BOURU FRIAR-BIRDBOURU FRIAR-BIRDLike most of the group to which it belongs, this honey-eater (Tropidorhynchus bouruensis) is a soberly coloured bird, but is noisy, active, and aggressive.
BOURU FRIAR-BIRD
Like most of the group to which it belongs, this honey-eater (Tropidorhynchus bouruensis) is a soberly coloured bird, but is noisy, active, and aggressive.
BOURU ORIOLEBOURU ORIOLEThis “mimicking” oriole (Oriolus bouruensis) is of the same tone of colour as its supposed model the Friar-bird of the same island.
BOURU ORIOLE
This “mimicking” oriole (Oriolus bouruensis) is of the same tone of colour as its supposed model the Friar-bird of the same island.
In some cases these brilliantly coloured insects may be survivals of an age in which there were no birds. When these came into being and began to prey upon insects, the conspicuously coloured species which were not inedible or very unpalatable would soon become extinct, while those that were inedible would survive as warningly-coloured insects. In other cases it is not improbable that these warningly-coloured creatures have arisen by mutations from more soberly-hued insects. It is conceivable that every now and again a mutation occurs which renders its possessor conspicuous. This will result in the early destruction of these aberrant individuals unless their newly-acquired gaudiness is either correlated with, or the result of, distastefulness.
In the case of warning colouration, the Neo-Darwinians have, as usual, pursued their theory to absurd lengths. Professor Poulton, for example, extends it to sounds and attitudes. “Sound,” he writes, on page 324 ofEssays on Evolution, “may be employed as an Aposematic character, as in the hiss of some snakes and some lizards. Certain poisonous snakes when disturbed produce by an entirely different method a far-reaching sound not unlike the hiss. Thus the rattle-snake (Crotalus) of America rapidly vibrates the series of dry, horny, cuticular cells, movably articulated to each other and to the end of the tail. The stage through which the character probably arose is witnessed inanother genus which vibrates its tail among dry leaves, and thus produces a warning sound. The deadly little Indian snake (Echis carinata) (‘the Kuppa’) makes a penetrating swishing sound by writhing the coils of its body one over the other. Special rows of the lateral scales are provided with serrated keels which cause the sound when they are rubbed against each other. Large birds, when attacked, often adopt a threatening attitude, accompanied by an intimidating sound which usually suggests more or less closely the hiss of a serpent, and thus includes an element of mimicry. . . . The cobra warns an intruder chiefly by attitude and by the broadening of its flattened neck, the effect being heightened in some species by the ‘spectacles.’ In such cases we often witness a combination of cryptic and Aposematic methods, the animal being concealed until disturbed, when it instantly assumes a warning attitude.
“The benefit of such intimidating attitudes is clear: a venomous snake gains far more advantage by terrifying than by killing an animal it cannot eat. By striking, the serpent temporarily loses its poison, and with this a reserve of defence. Furthermore, the poison does not cause immediate death, and the enemy would have time to injure or destroy the snake.”
At first sight this reasoning may seem very convincing. But consider for a moment theprocess by which the hiss originated and gradually increased by natural selection. We must suppose that the rattle-snake was formerly incapable of making any sound. One day a variety appeared in which the skin was slightly hardened, so that when the creature moved its body rapidly there issued a slight sound. This must have caused an enemy to refrain from attack; it thus lived to transmit this peculiarity to its offspring, and those which made more noise than their ancestors escaped, while those that made less succumbed to their enemies. For ourselves, we find it quite impossible to believe that the rattle was thus gradually evolved by means of natural selection. Indeed, we are inclined to think that neither the hiss of the cobra nor its “intimidating attitude” has any terrifying effect on its adversary. In the case of the cobra we are able to cite positive evidence that dogs and cattle show no alarm at the attitude.
“Dogs,” writes D. Dewar of this display, “regard it as a huge joke. Of this I have satisfied myself again and again, for when out coursing at Muttra we frequently came across cobras, which the dogs used invariably to chase, and we sometimes had great difficulty in keeping the dogs off, since they seemed to be unaware that the creature was venomous.”
Colonel Cunningham writes, on page 347 ofSome Indian Friends and Acquaintances: “Sportingdogs are very apt to come to grief where cobras abound, as there is something very alluring to them in the sight of a large snake when it sits up nodding and snarling; and it is often difficult to come up in time to prevent the occurrence of irreparable mischief.”
Colonel Cunningham also states that many ruminants have a great animosity to snakes, and are prone to attack any that they may come across.
We may therefore well be sceptical as to the value of intimidating attitudes to those creatures which are in the habit of striking them.
In a work of this kind it is neither possible nor necessary to consider in great detail the mass of evidence which has been advanced in favour of the theory of mimetic resemblance.
Chapters vii. and viii. of Professor Poulton’sEssays on Evolutioncontain an up-to-date statement of the facts in favour of the theory. Professor Poulton believes that in all cases mimetic resemblance is the result of the action of natural selection.
