Conscious Protective Resemblance (Marshall 1900)
CONSCIOUS PROTECTIVE RESEMBLANCE.
By Guy A.K. Marshall, F.Z.S.
In the second portion of his "Biological Suggestions" (Zool. (1899) pp. 289, 341, 443, 529; (1900) p. 116), Mr. Distant has dealt at some length with the phenomena of animal colouration, generally described under the terms of Protective and Aggressive Resemblance. It is not altogether obvious why these phenomena should have been ranked by him under the term Mimicry. I am aware that this latter word, as first used by Kirby and Spence at the beginning of the century, included all cases of resemblance of what kind soever; but seeing that, with our increasing knowledge of the subject, students of animal colouration have found it both useful and advisable to discriminate between resemblance in order to attract attention (Mimicry) and resemblance in order to obtain concealment (Protective and Aggressive Resemblance), there seems to be no sufficient reason why we should revert to an earlier and less exact definition, which is only apt to cause confusion.(n1) (These numerals refer to some concluding notes by Prof. Poulton.) In his review of the matter, Mr. Distant has brought together a large number of interesting facts and observations bearing on the subject of general and special resemblance (a distinction, however, which he overlooks), containing examples from all classes of animal life. A consideration of these facts has led him to offer the suggestion that "animals of their own volition, and in their efforts to avoid their enemies, place themselves where possible in such adaptation to their surroundings, that protective resemblance and some forms of mimicry are due to animal intelligence, and not so entirely to what is generally understood as the unconscious process of natural selection" (l.c. 1899, p. 465). It is proposed to designate this conscious action by the somewhat unsatisfactory name of "active mimicry"; it seems doubtful whether any special name is really required for this process, but, if it be so, I would suggest that "conscious resemblance" is more suitable and more in conformity with the recognised terminology.
This "active mimicry" is apparently regarded by Mr. Distant as something apart from natural selection, a separate factor in evolution, for he says: "If the process of natural selection was to be applied, according to a very frequent method, as universal, then birds arising from these white and prominent eggs would seem in course of time to be doomed to destruction. But we find nothing of the kind. Natural selection is here replaced by the evolution of intelligence or active mimicry. True, it may be argued that birds laying white eggs would become extinct without they had gradually acquired the intelligent or automatic powers of concealment through a process of natural selection. But this is only begging the question" (l.c., 1899, p. 546). (The italics are my own.) Seeing that this attitude permeates the whole discussion, it is somewhat disconcerting to read in the concluding remarks that, "to fully understand mimicry, we must appreciate general animal intelligence, and then we shall probably comprehend how much activity has been displayed by animals seeking protection by adaptive and assimilative efforts. This in no way contradicts, but supports, the doctrine of Natural Selection. The animal survives which can best hide from its enemies, and this implies that the variations which tend to adaptive and assimilative efforts, not only succeed in the battle of life, but by the selective process become dominant, and more and more accentuated with a greater need" (l.c., 1900, p. 124). It is scarcely necessary to point out that the latter position, which is essentially that of those very selectionists(n2) whose views Mr. Distant is combatting, is quite at variance with the former. It will therefore be necessary, for the purpose of this discussion, to neglect this remarkable contradiction.
The whole question of conscious resemblance must necessarily depend upon our ideas of animal intelligence, and in the present state of our knowledge these are unavoidably hazy and obscure. It must be that our conception of mind, even in our fellow men, is based entirely on analogy, and thus the further we depart from the human type, the lower we go in the organic scale, the weaker and weaker must that analogy become, and the more careful must we be to avoid the conception that any apparently purposive actions we may observe in these lower organisms must be due to trains of reasoning such as we find in ourselves. The whole subject is, at present, merely hypothetical; but, on the other hand, we must not forget that even our most definite scientific facts are only very high probabilities.
It is probable that no evolutionist would deny that there must be a certain measure of truth in the contention that some animals are capable of appreciating the protective value of their colouring;(n3) for, apart from observational evidence on the point, we should antecedently expect a certain amount of reasoning power in this direction according to the ordinary principles of evolution. The question is, however, whether, in the suggestions under consideration, this power has not been considerably over-estimated. An examination of the examples referred to "active mimicry" would certainly lead to this conclusion, for the arguments used in these instances are equally applicable to every case of protective or aggressive resemblance. There would be little difficulty in demonstrating the untenability of such a position, but this is unnecessary, as we are expressly warned that the suggestion of active mimicry must not be made too absolute, although no suggestion is offered as to its probable limits.
