Darwinism (1889) Part 16

_Summary of the Preceding Exposition._

Before proceeding to discuss some of the more recondite phenomena of animal coloration, it will be well to consider for a moment the extent of the ground we have already covered. Protective coloration, in some of its varied forms, has not improbably modified the appearance of one-half of the animals living on the globe. The white of arctic animals, the yellowish tints of the desert forms, the dusky hues of crepuscular and nocturnal species, the transparent or bluish tints of oceanic creatures, represent a vast host in themselves; but we have an equally numerous body whose tints are adapted to tropical foliage, to the bark of trees, or to the soil or dead leaves on or among which they habitually live.

Then we have the innumerable special adaptations to the tints and forms of leaves, or twigs, or flowers; to bark or moss; to rock or pebble; by which such vast numbers of the insect tribes obtain protection; and we have seen that these various forms of coloration are equally prevalent in the waters of the seas and oceans, and are thus coextensive with the domain of life upon the earth. The comparatively small numbers which possess "terrifying" or "alluring" coloration may be cla.s.sed under the general head of the protectively coloured.

But under the next head--colour for recognition--we have a totally distinct category, to some extent antagonistic or complementary to the last, since its essential principle is visibility rather than concealment. Yet it has been shown, I think, that this mode of coloration is almost equally important, since it not only aids in the preservation of existing species and in the perpetuation of pure races, but was, perhaps, in its earlier stages, a not unimportant factor in their development. To it we owe most of the variety and much of the beauty in the colours of animals; it has caused at once bilateral symmetry and general permanence of type; and its range of action has been perhaps equally extensive with that of coloration for concealment.

_Influence of Locality or of Climate on Colour._

Certain relations between locality and coloration have long been noticed. Mr. Gould observed that birds from inland or continental localities were more brightly coloured than those living near the sea-coast or on islands, and he supposed that the more brilliant atmosphere of the inland stations was the explanation of the phenomenon.[90] Many American naturalists have observed similar facts, and they a.s.sert that the intensity of the colours of birds and mammals increases from north to south, and also with the increase of humidity.

This change is imputed by Mr. J.A. Allen to the direct action of the environment. He says: "In respect to the correlation of intensity of colour in animals with the degree of humidity, it would perhaps be more in accordance with cause and effect to express the law of correlation as a _decrease_ of intensity of colour with a _decrease_ of humidity, the paleness evidently resulting from exposure and the blanching effect of intense sunlight, and a dry, often intensely heated atmosphere. With the decrease of the aqueous precipitation the forest growth and the protection afforded by arborescent vegetation gradually also decreases, as of course does also the protection afforded by clouds, the excessively humid regions being also regions of extreme cloudiness, while the dry regions are comparatively cloudless districts."[91] Almost identical changes occur in birds, and are imputed by Mr. Allen to similar causes.

It will be seen that Mr. Gould and Mr. Allen impute opposite effects to the same cause, brilliancy or intensity of colour being due to a brilliant atmosphere according to the former, while paleness of colour is imputed by the latter to a too brilliant sun. According to the principles which have been established by the consideration of arctic, desert, and forest animals respectively, we shall be led to conclude that there has been no direct action in this case, but that the effects observed are due to the greater or less need of protection. The pale colour that is prevalent in arid districts is in harmony with the general tints of the surface; while the brighter tints or more intense coloration, both southward and in humid districts, are sufficiently explained by the greater shelter due to a more luxuriant vegetation and a shorter winter. The advocates of the theory that intensity of light directly affects the colours of organisms, are led into perpetual inconsistencies. At one time the brilliant colours of tropical birds and insects are imputed to the intensity of a tropical sun, while the same intensity of sunlight is now said to have a "bleaching" effect. The comparatively dull and sober hues of our northern fauna were once supposed to be the result of our cloudy skies; but now we are told that cloudy skies and a humid atmosphere intensify colour.

In my _Tropical Nature_ (pp. 257-264) I have called attention to what is perhaps the most curious and decided relation of colour to locality which has yet been observed--the prevalence of white markings in the b.u.t.terflies and birds of islands.