He admits that there is no direct evidence in its favour, but asserts that “the facts of the cosmos, so far as we know them, are consistent with the theory, and none of them inconsistent with it” (page 271).
We are not at all sure that no facts are against the theory of protective mimicry. We shall presently set forth some which to us seem, if not actually inconsistent with the theory, at least to point to the conclusion that the phenomenon may be explained otherwise than as a product of natural selection.
Let us first briefly state the case for the theory of protective mimicry.
1. It is asserted that the mimicking species and that which is mimicked are often not nearly related. For example, the unpalatable larva of the Cinnabar Moth (Euchelia jacobaeæ) is said to mimic a wasp, because it has black and yellow rings round its body.
“The conclusion which emerges most clearly,” writes Poulton (p. 232), “is the entire independence of zoological affinity exhibited by these resemblances.” This is supposed to be proof that Darwin was wrong when he asserted that the original likeness was due to affinity. Says Poulton: “The preservation of an original likeness due to affinity undoubtedly explains certain cases of mimicry, but we cannot appeal to this principle in the most remarkable instances.”
2. It is asserted that species which are mimicked are invariably either armed with a sting, well defended, or unpalatable, so that it is against the interest of insectivorous creatures to attack them. It is further asserted that thespecies imitated are “even more unpalatable than the generality of their order.”
3. It is pointed out that the most distasteful groups of butterflies—theDanaidæ, theAcræinæ, theIthomiinæ, and theHeliconinæ—consist of large numbers of species which closely resemble one another. This is said to be due to Müllerian mimicry. Mayer states that in South America there are 450 species of inedibleIthomiinæwhich display only 15 distinct colours, while the 200 species ofPapilio, which are edible, exhibit 36 distinct colours. Nevertheless, he says, there is no lack of individual variability among the former hence their conservatism as regards colour cannot be attributed to their having but little tendency to vary.
4. It is asserted that although in many cases the mimetic resemblances extend to the minutest detail, nevertheless they are not accompanied by any changes in the mimetic species except such as assist in the production or strengthening of a superficial likeness.
Pictures illustrating such cases of mimicry are figured on pp. 241, 247, and 251 of Wallace’sDarwinism(1890 edition).
5. It is stated that mimetic resemblance is not confined to colour, but extends to pattern, form, attitude, and movement; that deep-seated organs are affected when the superficial resemblance is intensified, but not otherwise. Poulton citesClytus arietis, the “wasp-beetle,” as an example of this.
6. It is asserted that mimetic resemblances are produced in the most diverse ways; that the modes whereby the similarity in appearance is brought about are varied, but the result is uniform.
“A lepidopterous insect,” writes Poulton (p. 251), “requires above all to gain transparent wings, and this, in the most striking cases that have been studied, is produced by the loose attachment of the scales, so that they easily and rapidly fall off and leave the wing bare except for a marginal line and along the veins (Hemaris,Trochilium).”
7. It is alleged that the imitator and imitated are always found in the same locality. If they did not do so no advantage would be derived from the resemblance. It is further alleged that where the mimicking species is edible it is invariably less abundant where it occurs than the species it imitates.
8. It is pointed out that it sometimes happens that where in the mimic the sexes differ in appearance, the male copies one species, the female quite a different one. This is said to be because the deception would be liable to be detected if the mimicking species became common relatively to that which is imitated. “We therefore find that two or more models are mimickedby the same species” (Essays on Evolution, p. 372).
Occasionally the female mimics two other species,i.e.she occurs in two forms, each like a different species.
It sometimes happens that the female alone mimics. This is said by Wallace to be due to her greater need of protection. When she is laden with eggs her flight is slow, and therefore she requires a special degree of protection.
9. It is said that in some species we find a non-mimetic ancestor preserved on islands where the struggle for existence is less severe, while on the adjacent continent mimicry has been developed.
10. It is alleged that in the cases where moths resemble butterflies the former are either as diurnal as the butterflies or are species which “readily fly by day when disturbed.”
11. It is asserted that some seasonally dimorphic forms are examples of mimicry only in one state, in the form that comes into being at the time when the struggle for existence is most severe; that is to say, in the dry season, in Africa, when insect life is far less abundant than in the rainy season.
In other cases the mimicry of the dry-weather form is said to be far more perfect.
Instances of this phenomenon are set forth in Professor Poulton’sEssays on Evolution.
It will be observed that we have quoted very largely from Professor Poulton’s work. Our reason for so doing is that he appears to be the most prominent advocate of the theory of protective mimicry, and his work, which was published in 1908, may be taken as the latest Neo-Darwinian pronouncement on the subject.
Hence if we can show, as we believe we can, that his arguments are not sound, we may take it that we have demonstrated that the theory in its present form is untenable.
It is worthy of notice that Professor Poulton sets forth three other suggestions which have been proposed as substitutes for natural selection as an explanation of the phenomena of mimicry.
The first is the theory of External Causes, namely, that the resemblance is due to some external cause, such as food or climate.