It may, perhaps, be possible to define roughly certain limits within which such consciousness cannot be recognised. Resemblances have been aptly divided by Prof. Poulton into two categories, viz.: "Special Resemblance, in which the appearance of a particular object is copied in shape and outline as well as in colour; and General Resemblance, in which the general effects of surrounding colours are reproduced" ('Colours of Animals,' p. 24); and in connection with this distinction it is interesting to note that in the most intelligent section of the animal kingdom, namely, the higher vertebrates, we find little but general resemblances, and the lower we go in the scale of intelligence, the more frequently do we observe special resemblances, that is, where colour is utilised for protective purposes.(n5) But, quite apart from this, it is evident that it is practically impossible to include cases of special resemblance under the term "active mimicry," as here discussed. For instance, in those wonderful cases which are found so frequently among insects, the habits of each species are so intimately correlated with its abnormal structure and colouring,(n4) that it is unreasonable to believe that these characters have been developed independently by different factors; the latter by natural selection, and the former by the "evolution of active mimicry," whatever that may mean. These special structures cannot be accounted for by "active mimicry," neither can they be explained by any general theory of internal or external causes, for, as the late Mr. Romanes has well remarked, "Were it not that some of Darwin's critics have overlooked the very point wherein the great value of protective colouring as evidence of natural selection consists, it would be needless to observe that it does so in the minuteness of the protective resemblance which in so many cases is presented. Of course, where the resemblance is only very general, the phenomena might be ascribed to mere coincidence, of which the instincts of the animal have taken advantage. But in the measure that the resemblance becomes minutely detailed, the supposition of mere coincidence is excluded, and the agency of some specially adaptive cause demonstrated" ('Darwin and after Darwin,' p. 318, note).
Thus a strong objection may be lodged against the whole suggestion of active mimicry, as opposed to that of natural selection, in that the former suggestion is essentially incomplete and cannot explain all the facts of the case. Let us take the instance of the leaf-butterflies of the genus Kallima, of which Mr. Distant says: "The partiality of this insect for settling on dry and withered leaves appears a true instance of active mimicry " (l.c., 1899, p. 531).(n6) Upon the theory of natural selection (granted the undisputed facts of variation and the struggle for existence), it is easy to understand that any marked variations in the direction of leaf-like shapes or markings, which would afford better concealment, would tend to be preserved and further augmented, both by heredity and by the increased keenness of enemies, until the present admirable resemblance had been arrived at. "But, as Mr. Badenoch has well enquired, 'Of what avail would be the disguise were the insect prone to settle upon a flower, or green leaf, or other inappropriate surface?'"(l.c.). Quite true; and the fact that the insect is not so inclined is readily explainable by the Darwinian theory; for it is clear that a much greater proportion of those individuals which were prone to render themselves conspicuous by settling on inappropriate surfaces would be picked off by their enemies than of those which selected suitable resting places; and thus, by a gradual process of elimination, the progeny of those individuals, which possessed a well-defined instinct to settle upon withered leaves, &c, would eventually supersede those whose instincts were not so well in harmony with their colouration. On the other hand, for the suggestion of "active mimicry," it is contended that the actions of these insects are apparently so purposive that it is difficult to believe that they are not due to "conscious volition" on their part; and, in support of this contention, a large number of other similar cases are adduced, all, be it noted, equally, or more fully, I consider, explicable on the theory of natural selection. But when we stop to enquire why, or how, these butterflies have developed this peculiar colouration, the supporters of the suggestion of "active mimicry" can vouchsafe us no reply. According to this suggestion, the tiger selects the bamboo-thicket, the leopard the leafy forest, and the lion the open veldt, simply because they have individually discovered, by their own reasoning powers, that these respective localities are best suited to their particular styles of colouration; and the question why one is striped, another spotted, and the third unicolourous, reverts to an open problem. Thus all the beautiful explanations of adaptive colouring, afforded us by Darwin's grand conception, are to be thrown to the winds if "active mimicry" be logically applied.