So many cases are adduced from so many different islands, both in the eastern and western hemisphere, that it is impossible to doubt the existence of some common cause; and it seems probable to me now, after a fuller consideration of the whole subject of colour, that here too we have one of the almost innumerable results of the principle of protective coloration. White is, as a rule, an uncommon colour in animals, but probably only because it is so conspicuous. Whenever it becomes protective, as in the case of arctic animals and aquatic birds, it appears freely enough; while we know that white varieties of many species occur occasionally in the wild state, and that, under domestication, white or parti-coloured breeds are freely produced. Now in all the islands in which exceptionally white-marked birds and b.u.t.terflies have been observed, we find two features which would tend to render the conspicuous white markings less injurious--a luxuriant tropical vegetation, and a decided scarcity of rapacious mammals and birds. White colours, therefore, would not be eliminated by natural selection; but variations in this direction would bear their part in producing the recognition marks which are everywhere essential, and which, in these islands, need not be so small or so inconspicuous as elsewhere.

_Concluding Remarks._

On a review of the whole subject, then, we must conclude that there is no evidence of the individual or prevalent colours of organisms being directly determined by the amount of light, or heat, or moisture, to which they are exposed; while, on the other hand, the two great principles of the need of concealment from enemies or from their prey, and of recognition by their own kind, are so wide-reaching in their application that they appear at first sight to cover almost the whole ground of animal coloration. But, although they are indeed wonderfully general and have as yet been very imperfectly studied, we are acquainted with other modes of coloration which have a different origin. These chiefly appertain to the very singular cla.s.s of warning colours, from which arise the yet more extraordinary phenomena of mimicry; and they open up so curious a field of inquiry and present so many interesting problems, that a chapter must be devoted to them. Yet another chapter will be required by the subject of s.e.xual differentiation of colour and ornament, as to the origin and meaning of which I have arrived at different conclusions from Mr. Darwin. These various forms of coloration having been discussed and ill.u.s.trated, we shall be in a position to attempt a brief sketch of the fundamental laws which have determined the general coloration of the animal world.

FOOTNOTES:

[Footnote 65: _Proceedings of the Royal Society_, No. 243, 1886; _Transactions of the Royal Society_, vol. clxxviii. B. pp. 311-441.]

[Footnote 66: _A Naturalist's Wanderings in the Eastern Archipelago_, p.

460.]

[Footnote 67: _Trans. Phil. Soc._ (? _of S. Africa_), 1878, part iv, p.

27.]

[Footnote 68: _Proc. Zool. Soc._, 1862 p. 357.]

[Footnote 69: With reference to this general resemblance of insects to their environment the following remarks by Mr. Poulton are very instructive. He says: "Holding the larva of Sphinx ligustri in one hand and a twig of its food-plant in the other, the wonder we feel is, not at the resemblance but at the difference; we are surprised at the difficulty experienced in detecting so conspicuous an object. And yet the protection is very real, for the larvae will be pa.s.sed over by those who are not accustomed to their appearance, although the searcher may be told of the presence of a large caterpillar. An experienced entomologist may also fail to find the larvae till after a considerable search. This is general protective resemblance, and it depends upon a general harmony between the appearance of the organism and its whole environment. It is impossible to understand the force of this protection for any larva, without seeing it on its food-plant and in an entirely normal condition.

The artistic effect of green foliage is more complex than we often imagine; numberless modifications are wrought by varied lights and shadows upon colours which are in themselves far from uniform. In the larva of Papilio machaon the protection is very real when the larva is on the food-plant, and can hardly be appreciated at all when the two are apart." Numerous other examples are given in the chapter on "Mimicry and other Protective Resemblances among Animals," in my _Contributions to the Theory of Natural Selection_.]

[Footnote 70: _The Naturalist in Nicaragua_, p. 19.]

[Footnote 71: R. Meldola, in _Proc. Zool. Soc._, 1873, p. 155.]

[Footnote 72: _Nature_, vol. iii. p. 166.]

[Footnote 73: _Trans. Ent. Soc. Lond._, 1878, p. 185.]

[Footnote 74: _Ibid._ (_Proceedings_, p. xlii.)]