The second is the theory of Internal Causes, which states that mimetic resemblance is due to internal developmental causes.
The third is the suggestion that sexual selection has caused the origin of these resemblances.
He then proceeds to demolish these to his own satisfaction, and adds triumphantly, “The conclusion appears inevitable that under no theory, except natural selection, do the various resemblances of animals to their organic andinorganic environments fall together into a natural arrangement and receive a common explanation” (p. 228).
To reasoning of this description there is an obvious reply. Even if it be granted that the alternatives to the theory of natural selection as set forth by Professor Poulton are untenable, it does not follow that natural selection affords an adequate explanation. If A, B, C and D are charged with theft and the prosecutor proves that neither A nor B nor C committed the theft, this will not suffice to secure the conviction of D. It is quite possible that a fifth person, E, may be the culprit.
Much of the popularity of the theory of natural selection is due to the fact that biologists have not yet been able to discover a substitute for it.
It seems to us that the proper method of making progress in science is not to bolster up natural selection by ingenious speculations, but to look around for other hitherto undiscovered causes.
KING-CROW OR DRONGOKING-CROW OR DRONGOThis very conspicuous black bird (Dicrurus ater), ranging from Africa to China, is a striking feature of the landscape wherever it occurs.
KING-CROW OR DRONGO
This very conspicuous black bird (Dicrurus ater), ranging from Africa to China, is a striking feature of the landscape wherever it occurs.
DRONGO-CUCKOODRONGO-CUCKOOThe fork of the tail in this bird is unique among cuckoos, but is nevertheless much less developed than in the supposed model, and may be an adaptation for evolution in flight, as such tails usually appear to be.
DRONGO-CUCKOO
The fork of the tail in this bird is unique among cuckoos, but is nevertheless much less developed than in the supposed model, and may be an adaptation for evolution in flight, as such tails usually appear to be.
It is obvious that for one creature to resemble another can be of little or no benefit to either until the resemblance is tolerably close. It is,therefore, insufficient to prove the utility of the perfected resemblance. We may readily grant this and yet maintain that the origin of the resemblance cannot be due to the action of natural selection.
The Drongo-cuckoo (Surniculus lugubris) displays so great a likeness to the King Crow (Dicrurus ater) that it is frequently held up by Neo-Darwinians as an excellent example of mimicry among birds. But D. Dewar writes, on page 204 ofBirds of the Plains: “I do not pretend to know the colour of the last common ancestor of all the cuckoos, but I do not believe that the colour was black. What then causedSurniculus lugubristo become black and assume a king-crow-like tail?
“A black feather or two, even if coupled with some lengthening of the tail, would in no way assist the cuckoo in placing its egg in the drongo’s nest. Suppose an ass were to borrow the caudal appendage of the king of the forest, pin it on behind him, and then advance among his fellows with loud brays, would any donkey of average intelligence be misled by the feeble attempt at disguise? I think not. Much less would a king-crow be deceived by a few black feathers in the plumage of a cuckoo. I do not believe that natural selection has any direct connection with the nigritude of the drongo-cuckoo.”
Darwin was fully alive to this difficulty whenhe wrote: “As some writers have felt much difficulty in understanding how the first step in the process of mimicry could have been effected through natural selection, it may be well to remark that the process probably commenced long ago between forms not widely dissimilar in colour” (Descent of Man, 10th Ed., p. 324). Such a statement is of course quite inconsistent with the Neo-Darwinian position. “The conclusion which emerges most clearly,” writes Poulton (Essays on Evolution, p. 232), “is the entire independence of zoological affinity exhibited by these resemblances; and one of the rare cases in which Darwin’s insight into a biological problem did not lead him right was when he suggested that a former closer relationship may help us to a general understanding of the origin of mimicry. The preservation of an original likeness due to affinity undoubtedly explains certain cases of mimicry, but we cannot appeal to this principle in the most remarkable instances.”
It is unnecessary to labour this point. It is surely evident to everyone with average intelligence that, until the resemblance between two forms has advanced a considerable way, the likeness cannot be of utility to either, or at any rate of sufficient utility to give its possessor a survival advantage in the struggle for existence. Until it reaches this stage, natural selection cannotoperate on it. It is therefore absurd to look upon natural selection as the direct cause of the origin of the likeness. When once a certain degree of resemblance has risen, it is quite likely that in some cases natural selection has strengthened the likeness.
The second great objection to the Neo-Darwinian explanation of the phenomenon known as mimicry is that in many cases the resemblance is unnecessarily exact. Even as we saw how the Kallimas, or dead-leaf butterflies, carried their resemblance to dead leaves to such an extent as to make it appear probable that factors other than natural selection have had a share in its production, so do we see in certain cases of mimetic resemblance an unnecessarily faithful likeness.