It will be thus seen that it is only among the most generalised types of resemblance that we may seek for signs of conscious adaptation, as opposed to quasi-mechanical instincts. But even here the foregoing objection also applies, though with less force, since the contention of coincidence may be put forward in some cases, as indicated by Mr. Romanes. But it must be borne in mind that this contention is nothing but an argument from ignorance, and, as such, is not scientifically permissible where any other reasonable and adequate explanation can be advanced. The mere citation of a number of instances of protective colouring, however purposive the actions of the animals may appear, are in themselves no proof of conscious resemblance; neither do they in any way weaken the theory of natural selection in this regard; for this theory not only consistently explains the reasons for, and the development of, the colouration, but also accounts for that very purposiveness upon the occurrence of which the former proposition is alone based. Again, in the case of special resemblances, if it be conceded, as a result of the arguments adduced above, that both the morphological and psychological characters have been contemporaneously perfected through the mechanical action of natural selection (and in fact the structural peculiarities cannot well be explained on this principle without the instincts), then this alone would form strong presumptive evidence that, at least, the great majority of cases of general resemblance are due to the same factor. For it is evident that all cases of special resemblance must, at some time or other, have passed through a general phase, and therefore we must necessarily apply the same explanation in both categories.
Nevertheless, while the orthodox Darwinist may maintain that protective colouration, together with the appropriate instincts which are necessary to render it of any use, have been ultimately developed through natural selection (save, perhaps, in a very few exceptional cases),(n7) yet it is competent for him, without any contradiction, to admit that probably some few of the most intelligent animals may, in the course of their mental evolution, have arrived at such a standard as to be able to appreciate the value of their own protective actions, which were Originally merely instinctive—a very different position, however, from that suggested by Mr. Distant.
But even for such an admission some definite proof is required. On looking through the large number of instances quoted by Mr. Distant in support of his suggestions, there appears to be only one case which affords anything like real proof, as opposed to mere suggestion. I refer to Mr. E.S. Thompson's account of the actions of a fox: "A fire had swept the middle of the pasture, leaving a broad belt of black; over this he skurried until he came to the unburnt yellow grass again, when he squatted down and was lost to view. He had been watching us all the time, and would not have moved had we kept to the road. The wonderful part of this is, not that he resembled the round stones and dry grass, but that he knew he did, and was ready to profit by it" ('Wild Animals I have known,' p. 193).(n8) This is a good example from Mr. Distant's point of view, but the fox is notoriously one of the most sagacious and cunning of animals, and, even if we believe that many of its actions are due to conscious intelligence, this does not in any way prove the occurrence of such intelligence in insects, fishes, or even other mammals, each of which cases would require independent proof. Further, it may be as well to point out that probably the process of reasoning in the fox would be quite different from that which would prompt a man to put on a khaki-coloured shirt when going out to shoot buck. It is improbable that any of the lower animals have any real conception of their own appearance, and it is likely that any consciousness they may exhibit in their protective actions consists rather in the general recognition that they are freer from attack in certain particular spots or types of country, than from any true appreciation of the optical phenomena to which they really owe their safety.
But it must be noted that a mere desire to hide, apart from any colour consideration, cannot be regarded in itself as any evidence of conscious resemblance. For example: if we break a piece off a termite-heap and see that the inmates at once run back into the nest or avail themselves of the nearest cover they can find, we cannot assume that this is due to their intelligent recognition that their colours are out of harmony with their then surroundings, but we should rather attribute it to the instinctive avoidance of light shown by all such nocturnal creatures, an instinct which is preferably explained by natural selection.(n9)
As a matter of fact, the most satisfactory style of evidence would probably consist in a careful and exact observation of the demeanour of protectively-coloured animals, which find themselves, by a natural accident or necessity, in an environment to which their colour is quite unsuited; or, conversely, of the behaviour of striking sports or variations of such species, when occurring in their normal surroundings. If, in such cases, the animals show a distinct appreciation of the danger of their position and alter their normal habits accordingly, then the suggestion of active mimicry will be sufficiently proved, so far as those animals and their immediate allies are concerned. But if, on the other hand, they show no such appreciation and merely adopt their usual attitudes of concealment, which in that case would egregiously fail in their purpose, then this suggestion will be very strongly discounted. It seems that a careful collection and discussion of all the authenticated observations of this description would add considerably to our knowledge of animal psychology. Perhaps, however, this has been already done, for it is impossible to keep abreast of scientific thought and work when living on the very outskirts of civilisation. I may here refer to one or two examples of this kind which tend to show that many cases of protective actions on the part of the higher vertebrates must be attributed to unreasoning instinct rather than to conscious volition.
