from India: if that were so, then P. Pammon would exhibit an instance of trimorphism. P. Ormenus was certainly trimorphic, for three distinct forms of female were found, all differing greatly from the male; and Mr. Wallace had reason to believe that to some males of Papilio as many as four distinct wives must be assigned. These forms had hitherto been classed as varieties, but the physiological differences presented by them were striking.
Mr. Wallace further distinguished between a "variety" and a "local form" or [[p. 16]] "race," denoting by the latter term a form which kept distinct from the parent stock and propagated itself independently: such a form was not a "variety" by reason of its constancy and the absence of intermediate links, and could not be considered a "species" by reason of the comparative unimportance of the characters in which it differed by the type-form. The subject of Mr. Wallace's remarks had been treated by him at length in a paper recently read before the Linnean Society: he further illustrated the phenomenon of polymorphism by the following hypothetical case:--Imagine the discovery of an island inhabited by white men, and black (negro), red (Indian), and yellow (Chinese) women, and in which the union of these variously-coloured parents produces children which always resemble one or other of the four above-mentioned forms, no intermediate forms ever occurring; the boys are always white, whilst the girls are black, red or yellow, but without any necessary connexion with the colour of the mother, so that a black girl may be the offspring of a white father and of either a black, red or yellow mother. Such a phenomenon would certainly be an extraordinary one, but it was exactly parallel to what took place with the exhibited species of Papilio.
Gen. Sir John Hearsey remarked that so long ago as 1831 he had captured Papilio Pammon and P. Polytes in copulâ, had killed the pair in situ, and so sent them to Prof. Westwood; in which position they were to be seen at Oxford to this day.
Mr. Waterhouse enquired whether there was any evidence of the fertility of the abnormal females, since on theoretical grounds it might almost be expected that they would be unproductive.
Mr. Wallace replied that he believed one of the abnormal females then exhibited could be seen to be full of eggs.
Prof. Westwood exhibited a butterfly recently received at the Oxford Museum from M. Snellen van Vollenhoven; the specimen was from Ceram, and it appeared to him to differ from Papilio Peranthus only in its large size and to some extent in its colouring; he found no structural difference whatever, and he begged to exhibit it as P. Peranthus, var.; it seemed to him to be precisely one of Mr. Wallace's "local forms," but Mr. Wallace informed him that he should describe it as a new species.
Mr. A. R. Wallace said that if structural difference was necessary to constitute a species, two-thirds, or probably nine-tenths, of the existing species must be abolished; the difference between a local form and a species was one of degree only, and in his opinion the butterfly exhibited possessed characters sufficient to entitle it to species rank.
[[p. 21]] Mr. A. R. Wallace exhibited various species of Papilio, Eronia and Pieris, with the intention of showing the effect of locality in producing change of form in insects. Numerous species inhabiting the island of Celebes were produced, and in each case in juxta-position therewith was its nearest ally from the adjoining islands. The Celebes insects, in every instance exhibited, had the costa of the anterior wings much more strongly arched than was the case with their congeners with which they were compared. Mr. Wallace remarked that changes in colour which were due to locality had been frequently noticed, but of instances of a change of form only few had been recorded; Mr. Bates, however, had mentioned some as having come under his observation in South America. In the Eastern Archipelago, he (Mr. W.) had found that the butterflies inhabiting the islands which formed the eastern half were generally larger than those in the western half; and, especially in the Papilionidæ, that Celebes and Amboyna produced the largest specimens. Some species which in India were found with a large tail appended to their hind wings, possessed only a small tail in the Indian Islands, which ceased altogether or was reduced to a mere tooth in the islands of the Pacific; of this Papilio Agamemnon was an example. The island of Celebes was as nearly as might be the centre of the Eastern Archipelago, and the butterflies of that island, especially the Papiliones, possessed the peculiarity in the form of their wings to which he had invited attention; sixteen or seventeen species of Papilio were found in Celebes, and, with a single exception to be afterwards noticed, all were distinguished from their allies in the other islands by having the upper wings strongly arched, in lieu of a gentle and gradual curve, or, if he might so term it, a flat bend. The same distinction existed also in many Pieridæ and in a few Nymphalidæ, but he had not observed it in other groups of butterflies. Mr. Wallace had a theoretical explanation to offer of this phenomenon; he conceived that the insects had become modified in form by the eternal circumstances to which they had been subjected, and that this modification was to be accounted for by some physical or organic change which had occurred in Celebes, but not in the now adjacent isles. Rapidity of flight was generally supposed to be the consequence or a property of the falcate form of wing; he (Mr. W.) was inclined to think that the falcate form gave greater facility in twisting or turning about; if that were so, the Celebes form of butterfly-wing would give the insect this additional facility, and thus enable it more easily to escape from its enemies. If, then, the Celebes insects were supposed to have been formerly subject to great persecution, those with the arched form of wing would have the best chance of escaping, the less favoured forms would be gradually killed off, those that survived would owe their existence to the form of their wings, their offspring would resemble and some few would exceed them in the possession of the advantageous shape, and the specimens with the best developed wings being naturally selected in each succeeding generation would lead to the gradual and regular increase of the peculiarity. But then it might be asked, How is it that only a few of the butterflies have the peculiarity in question? [[p. 22]] The answer was, that different insects preserve their existence by different means; one may escape by means of its power of flight, the habits of a second may enable it to avoid its foe, a third may owe safety to its colour, whilst a fourth may be positively distasteful to animals which prey upon insects. Thus the Danaidæ, though slow of flight, were very abundant and were the subjects of mimicry or imitation by other species; in all probability they owed their immunity from destruction to the strong and unpleasant odour which they indubitably possessed. It was manifest that where another means of escape already existed, the principle of selection would not be brought into play; the peculiar form of wing would not be wanted, and therefore would not be acquired. The obscure Satyridæ were doubtless protected by their colour; the majority of the Nymphalidæ had already sufficient power of flight, and those of that group which had acquired the arched form of wing were precisely the species which possessed only about the same power of flight as a Papilio. The single Celebesean Papilio which had not assumed the arcuate form of wing was one belonging to the Polydorus group, a group which was itself imitated by other Papiliones, and which therefore was doubtless provided with some special defence, though the nature of it was unknown to us.