The common Hawk Cuckoo of India (Hierococcyx varius) furnishes an example of this: “The brain-fever bird,” writes Finn, on page 58 ofOrnithological and Other Oddities, “is the most wonderful feather copy of the Indian Sparrow-hawk or Shikra (Astur badius). All the markings in the hawk are reproduced in the cuckoo, which is also of about the same size, and of similar proportions in the matter of tail and wing; and both hawk and cuckoo having a first plumage quite different from the one they assume when adult, the resemblance extends to that too. Moreover, their flight is so much the same thatunless one is near enough to see the beak, or can watch the bird settle and note the difference between the horizontal pose of the cuckoo and the erect bearing of the hawk, it is impossible to tell them apart on a casual view.” Moreover, the tail of the cuckoo sometimes hangs down vertically, thus intensifying the likeness to the hawk.
It is quite possible that the brain-fever bird derives some benefit from the resemblance; indeed, it has been seen to alarm small birds, even as the hawk-like common cuckoo frightens its dupes, but, as D. Dewar pointed out, on page 105 of vol. 57 of theJournal of the Society of Arts, “this is not sufficient to explain a likeness which is so faithful as to extend to the marking of each individual feather. When a babbler espies a hawk-like bird, it does not wait to inspect each feather before fleeing in terror; hence all that is necessary to the cuckoo is that it should bear a general resemblance to the shikra. The fact that the likeness extends to minute details in feather marking, points to the fact that in each case identical causes have operated to produce this type of plumage.” This conclusion is still further strengthened by the fact that the likeness extends to the immature plumage, that is to say, exists at a time when it cannot assist the cuckoo in its parasitical work.
Poulton meets this objection as follows:
SHIKRA HAWKSHIKRA HAWKThe upper surface of the tail, not shown in this drawing, exactly corresponds with that of the cuckoo “mimic.”
SHIKRA HAWK
The upper surface of the tail, not shown in this drawing, exactly corresponds with that of the cuckoo “mimic.”
HAWK-CUCKOOHAWK-CUCKOOThis species (Hierococcyx varius) is commonly known in India as the “Brain-fever bird.”
HAWK-CUCKOO
This species (Hierococcyx varius) is commonly known in India as the “Brain-fever bird.”
“All such criticism is founded on our imperfect knowledge of the struggle for existence. The impressions and judgments of man are immensely influenced by the ‘corroborative detail,’ giving ‘artistic verisimilitude to a bold and unconvincing narrative.’ Indeed, the laughter which is invariably raised by this passage fromThe Mikadois, I have always thought, not only or chiefly due to the humour of the application, but to the way in which a great and familiar truth breaks in upon the listener with all the pleasing surprise which belongs to epigram. Birds, the chief enemies of insects, are known to have powers of sight far superior to those of man, and, from our experience of them in captivity, it may be safely asserted that their attention is attracted by excessively minute detail. Until our knowledge of the struggle for life is far more extensive than at present, the argument founded on Hypertely may be left to contend with another argument often employed against the explanation of cryptic and mimetic resemblance by natural selection. Hypertely assumes that there are unnecessary details in the resemblance, that the resemblance is perfect beyond the requirements of the insect; the second argument maintains that birds are so supremely sharp-sighted that no resemblance, however perfect, is of any avail against them. In the meantime the majority of naturalists will probably reject both extremes, and believe thatthe enemies are certainly sharp-sighted and successful in pursuit, but that perfection in detail makes their task a harder one, and gives to the individuals possessing it in a higher degree than others, increased chances of escape, and of becoming the parents of future generations.” (Essays on Evolution, p. 302.)
This long quotation requires careful consideration, since to us it appears to be typical of the kind of reasoning resorted to by Neo-Darwinians.
Note the reference to our “imperfect knowledge of the struggle for existence.” This is almost invariably the last refuge of the Neo-Darwinian when worsted in argument. We fully admit that there is still much to be learned of the nature of the struggle for existence, but such a statement sounds very curious when uttered to those who pin their faith to the theory which sees in the principle of natural selection an explanation of all the phenomena of the organic world. Natural selection, be it remembered, is but a name for the struggle for existence.
“Birds,” says Professor Poulton, “are the chief enemies of insects.” This may be so. But we greatly doubt whether they are the chief enemies of butterflies and moths, among which the most perfect examples of mimicry are supposed to occur.
We have watched birds closely for some years, but believe that we could almost count on ourfingers the cases in which we have seen a bird chase a butterfly.
Professor Poulton, being aware of this objection, sets forth, on pp. 283-292 ofEssays on Evolution, the evidence he has gathered in favour of the view that birds are the chief enemies of butterflies and other lepidoptera.
As the result of five years’ observation in S. Africa, Mr G. A. K. Marshall was able to record some eight cases of birds capturing butterflies. In three cases the butterfly seized was warningly coloured, or, at any rate, conspicuous! In two of these eight cases the bird failed to capture its quarry!
Says Mr Marshall, “the fact that birds refrain from pursuing butterflies may be due rather to the difficulty in catching them than to any widespread distastefulness on the part of these insects.”