The late Mr. Romanes very truly remarked, that "Every sportsman must have noticed that the somewhat rare melanic variety of the common Rabbit will crouch as steadily as the normal brownish-grey type, notwithstanding that, owing to its normal colour, a 'nigger rabbit' thus renders itself the most conspicuous object in the landscape. In all such cases, of course, there has been a deviation from the normal type in respect of colour, with the result that the inherited instinct is no longer in tune with the other endowments of the animal" ('Darwin and after Darwin,' p. 320). Again, to quote Mr. Distant himself, in reference to the crouching habits of the South African Francolinus, he says: "Subsequently I observed how this action could become habitual without a suitable environment. I flushed a pair of Francolinus subtorquatus, one of which squatted in the same manner, but, by force of circumstances, among the short, black and charred remains of a grass fire. Here its colour stood out in bold relief, and I easily bagged it" (Zool. 1899, p. 545, note). I have on several occasions observed a similar behaviour on the part of this same bird in Mashonaland; and, indeed, the blackening of the veldt by grass fires not unfrequently gives one opportunities of realising that at least some protectively coloured animals have no mental appreciation whatever of the real relation between their own colouring and that of their environment.
There are few birds in this country which show a stronger apparent reliance on their protective colouring than the little Rufous-capped Lark (Tephrocorys cinerea) or the Cape Long-claw (Macronyx capensis); they will readily permit one to approach within a few yards of them, and will then merely run on ahead in their curious, crouching, rat-like manner. This action is certainly of considerable protective value in their ordinary surroundings, but they will do precisely the same on the open "burns," where it must be rather detrimental than otherwise. Did they really comprehend the contrast exhibited by their plumage in such spots, they would assuredly escape by flight instead of by running. Not long ago I noticed a similar case on the part of our common Side-striped Jackal (Canis lateralis). While travelling on a post-cart we passed a fire burning not far from the road, and strongly outlined against the burnt grass we saw the forms of two Jackals. They were a little distance apart, one sitting on its haunches, the other standing, and they were evidently watching for the rats, young birds, &c, which the fire would disturb. At our approach they merely looked round at us without concern, and so, without stopping the cart, one of my companions tried a shot with his rifle. The bullet whizzed close over the head of the standing animal, which promptly bounded into the long, unburnt grass; the other, however, which had only heard the report without feeling the shock of the bullet, merely crouched to the ground, when it was quite as conspicuous as before, and did not move until a second bullet knocked up the dust close by its side. I have further seen an identical instance of the misapplication of the protective crouching instinct on the part of the Aard Wolf (Proteles cristatus) in Natal; and, doubtless, such observations could be multiplied were special attention paid to them.
Anyone who has had many opportunities of observing animals must have been struck by the fact that even though they may possess a considerable amount of intelligence, this is curiously limited in many directions. This may even occur in an unexpected way, as in the observation of Col. Pollok, cited by Mr. Distant, that the Tiger has not yet learnt that in pursuit of game nothing can be done down wind. Considerations such as these must lend a certain measure of support to the mechanical conception of natural selection. Thus, in the matter of conscious resemblance, although many animals may show undoubted intelligence in other directions, it is highly probable that, in the great majority of cases, their reasoning powers would not be sufficient to enable them to decide whether, or no, their own colouring would have a protective value in any new or unusual environment, It is far more reasonable to suppose that such knowledge as they may have in this respect would be acquired through their experience of their liability to, or immunity from, attack under such conditions, quite apart from any colour considerations. The former process would be a true instance of "active mimicry," as defined by Mr. Distant, but the latter cannot be included under that term; indeed, in such cases, experience in the individual is the equivalent of natural selection in the species.
