The Zoologist/4th series, vol 4 (1900)/Issue 714/Conscious Protective Resemblance, Marshall/RemarksPoulton
REMARKS ON THE PRECEDING PAPER.
By Edward B. Poulton, M.A., F.R.S.
(Fellow of Jesus College, Oxford; Hope Professor of Zoology in the University.)
My friend Mr. Marshall has asked me to make any alterations or corrections in his paper. I find, however, that I so entirely agree with the whole of the argument that I have merely added a few confirmatory notes to certain passages in the paper, which are in each case marked by a number.
1 H.W. Bates, in his classical paper, also used the term mimicry in the wider sense employed by W.L. Distant. The majority of naturalists have since followed A.R. Wallace in keeping Protective and Aggressive Resemblance distinct from Mimicry—a course which appears to be convenient, inasmuch as the distinction in terms corresponds to a real distinction in the modes of defence. In the former, an animal resembles an object which is of no interest to its enemy, and in so doing becomes concealed; in the latter, an animal resembles an object which its enemy knows well and fears or dislikes, and in so doing becomes conspicuous. Other superficial resemblances—such as those produced by protective resemblances in common, warning colours in common (Müllerian mimicry), and functions in common (analogical or adaptive resemblances of Darwin)—are excluded from mimicry as here defined.
2 See the discussion on "Organic Selection," reported in 'Science,' N.S. vol. vi. No. 146, Oct. 15, 1897, where this view was sustained.
3 Probably most evolutionists would hesitate before committing themselves to such a conclusion. Highly intelligent animals, such as birds, crouch and hide when very young at every unusual sound. This action is performed instinctively and unintelligently, and is apparently an automatic response to stimulus. When the stimulus has been repeated, and no danger is apparent, the young birds cease to crouch. We are not justified in considering that their intelligence has done more than enable them to inhibit an unnecessary response. There is no reason to think that they have any understanding of the meaning of the response itself. See Lloyd Morgan's 'Habit and Instinct' (London, 1896).
4 It should be remembered that the structure and colouring are themselves made up of many complex factors, all of which must co-operate if the mimetic or protective resemblance is to be effective. See Linnean Soc. Journ., Zool., vol. xxvi. pp. 576–578.
5 That is to say, where the high cerebral development exists which would, according to W.L. Distant, tend to produce mimicry and protective resemblances, precisely there these adaptations are lowly developed as compared with Insecta, where we meet with far less intelligence and far more of the unvarying repetitions of pure instinct, incapable of improvement by learning, and, within their rigid limits, too perfect to require it. Where the conditions are most favourable for "active mimicry," mimetic and cryptic adaptations are least prominent; where they are least favourable, these adaptations become most conspicuous.
6 So far as I have been able to collect evidence, Kallima does not rest on dry and withered leaves, but in situations, such as trunks and branches, in which dead leaves would not attract attention. H.J. Elwes has stated that it freely expands its wings when settled, and looks anything but leaf-like; but this is probably when it is thoroughly on the alert, during the short pauses between successive flights. C. Swinhoe has informed me that it invariably rests head downwards, like a dead leaf hanging by its stalk, so that all the figures and preparations seen in this country representing its natural attitude are wrong.[1]
It is quite impossible to explain the protective attitude of this or any other insect on the principle of "active mimicry," unless we are going arbitrarily to assume that certain defensive activities are to be explained in this way, while others, equally necessary and equally elaborate, cannot be thus interpreted. Consider, for instance, the concealment often brought by the cocoon—the selection of an appropriate situation, the building into the walls of a part of the surrounding surface, &c, &c. Upon the principle of "active mimicry," "the view would be, I suppose, that the ancestral larva spun a cocoon which was not much of a success, and was in consequence attacked by enemies; that the larva observed these attacks, and accordingly improved its cocoon. But that is not the way in which the struggle for existence is waged with insects. If the larva failed, it failed, and that would be the end of the matter. It has no chance of improvement; it has no opportunity of learning by experience. Its only chance of survival is to avoid experience of foes altogether; experience is the most dangerous thing in the world for an edible insect. This becomes still more obvious when we remember that failure or success is almost always determined long after the cocoon is made. The caterpillar, perhaps, spins the cocoon in autumn, but the real stress of competition will come in winter, when insect-eating animals are pressed hard with hunger, and search high and low for food. But the caterpillar is by this time a chrysalis, and of course has no opportunity of improving the cocoon. The selective test is applied long after the operation has been performed, and when there is no possibility of gaining by experience. We are thrown back, then, solely upon natural selection, which acts on the nervous system of the caterpillar, and thus compels it to make the cocoon in a certain way. In other words, those caterpillars which are impelled by their nervous system to make ill-formed conspicuous cocoons have no chance of living, and, in future stages, producing offspring. Hence the selection caused by the keen sight of foes first raises, and then maintains at a high level, the standard of cocoon-making."
"This contention as to the uselessness and danger of experience applies to the whole of those smaller defenceless animals which have no chance of fighting with their enemies, or of escaping when once they have been detected" ('Proc. Boston Soc. Nat. Hist.' vol. xxvi. p. 391). It would be a most gratuitous indulgence in unnecessary hypothesis to insist that the appropriate attitude which gives a meaning to form and colour, and itself receives a meaning from these, originated in one way in the caterpillar, and in another and totally different way in the imago which develops from it.
7 See note (3).
8 The observation does not prove more than that the fox seeks cover and hides when he sees that he is observed by man. The burnt surface did not afford cover, and the fox sought it elsewhere. It would be very rash to assume from the observation that the fox knew anything about his own protective colouring.