During six years’ observation in India and Ceylon, Colonel Yerbury records some half dozen cases of birds capturing, or attempting to capture, insects. He writes: “In my opinion an all-sufficient reason for the rarity of the occurrence exists in the fact that in butterflies the edible matter is a minimum, while the inedible wings, etc., are a maximum.”
Colonel C. T. Bingham in Burma states that between 1878 and 1891 he on two occasions witnessed the systematic hawking of butterfliesby birds, although he observed on other occasions some isolated cases.
This appears to be the sum total of the evidence adduced by Professor Poulton as regards the capture of butterflies by birds. This seems to us an altogether insufficient foundation upon which to build the theory that the cases of resemblance between unrelated species have been effected by natural selection.
It is, however, to be noted that probably among birds the most dangerous enemies of butterflies are not those that habitually catch insect prey on the wing. Such are experts in the art of fly-catching, and would despise the comparatively meatless butterfly. One often comes across butterflies with an identical notch in each wing, which leaves little room for doubt that those particular butterflies had been snapped at,while resting, by a bird. Among birds the chief enemies of butterflies and moths are probably to be found in those that hunt for their food in bushes and trees.
Thus, what we do know of the nature of the struggle for existence offers but poor support to the Neo-Darwinian explanations of the cases of so-called mimicry in nature.
Professor Poulton’s idea of pitting the argument of Hypertely against that of the alleged supreme sharp-sightedness of birds is ingenious, but is not likely to satisfy very many people savethose content to live in a fools’ paradise. If birds are supremely sharp-sighted, and pay attention to excessively minute detail, the difficulty of accounting for theoriginof protective mimicry on the natural selection hypothesis becomes all the greater.
The question whether or not birds are good observers is a most interesting one. Unfortunately, hitherto, but little attention has been paid to the subject. The evidence available seems to point to the fact that birds, like savages, have sharp eyes only for certain objects—that is to say, for the things they are accustomed to look out for. All observers of nature must have noticed how quick a butcher-bird is to catch sight of a tiny insect upon the ground at a distance of some yards from his perch.
On the other hand, it is said that when there is snow upon the ground wood pigeons will approach quite close to a man wearing white clothes and a white hat, provided he keep perfectly still. Finn once witnessed in Calcutta a sparrow pick up a very young toad, obviously by mistake, for it dropped it at once with evident distaste. Birds of prey are supposed to have remarkably good eyesight; yet they can readily be caught by a net stretched out before their quarry. They are not trained to be on the watch for such things as nets, and so do not appear to notice one when erected.
It is thus our belief that the very perfection and detail of some so-called mimetic resemblances are a very serious objection to the theory of protective mimicry as enunciated by Professor Poulton and other Neo-Darwinians.
There is yet a further objection to this theory, one which, in our opinion, is fatal to the hypothesis in its generally accepted form.
A number of cases occur where two species, in no way related, show close resemblance to one another under such circumstances that neither can possibly derive any benefit from the likeness. The theory of protective mimicry is quite unable to explain these cases. This fact leads to a suspicion that, in the instances where the theory does at first sight appear to offer an explanation, the resemblance may also be due to mere coincidence.
We may perhaps call the cases which the theory of mimicry is unable to account for “false mimicry,” but in so doing we must bear in mind the possibility that some, at any rate, of the examples of so-called mimicry may, on further investigation, prove to be nothing of the kind.
The Cacomistle of Mexico (Bassaris astuta), one of the raccoon family, has a grey body and long black-and-white ringed tail, just like the ring-tailed Lemur of Madagascar (Lemur catta);both are arboreal and about the same size, and this lemur’s colouration is exceptional in its family.
The banded Duiker-buck of West Africa (Cephalophus doriae), has the same very unusual colouration as the thylacine or marsupial wolf of Tasmania, light brown, with bold black bands across the hinder part of the back, and the animals are about the same size.
The dormouse of Europe closely resembles a small American Opossum (Didelphys murina), and a larger opossum (D. crassicaudata) is very like the Siberian Mink (Mustela sibirica).
The Flying Squirrel of North America (Sciuropterus volucella) is closely copied by the Flying Phalanger (Petaurus breviceps) of Australia.
It will be readily seen that in no one of these cases can the likeness be of utility to either the “model” or the “copy.”
There are many instances of this phenomenon among birds. The New Zealand Cuckoo (Urodynamis tritensis) shows a far closer resemblance to the American Sparrow-hawk (Accipiter cooperi) than to any New Zealand hawk, and in fact closely mimics this quite alien bird.
The stormy petrel, a purely oceanic bird, closely resembles in size, colour, and style of flight the Indian Swift (Cypselus affinis), a purelyinland creature; both are sooty black, with a conspicuous white patch on the lower back.
The Pied Babbling Thrush (Crateropus bicolor) of Africa is singularly like the Pied Myna (Græulipica melanoptera) of Java, both being of about the same size, with white body and black wings and tail quills. This, we may add, is a very unusual colouration among small birds.