In the preliminary portion of his paper, Mr. Distant has given us many excellent examples and arguments to show that mimicry and protective resemblance probably existed in very remote antiquity;(n10) and he has done well in drawing attention to the matter, which is apt to be overlooked. But I must certainly join issue with him when he states that: "The present attitude of many champions of the cause, who seek to find, or to invent, present factors for producing these phenomena, seems fraught with peril for the whole theory; and, with the same weariness and perseverance with which the original promulgators thought out the doctrine, we must go on searching for further proofs, which will necessitate our appealing to the Caesar of the past—the ever-growing science of palaeontology" (l. c., p. 302). I must confess that this appears to me to be a very remarkable assertion. In the first place, the vast majority of cases generally referred to mimicry and resemblance are concerned with colour and movement alone, structure playing but a very subordinate part therein. Mr. Distant has himself been at some pains to show the very evident futility of appealing to palæontology for evidence as to these phenomena; thus, if we are denied the right of attempting to explain them by causes acting at the present time, we shall have to abandon the whole question in despair. But, what is more important, mimicry and resemblance are only particular aspects of the principle of natural selection, and therefore if the factors of mimicry do not exist to-day, then, a posteriori, neither do those of natural selection. A single glance at nature is sufficient to justify the rejection of such a conclusion, and we must, therefore, admit that the factors of mimicry are in actual operation now; were they not, we should have no grounds for assuming that they had operated in past geological epochs. If, therefore, we find that certain cases appear difficult of exact explanation in the present state of our knowledge, we are by no means justified in disposing of the difficulty by referring them to causes operating only in the dim past, which we can neither prove nor disprove. Rather must we continue the laborious search for further evidence, not by a study of the anatomy of extinct animals, but by seeking a deeper and more intimate knowledge of the real life-histories of living organisms; for we are still profoundly ignorant of the immensely complex factors which go to make up the conditions of life of the very commonest animal upon this earth.
Nevertheless, it must be conceded as possible that there may be certain cases of mimicry or resemblance which cannot be attributed to exact causes acting at the present time; but these would be only exceptional, and would probably be due to a recent change in the enemies or the general environment of the species. I say "recent" advisedly, for we have very good grounds for believing that complicated protective characters would gradually disappear soon after the need for them ceased, whether this disappearance be attributed to pammixis or to disuse.
Later on, in the papers under consideration, we find an excellent suggestion that all examples of mimicry and resemblance should be classified under various headings, such as—Demonstrated—Suggested or Probable—Disputed or Mistaken—Purposeless—or Active. If such an arrangement could be thoroughly and carefully carried out, it would be of considerable value to students of these phenomena. Mr. Distant could, of course, only give us a mere sketch of the subject; but it is remarkable that there is not even a reference to the lengthy and important paper by Prof. Poulton, who has so thoroughly identified himself with this line of research, on "The Experimental Proof of the Protective Value of Colour and Markings in Insects in reference to their Vertebrate Enemies" (Proc. Zool. Soc. 1887, pp. 191-274), in which all the reliable experiments on British insects, up to that date, are tabulated and discussed. Unfortunately, however, the classification of several of the cases given by Mr. Distant is open to criticism. For example: in the instances of resemblances in birds, given by Mr. J.H. Gurney (l.c., 1899, p. 460), every case relates to species of the same genus inhabiting different areas—in fact, representative species, or even local races; and the resemblances between them are simply due to close kinship, and have nothing whatever to do with the subject of mimicry. Again, a reference to the suggested mimicry of the Cape Hunting Dog (Lycaon pictus), of the Spotted Hyaena (Hyæna crocuta), is placed under the heading of "Suggested or Probable Mimicry" (l. c., p. 449), although Mr. Lydekker's remarks, showing the difficulty of accepting this proposition, are quoted. Indeed, I have always been at a loss to understand how such a strong and fearless animal as the former—of which Selous has recorded that it "is capable of overtaking and attacking single-handed such a powerful animal as a male Sable Antelope" ('Hunter's Wanderings in Africa,' p. 357)—could be supposed to derive any benefit from resembling a cowardly brute like the Hyæna. To anyone acquainted with the two animals in nature, it is abundantly evident that, whatever mimicry there may be between them, it would be in just the reverse direction; that is, the skulking Hyaena would materially benefit by being mistaken for the bold and gregarious Hunting Dog.(n11)
I need only refer to one more example—namely, that of the Honey Bee (l.c, p. 356). It is well known that various species of the dipterous genus Eristalis mimic Bees; and Mr. Distant quotes the experiments of Prof. Lloyd Morgan with Chickens, and Mr. R. J. Pocock with Spiders, which demonstrate the value of this mimicry. Yet this instance is not placed in the "Demonstrable" category, but in that of "Suggested or Probable," on the ground that "the Bee itself is not absolutely protected by its sting." If such a classification were adhered to, there never would be a case of demonstrated mimicry; but it must be noted that, on the same page, it is explained that: "By the term 'Demonstrable' is implied all those instances where protection, absolute or partial, has been, or can be, demonstrated by experiment or actual observation."(n12)