9 Or the numberless examples of insects which fall motionless when their food-plant is shaken.
10 There are many reasons for considering that colours and patterns change very rapidly when no longer sustained by natural selection. When animals become cave-dwellers, or inhabit the greatest depths of the ocean, their colours are profoundly modified and often tend to disappear. This happens in forms closely allied to others which still retain the normal colouring and live in the light.
The majority of domestic animals have been immensely modified in this respect in a measurable number of years. In some cases these changes have been brought about without the aid of specially directed artificial selection. Thus a large proportion of our fowls produce white eggs instead of the brown of the ancestral species.
Again, the enormous difference between the colours and patterns of certain closely-allied species is evidence for ease and rapidity of change rather than stability in this element of structure. The argument becomes stronger when we consider the cases of sexual and seasonal, and other di- or poly-morphism in the different individuals of the same species. A single instance will make this clear. There are certain genera of butterflies, such as Dismorphia (in the wide sense), Pseudacræa, and Hypolimnas (also in the wide sense), of which almost the whole of the numerous species are mimetic. Within the limits of each genus the most divergent models have been followed, so that utterly different colours and patterns have been produced in forms which are still closely related, and in other structural features exhibit no corresponding differences. In the most extreme case known to me, immense differences occur in the different races of a form which systematists consider as a single species, viz. Hypolimnas bolina. If we compare the Indian form of female with those of the Malayan region, Australia, and Polynesia, including Fiji (in which the local race itself contains the most widely divergent forms), and remember that no corresponding differences exist which would justify us in conferring specific rank in any of the cases, we are driven to the conclusion that colour and pattern are the most superficial of all specific characters,—of all the least likely to persist unchanged when the models upon which they were founded have long since disappeared.
In one special case which I have observed, there is evidence that changes in the nervous system have outlasted the markings which once gave a meaning to them. Some of the remarkable larvæ of the genus Ophideres have two eye-spots at the junction of the anterior and middle third of the body. They have the instinct of bending the anterior third so that it rests under the middle one, and thus the eye-spots are brought into an appropriate position apparently at the anterior end of a somewhat snake-like body. But a caterpillar of this genus which I found in Teneriffe assumed the attitude, on irritation, although the eye-spots were almost completely wanting.
11 It is worth considering whether the Müllerian principle may have been operative in this case.
12 Of course, no natural selectionist has ever been so unreasonable as to contend for absolute protection. In every species, whether defended by the most distasteful or dangerous qualities, or the most effective concealment, no more can be achieved than to keep up the average numbers under average conditions, and this means that an immense majority of individuals are doomed to failure. As regards concealment, success merely means that enemies have so far to work for their living that in the time at their disposal they cannot do more than reduce the number of individuals to the average. Warning colours and unpalatable or otherwise unpleasant qualities are more complex as a means of defence, depending as they do for their success upon the co-existence of other more desirable food. Their operation, under favourable circumstances, is probably to reduce the number of enemies, this success being compensated, however, by the more persistent attacks of certain special enemies—the result being the same as in the cryptic colouring, namely, to keep up the average number of individuals.
13 Darwin remarks on the sound made by this species ('Voyage of the Beagle'), which he witnessed during his travels in South America. He believed that the sound was of sexual significance, and in his essay on sexual selection compared it to that made by the males of Halias prasinana during courtship—a sound which I have myself once heard. The display or exercise of secondary sexual characters is probably often a danger to the individual, although I fail to see how it is possible to argue from this that the cryptic colouring and attitudes of other phases of life are thereby rendered inoperative and valueless. The sound-producing time is one of high activity and rapid movement in both the species of Lepidoptera mentioned; in the case of the common English moth it is indulged in so rarely, that comparatively few naturalists have ever heard it, while in Ageronia it is not likely to be produced during more than a very small proportion of the life of the male. As to its cryptic colouring and, of even more importance, the corresponding instinctive attitudes and movements, Darwin made special remark in the volume already mentioned.
14 1 have noticed the same thing in North America. Not only was the distance very difficult to estimate, but the direction from which the sound came equally hard to trace.
[In closing this discussion, which has now extended beyond the limited space of 'The Zoologist,' as writer of the incriminated "Suggestions," I ought perhaps to make some rejoinder. This is unnecessary to my friend Mr. Marshall's objections, as they principally express an ably stated difference of opinion, and I have merely added footnotes to make his quotations from my suggestions a little more ample and representative. Prof. Poulton, in forwarding his "Notes," with his usual fairness, wrote: "My remarks are more of a reinforcement of Marshall's arguments than a direct answer to your paper, which I have not seen. I expect, however, from Marshall's MS., that they do affect the drift of your argument, and are therefore in the nature of a reply." This statement of course disarms any rejoinder. Besides which a comparison of Poulton's notes to Marshall's opinions also discloses a diversity of view, though the first named states he entirely agrees with Mr. Marshall's argument. Thus Mr. Marshall writes (ante, p. 538), "It is possible no evolutionist would deny," and Prof. Poulton to this adds the note, "Probably most evolutionists would hesitate before committing themselves to such a conclusion." Again, they both differ as to the active mimicry of the Fox (cf. pp. 541, 552). A triangular discussion is therefore out of the question, and we may continue to differ in opinion and search together for facts.—Ed.]
- ↑ Cf. Eha, 'Natural Science,' vol. ix. p. 299.— Ed.