The black-headed Oriole (Oriolus melanocephalus) of India is very similar in appearance to the common Troupial (Icterus vulgaris) of Brazil; indeed, the troupials, a purely American group, are so like the old world orioles in colour that they usurp their name in America.
The little insectivorous Iora (Ægithina tiphia) of India strongly resembles in size and colour a Siskin (Chrysomitris colambiana) from South America, the males in both being black above and yellow below, while in the females the black is replaced by olive-green.
Another Indian babbler (Cephalopyrus flammiceps), yellowish-green, with orange forehead, is closely copied by, or copies, the well-known Brazilian Saffron-finch (Sycalis flaveola).
In Fergusson Island, near New Guinea, there is a ground pigeon (Otidiphaps insularis) which is black with chestnut wings, like several of the powerful ground cuckoos of the genusCentropus, but no species of these cuckoos so coloured appears to inhabit the island.
In Africa there is a tit (Parus leucopterus) which has the same very unusual colouration as an East-Indian bulbul (Micropus melanoleucus), both being black with a white patch on the wing-coverts. These two birds are about the same size. As showing the purely coincidental character of such resemblances, we may mention that this same rare pattern occurs again in our Black Guillemot (Uria grylle) and in the Muscovy Duck (Cairina moschata).
We have already quoted Gadow (p. 198) on “false mimicry” among snakes. He also gives, on p. 110 ofThrough Southern Mexico, an example of this phenomenon among amphibia. It is, he writes, “impossible to distinguish certain green tree-frogs of the African genusRappiafrom aHyla, unless we cut them open. If they lived side by side, which they do not, this close resemblance would be extolled as an example of mimicry.”
We should be very greatly surprised if abundant examples of “false mimicry” are not found among insects. We trust that this remark will stimulate some entomologist to pay attention to the subject.
It is the essence of Müllerian mimicry that both model and copy are immune from attack from enemies. Unfortunately for the theory, similar resemblances occur among birds of prey,where neither party can benefit from the association. This gives rise to what we may perhaps call false Müllerian mimicry. Thus the goshawk and peregrine falcon resemble each other in being brown above and streaked below in immature plumage, and having barred underparts and a grey upper plumage when adult.
Having stated the more important objections to the theory of protective mimicry, it now remains for us to deal specifically with each head of evidence offered in its favour.
1. With regard to the assertion that the model and its copy are often not nearly related, we have shown that among mammals and birds instances of resemblance between widely-separated groups occur under such circumstances that neither party can derive any benefit therefrom.
2. As regards the assertion that species which are mimicked are either well-defended or unpalatable, this certainly does not hold good with regard to some at any rate of the coincidental resemblances among birds which we have pointed out; even if these pairs of similar species lived in the same country it would require considerable ingenuity to say why one should mimic the other.
3. As regards the argument that the inedible species ofIthomiinæ, etc., display only fifteen colours, while the less numerous ediblePapiliosdisplay more than double this number of colours, we may draw attention to the fact that those birds which are most immune from attack are precisely those which display the smallest range as regards colour, e.g., hawks, owls, crows, gulls, storks, and cranes. As we have already submitted, no question of Müllerian association comes in here.
On the other hand, the eminently edible families of game-birds and ducks display great variety of colour, in the males at all events.
4. As regards the statement that although in many cases the mimetic resemblances extend to the minutest detail, they are not accompanied by any structural changes except such as assist in the production of a superficial likeness, we may refer to the case we have already cited of the New Zealand cuckoo, which, though it so closely copies an American hawk, is typically cuculine in structure. Here, of course, there can be no question of advantage to the “mimicking” cuckoo in the resemblances.
5. In answer to the argument that mimetic resemblance extends to form, attitude, and movement, as well as colour, and that deep-seated organs are affected only when the superficial resemblance is thereby intensified, we may draw attention to such cases as the following:—
(a) The harmless Indian Snake (Lycodon aulicus) is closely similar to the well-known Krait (Bungarus cœruleus),also Indian; but the resemblance extends to a structural detail which can hardly have mimetic value—namely, the harmless snake has long, fang-like front teeth, though these are unconnected with poison-glands. Animals which come into contact with the krait and its mimic are hardly likely to inspect their teeth.
(b) A considerable number of birds of the shrike group—known as Cuckoo-Shrikes (Campophaga)—closely resemble cuckoos in plumage; but even if they derive any benefit from mimicking birds which are credited with being mimics already, they cannot profit by the fact that the shafts of the rump-feathers in both groups are stiffened; this being a peculiarity which would not be perceptible until the bird was in the grasp of an aggressor.
(c) As a third case of coincidence we may refer to the tubercle in the nostril of the Brain-fever-bird (Hierococcyx varius), as a minute detail of hawk-like appearance, though not present in the particular species imitated.
6. The argument that mimetic resemblances are produced in the most diverse ways, but the result is uniform, loses much of its force when we consider the various methods by which short-tailed birds appear to have long caudal appendages.