It now only remains to discuss the objections raised by Mr. Distant to certain cases which are generally referred to protective resemblance. After stating that "colour alone may prove a false analogy to protection" (l.c., p. 350), and referring to the strongly protective colouring of a certain South American butterfly, Ageronia feronica, he says:
"This observer, however, at the same time refers to the statement of Bigg Wither, that this very insect is called the Whip-Butterfly, owing to the sharp whip-cracking sound made by its wings when battling by its fellows in the air,(n13) and that this sound makes it the easy prey of a forest bird, locally known as 'the Suruqua,' who thus detects and secures it. Here the apparent protection by "protective resemblance " is invalidated by a peculiar and unusual sound-producing quality, which is as equally dangerous as its colour is reported protective. A similar remark may be made as to the musical Cicadidæ. How often have the usual green and brown colours of these insects been adduced as an example of protective resemblance;... but when we desire to capture them the shrill noise proclaims their retreat, and their assimilative colouration avails them little." Again, in commenting upon Mr. Tutt's graphic account of the protective colouring of the Lappet Moth (Lasiocampa quercifolia), he says: "Here the expression, 'trained eye,' of the entomologist, would suggest a more developed 'trained eye' of the moth's natural enemies, and hence any theory of protective mimicry is much discounted (l.c. p. 455). From these quotations it may be gathered that Mr. Distant's attitude towards the subject is somewhat as follows:—When we find that the colouring of any animal assimilates well with that of its environment, but, at the same time, that this animal is apt to render itself more or less noticeable by certain movements or noises, then we are not justified in regarding its colouration as an efficient protection, and the case must therefore be removed from the category of protective resemblance. Tempting as such a conclusion may be to the opponents of Darwinism, it appears to me to be wholly erroneous. The fundamental fallacy lies in the gratuitous assumption that the protection afforded must be absolute; for otherwise there is no ground whatever for the objection raised. In the first place, I am not aware that such absolute protection has been anywhere observed in nature, and, indeed, were the above proposition a sound one, the principle of protective resemblance would have to be entirely abandoned. But, as a matter of fact, this principle predicates no such complete immunity from attack; in truth, the very essence of the theory of natural selection negatives any such supposition; for, according to this theory, protective resemblance, as we now see it, has been arrived at by the gradual accumulation and improvement of colour variations which make for concealment, and the protective value of such variations must essentially be, or have been, of only a partial character. Admitting that the gambols of the Whip Butterfly (presumably of a sexual character) lead the insect into a certain amount of danger, yet, to ask us to believe that it thereby "invalidates" the protection afforded it, when at rest, by its assimilative colouring, against other enemies, and perhaps even against the "Suruqua" itself, is, as Mr. Bateson puts it, referring to a different assumption, "to ask us to abrogate reason." Further, the not unusual fact, that animals exhibiting a very high grade of resemblance are yet subject to a considerable amount of persecution, in no way invalidates, but rather strengthens, this principle; for it is evident that such a degree of resemblance can only have been developed in response to a similarly high degree of persecution, acting either now, or within recent times.
It will thus be seen that, on general considerations alone, the above objections to the principle of protective resemblance must be at once ruled out of court. It may be as well, however, to discuss the case of Cicadas in more detail. In the first place, I cannot agree with Mr. Distant that these insects are easily captured owing to their shrill cries. All high-pitched, vibrating sounds of this kind are very difficult to localise exactly, and with Cicadas I have noticed very frequently, both with myself and others, that the distance of the insect is invariably much underestimated.(n14) But even when the tree on which the Cicada is sitting has been ascertained, it must be very cautiously approached, for many species are able to detect one's presence at a distance of fifteen to twenty yards in open country, and, on so doing, they will at once cease their call; and although they will generally permit a much closer approach than this, yet it is always extremely difficult to locate the exact position of the sound on the tree. Their habits, however, vary in this respect, and among the dozen or more species which I have observed in various parts of South Africa, I have found it to be a very general rule that their wariness is inversely proportionate to their protective resemblance; those species which live on rough, knotted bark, or among dense foliage, permitting one to approach much nearer than do those that rest on bare, smooth trunks or small twigs. The above remarks apply to the calling of a single insect; but, when a number are calling together, it is still more difficult to localise any particular cry; and, indeed, I have on several occasions been driven out of a patch of machabel bush by the continuous ear-piercing scream of a number of the large Pœcilopsaltria horizontalis, Karsch, which seems to make the whole air pulsate, without betraying the exact locality of a single individual. Although in many cases I have actually tracked down individuals by their cry, in order to learn the calls of the different species, yet such a method is far too laborious for ordinary collecting purposes. So experienced a collector as Dr. Percy Kendall says: "In the Transvaal I have also taken them at rest on tree-trunks, but I do not think they were taken in consequence of their song having thus localized them. At Zomba I caught a large species by actually localizing its noise, but that was the only instance of the kind that I remember" ('Zoologist,' 1897, p. 520).