Prof. Westwood, after remarking upon the pleasure he always derived from Mr. Wallace's speculations, whether he agreed with them or not, said that he was unable to follow Mr. Wallace in tracing the phenomenon to the causes assigned by him; arched wings were not necessarily, or even generally, accompanied by the greatest rapidity of flight; and if the original form of the butterfly must be varied, if one species must imitate another, he (Prof. Westwood) thought it far more probable that the variation would consist in getting stronger muscles to their bodies, as in the species of Charaxes he had that evening exhibited, rather than in a minute change in the curve of the wing.
Captain Cox thought that the swiftest fliers had the straightest wings, as, e.g., the Sphinges, where the costa was scarcely arched at all.
Mr. Newman also was unable to connect an arcuate wing with rapidity of flight, and instanced the straight costæ of the swiftly-flying Diptera.
Mr. F. Smith referred to some of the swiftest Hymenoptera, which had pointed wings, but with perfectly straight costæ.
Mr. Baly said that the arched form of wing might be advantageous in giving a greater power of twisting and turning about, rather than in giving greater rapidity of flight; and this, he thought, was what Mr. Wallace suggested.
The President observed that the theory seemed to be based upon the supposition of a gigantic persecution to which the butterflies had been subject; but was there any evidence of the existence of such persecution? had Mr. Wallace ever seen the insects actually pursued by birds or other enemies? It seemed to him that persecution, to the extent to which it now occurred, was altogether inadequate to account for the magnitude of the result attributed to it.
Mr. Bates said that he had frequently observed in South America that the paths were strewn with the wings of butterflies; and in this country it was not an uncommon sight to see the Pontiæ pursued by birds, and sometimes escaping by means of their tortuous or "dodging" flight.
Captain Cox had recently seen forty or fifty specimens of Brephos notha destroyed by tom-tits in a single morning.
Mr. F. Smith remarked that, on the persecution principle, great change must be impending in the shape of wing of the butterflies of the London district.
[[p. 23]] Prof. Westwood asked whether Mr. Wallace had observed the twisting or "dodging" flight to be conspicuous in the butterflies which have largely-developed wing-tails? or whether the tails contributed to or affected the flight in any, and, if any, in what manner?
Mr. Wallace replied that he had not observed that the wing-tails had any influence upon the flight of the insect; he did not think they affected the rapidity, or that they acted as a rudder, and he was at a loss to assign any use to those appendages. With respect to the other points which had been touched upon, there was in some cases an increase of muscular power as well as an increase of curvature of the wings, but the wing of a butterfly was far more liable to variation than the body of the insect; that organ was therefore more accessible to the operation of the principle of selection, and a modification of its form was consequently more readily produced. Most of the instances of swift-fliers with straight wings which had been adduced (as e.g. the Sphinges) were insects with the strongest bodies and the greatest muscular power; moreover, he did not maintain that any insect with arched wings would have a more rapid flight than any other with straight wings; all that he had argued was, that, as between two closely allied species, the one which had the wings most arched would have the flight most rapid; for instance, he should expect a Sphinx with arched wings to fly more rapidly than a Sphinx with straight wings. But in truth, though he had referred to the supposed concomitancy of falcate wings and a rapid flight, his own notion was (as had been correctly stated by Mr. Baly) that the arched form was chiefly useful in giving greater power of turning or twisting. As to the enquiry whether he had seen the butterflies actually pursued, he admitted that he had not to any great extent; he offered his explanation as a theoretical one; but he thought it could scarcely be doubted that butterflies were subject to the attacks of numerous depredators.