In the peacock it is the upper tail coverts which are elongated; in the Stanley Crane(Tetrapteryx paradisea) it is the innermost or tertiary quills of wing; in one of the egrets some of the feathers of the upper back grow to a great length and form a train; in the Bird of Paradise (Paradisea apoda) the long flank plumes are commonly mistaken for the tail.
In these cases there can be no question of mimicry.
7. We have shown that the idea that imitator and imitated are always found in the same area is absolutely fallacious. In birds, for example, the most striking resemblances appear to occur between species that dwell far apart.
8. We can cite, as parallel to the case of a mimicking species of which the male copies one model and the female another, the strange similarity between the barred brown plumage of the female blackcock and that of the female eider-duck. The males of these species, although both black and white, differ greatly in appearance; but the male blackcock is admittedly very like the male of another species of sea-duck—the scoter.
9. Against the supposed ancestral non-mimetic forms existing on islands we can pit the “mimetic” orioles in small islands and their non-mimetic cousins on the mainland. In Australia an oriole of what appears to be an ancestral style lives beside, but declines to mimic, a friar bird of a very pronounced type.
10. The case of certain diurnal moths mimicking butterflies appears to be explicable without the aid of the theory of protective mimicry. When two species adopt the same method of obtaining food, it not infrequently happens that a professional likeness springs up between them. Of this the swifts and swallows afford a striking illustration.
11. As a set-off to the cases where the alleged mimicry is confined to certain seasons of the year, we may cite the case of the pheasant-tailed Jaçana (Hydrophasianus chirurgus), which in its winter plumage might easily be mistaken, when on the wing, for the paddy bird or Pond Heron (Ardeola grayii), both being of like size and having a brown back, long green legs, and white wings. Moreover, they are to be found in the same localities in India. At the breeding season, however, they are absolutely different in plumage.
Yet another argument commonly adduced in favour of the theory of protective mimicry is that local variations of the imitated species are sometimes followed by the imitator; thus the butterflyDanais chrysippusshows a white patch on the hind wings in Africa, and this is followed by its mimic.
But the same thing occurs, quite irrationally, so to speak, among birds. The peregrine falcon and hobby of Europe are only winter migrantsto India, where they are replaced as residents by the Shaheen (Falco peregrinator) and Indian Hobby (F. severus). Both these differ from the migratory forms by being blacker above and chestnut below, instead of cream colour. Thus the resemblance occurs in each race. A similar distinction, as noted by Blyth, exists between the Common Swallow (Hirundo rustica) and the Swallow (H. tytleri) of Eastern Asia, the latter having the whole ventral surface rufous instead of only the throat. Yet no one will suggest that swallows mimic falcons, or that there is mimicry between the peregrine and hobby. It is obvious that such parallel changes occur independently of mimicry.
The Water-rail (Rallus aquaticus) and Baillon’s Crake (Porzana bailloni) of Europe are distinguished from their allies of Eastern Asia by having the sides of the head plain grey, whereas the Eastern Asiatic forms (R. indicusandP. pusilla) have a brown streak along each side of the face. Here, again, we have an instance of birds of the same family varying together with geographical distribution.
One of the prettiest conceits of the Wallaceian school of zoologists is the theory of recognition markings.
“If,” writes Wallace, on page 217 ofDarwinism,“we consider the habits and life-histories of those animals which are more or less gregarious, comprising a large proportion of the herbivora, some carnivora, and a considerable number of all orders of birds, we shall see that a means of ready recognition of its own kind, at a distance or during rapid motion, in the dusk of twilight or in partial cover, must be of the greatest advantage and often lead to the preservation of life. Animals of this kind will not usually receive a stranger in their midst. While they keep together they are generally safe from attack, but a solitary straggler becomes an easy prey to the enemy; it is therefore of the highest importance that, in such a case, the wanderer should have every facility for discovering its companions with certainty at any distance within the range of vision.
“Some means of easy recognition must be of vital importance to the young and inexperienced of each flock, and it also enables the sexes to recognise their kind and thus avoid the evils of infertile crosses; and I am inclined to believe that its necessity has had a more widespread influence in determining the diversities of animal colouration than any other cause whatever. To it may probably be imputed the singular fact that whereas bilateral symmetry of colouration is very frequently lost among domesticated animals, it almost universally prevails in a state of nature;for if the two sides of an animal were unlike, and the diversity of colouration among domestic animals occurred in a wild state, easy recognition would be impossible among numerous closely allied forms.”
As examples of recognition colouration, Wallace cites, among others, the white upturned tail of the rabbit—a “signal flag of danger,” the conspicuous white patch displayed by many antelopes, the white marks on the wing- and tail-feathers of the British species of butcher-birds, the stone-chat, the whin-chat, and the wheat-ear.
Wallace therefore asserts, firstly, that recognition marks not only help herbivorous animals to keep together, but act as a danger signal; the member of a flock which first catches sight of the enemy takes to its heels, displaying its white flag, which is the signal of danger to the other members of the flock. Secondly, that recognition marks prevent the evils of infertile crosses. Thirdly, that the necessity of being able to recognise one another has rigidly preserved bilateral symmetry among animals in a state of nature.