It must not be supposed that I do not recognize that the Cicada's cry must, under certain circumstances, be dangerous for individuals as, indeed, are many other secondary sexual characters; but Mr. Distant appears to have overestimated the danger, and the contention that this noise invalidates their admirably protective colouration appears to be an inverted way of looking at the question. It is more reasonable to suppose that the protective resemblance of these insects is so efficacious, that they have been able to develop these extraordinary cries through the process of sexual selection (or perhaps even natural selection, supposing aesthetic appreciation on the part of the female be denied), without unduly endangering the safety of the species. On this view, the Cicada's song, far from proving that the insect's colouring is inefficient for protective purposes, would stand as a testimony of its very high efficacy. In fact, I venture to think that, in the vast majority of cases in which animals produce conspicuously loud sounds, they will be found to possess either highly protective colouration or habits, or else distasteful or other qualities which render concealment unnecessary.
In conclusion, I can only hope that sufficient has been said to show that there are good grounds for opposing the suggestion that active mimicry is of any general occurrence in the animal kingdom; and, further, that the attempt to minimise certain phenomena of ordinary protective resemblance, in order to bring them within the scope of that principle, is not justifiable upon the evidence adduced. The subject, however, is such a wide one, that it is impossible to deal adequately with all its aspects within the limits of a paper such as this.
- I observe that Mr. Distant has strongly criticised (l.c., 1899, p. 361) a somewhat similar remark by Prof. Tyler, who says that "Natural Science does not deal in demonstrations, it rests upon the doctrine of probabilities; just as we have to order our whole lives upon this doctrine." To this Mr.
- "Conscious mimicry" was also a term stated to have been proposed by Prof. Henslow (cf. ib. p. 465).—Ed.
- I observe that Mr. Distant has strongly criticised (l.c., 1899, p. 361) a somewhat similar remark by Prof. Tyler, who says that "Natural Science does not deal in demonstrations, it rests upon the doctrine of probabilities; just as we have to order our whole lives upon this doctrine." To this Mr. Distant replies that "This is a cardinal doctrine in natural and apologetic theology, but is the very antithesis of science, natural or otherwise. The man who orders his whole life on probabilities will probably arrive at the conclusion that hope is a very good breakfast, but a most indifferent dinner." Prof. Tyler's remark appears to my mind as a sufficiently evident truth, but I may perhaps be permitted to adduce in its support the opinion of so virile a thinker as the late Prof. Huxley, who says: "We find, practically, that expectations, based upon careful observations of past events, are, as a rule, trustworthy. We should be foolish indeed not to follow the only guide we have through life. But, for all that, our highest and surest generalizations remain on the level of justifiable expectations, that is, very high probabilities." ('Collected Essays,' vol. v. p. 204.)
On the other hand, Prof. Huxley, in thanking Bateson for his well-known volume on 'Variation,' writes how glad he is to see "that we are getting back from the region of speculation into that of fact again" ('Life and Letters,' vol. ii. p. 372).— Ed.
- This is an apparent inference to Mr. Marshall, but no statement of the kind appears in the suggestions criticised.—Ed.
- The point discussed was the structural characters of the Phasmidæ. The exact quotation requires this antecedent: "We still have abundant reason for believing that, though the protective resemblance of these Phasmidæ was already acquired in Carboniferous times, the presence of Amphibia in an evolutionary sense is quite sufficient to account for it. This prompts two reflections: one, that we ought to look a long way back for the origins of these protective and mimetic guises; and the other, that we may reasonably hope to find them" (p. 302).—Ed.
- The writer may not have referred to every paper that Prof. Poulton has written, but he certainly did write (p. 451): "Poulton has focussed many observations respecting instances in the Insecta, largely augmented by information received from the well-known coleopterist C.J. Gahan" (cf. Journ. Linn. Soc. xxvi. pp. 558-612 (1898)); a much later paper than that referred to by Mr. Marshall.— Ed.
- Darwinism does not derive its sole support from theories of "mimicry," and the writer of the papers criticised was not aware that he was to be counted among "the opponents of Darwinism."—Ed.