As regards assertion number one, we would point out that where a flock of herbivora is being stalked by a beast of prey, the member of the flock nearest to the enemy—that is to say, the hindmost member—will probably be the first to observe him. As that creature will be more unfavourably situated for escape than the rest ofthe herd, it will not be to their advantage to follow the line it has taken. Moreover, being at the rear of the flock, it is not in a good position to take the lead, and its pursuer is likely to see the danger signal before its friends do. It would thus seem that “danger signals,” while possibly sometimes of service to their possessors, are on the whole ornaments which might profitably be dispensed with. Natural selection can scarcely be charged with the production of a character of such doubtful utility to the organism.
Moreover, flourishing species of many gregarious animals do not possess any “signal flag of danger,” while, on the other hand, a great many solitary species display markings that render them very conspicuous when in motion. Take the case of the famous Indian Paddy Bird (Ardeola grayii). This, when at rest, is coloured so as to be very difficult to distinguish from its surroundings, but flight transforms it, for it then displays its milk-white pinions, which would make a perfect danger signal, if only it were not peculiarly solitary in its habits. Its gregarious brethren, the Cattle Egrets (Bubulcus coromandus), on the other hand, display no danger signal.
That these recognition marks prevent the intercrossing of allied species and the production of infertile hybrids appears to be pure fiction. As we have already shown, hybrids between allied species are by no means always infertile.Moreover, species which differ only in colour seem usually to interbreed in those parts where they meet.
“This interbreeding,” writes Finn, on page 14 ofOrnithological and Other Oddities, “occurs where the carrion crow (Corvus corone) meets the hooded crow (Corvus cornix), where the European and Himalayan goldfinches (Carduelis carduelisandC. caniceps) encounter each other, and where the blue rollers of India and Burma (Coracias indicusandC. affinis) come into contact, to say nothing of other cases.”
Of these other cases, the Indian bulbuls of the genusMolpastesform a very remarkable one. In all places where two of the so-called species meet they appear to interbreed, and so freely do they interbreed that at the points where the allied species run into one another it is not possible to refer the bulbuls to either species. Thus William Jesse writes of the Madras Red-vented Bulbul (Molpastes hæmorrhous) (page 487 ofThe Ibisfor July 1902): “This bird, although I have given it the above designation, is not the trueM. hæmorrhous. I have examined numbers of skins and taken nests and eggs time after time, and have come to the conclusion that our type is very constant, and at the same time differs from all the red-vented bulbuls hitherto described. The dimensions tally with those given by Oates forM. hæmorrhous, while the black of the crownterminates rather abruptly on the hind neck, and is not extended along the back, as is the case withM. intermediusandM. bengalensis. On the other hand, as in the two last species, the ear coverts are chocolate. Furthermore, I may add—although I attach little importance to this—that the eggs of the Lucknow bird which I have seen are, without exception, far smaller than my eggs of genuineM. intermediusfrom the Punjab. My own opinion is that the Lucknow race is the result of a hybridisation between the other three species.”
Further, in Bannu, Mr D. Donald sawM. intermediusandM. leucogenyspaired at the same nest. That gentleman could not possibly be mistaken on the point, as the latter species has white cheeks and yellow under tail-coverts, while the cheeks of the former species are dark-coloured and the patch of feathers under the tail is red. Similarly, Whitehead and Magrath, writing of the birds of the Kurram Valley (Ibis, January 1909), record that the former shot no fewer than twelve bulbuls, which undoubtedly appear to be hybrids between these two species. As these hybrids differ considerablyinter se, there seems no room for doubt that they breed with one another and with the parent species.
Wallace’s third statement, that if the two sides of animals in a state of nature were alike, easy recognition would be impossible among numerousclosely allied forms, reminds us forcibly of the sad case of the boy whose tailor was his mother.Humanum est errare: she made her son one pair of trousers that fastened up behind, so that the poor boy when wearing them never knew whether he was going to or coming home from school! If animals are able to recognise their mates, their bilateral symmetry does not seem necessary to enable them to distinguish their fellows from allied species.
It is, indeed, true that asymmetrically marked animals are very rarely seen in the wild state, while they are the rule rather than the exception among domesticated species. But this appears to be due, not to the necessity of recognition markings in nature, but to the fact that those animals that display a tendency to massed pigment perish in the struggle for existence, since this massing of pigment appears to be correlated with weakness of constitution. In other words, this massing of pigment is an unfavourable variation, which under natural conditions dooms its possessor. In the easier circumstances of domestication, animals which are irregularly pigmented are able to survive, so that, among them, the almost universal tendency to the massing of pigment can be followed without let or hindrance.
It is unnecessary to say more upon this subject. The few facts we have set forth suffice to destroy this particular excrescence on the Darwinian theory.