[Footnote 75: Wallace's _Malay Archipelago_, vol. i. p. 204 (fifth edition, p. 130), with figure.]

[Footnote 76: Moseley's _Notes by a Naturalist on the Challenger_.]

[Footnote 77: _Proceedings of the Boston Soc. of Nat. Hist._, vol. xiv.

1871.]

[Footnote 78: _Nature_, 1870, p. 376.]

[Footnote 79: _A Naturalist's Wanderings in the Eastern Archipelago_, p.

63.]

[Footnote 80: A beautiful drawing of this rare insect, Hymenopus bicornis (in the nymph or active pupa state), was kindly sent me by Mr.

Wood-Mason, Curator of the Indian Museum at Calcutta. A species, very similar to it, inhabits Java, where it is said to resemble a pink orchid. Other Mantidae, of the genus Gongylus, have the anterior part of the thorax dilated and coloured either white, pink, or purple; and they so closely resemble flowers that, according to Mr. Wood-Mason, one of them, having a bright violet-blue prothoracic s.h.i.+eld, was found in Pegu by a botanist, and was for a moment mistaken by him for a flower. See _Proc. Ent. Soc. Lond._, 1878, p. liii.]

[Footnote 81: C. Dixon, in Seebohm's _History of British Birds_, vol.

ii. Introduction, p. xxvi. Many of the other examples here cited are taken from the same valuable work.]

[Footnote 82: See A.H.S. Lucas, in _Proceedings of Royal Society of Victoria_, 1887, p. 56.]

[Footnote 83: Professor Wm.H. Brewer of Yale College has shown that the white marks or the spots of domesticated animals are rarely symmetrical, but have a tendency to appear more frequently on the left side. This is the case with horses, cattle, dogs, and swine. Among wild animals the skunk varies considerably in the amount of white on the body, and this too was found to be usually greatest on the left side. A close examination of numerous striped or spotted species, as tigers, leopards, jaguars, zebras, etc., showed that the bilateral symmetry was not exact, although the general effect of the two sides was the same. This is precisely what we should expect if the symmetry is not the result of a general law of the organisation, but has been, in part at least, produced and preserved for the useful purpose of recognition by the animal's fellows of the same species, and especially by the s.e.xes and the young. See _Proc. of the Am. a.s.s. for Advancement of Science_, vol.

x.x.x. p. 246.]

[Footnote 84: _Descent of Man_, p. 542.]

[Footnote 85: It may be thought that such extremely conspicuous markings as those of the zebra would be a great danger in a country abounding with lions, leopards, and other beasts of prey; but it is not so. Zebras usually go in bands, and are so swift and wary that they are in little danger during the day. It is in the evening, or on moonlight nights, when they go to drink, that they are chiefly exposed to attack; and Mr.

Francis Galton, who has studied these animals in their native haunts, a.s.sures me, that in twilight they are not at all conspicuous, the stripes of white and black so merging together into a gray tint that it is very difficult to see them at a little distance. We have here an admirable ill.u.s.tration of how a glaringly conspicuous style of marking for recognition may be so arranged as to become also protective at the time when protection is most needed; and we may also learn how impossible it is for us to decide on the inutility of any kind of coloration without a careful study of the habits of the species in its native country.]

[Footnote 86: The principle of colouring for recognition was, I believe, first stated in my article on "The Colours of Animals and Plants" in Macmillan's _Magazine_, and more fully in my volume on _Tropical Nature_. Subsequently Mrs. Barber gave a few examples under the head of "Indicative or Banner Colours," but she applied it to the distinctive colours of the males of birds, which I explain on another principle, though this may a.s.sist.]

[Footnote 87: Quoted by Darwin in _Descent of Man_, p. 317.]

[Footnote 88: In the _American Naturalist_ of March 1888, Mr. J.E. Todd has an article on "Directive Coloration in Animals," in which he recognises many of the cases here referred to, and suggests a few others, though I think he includes many forms of coloration--as "paleness of belly and inner side of legs"--which do not belong to this cla.s.s.]

[Footnote 89: For numerous examples of this protective colouring of marine animals see Moseley's _Voyage of the Challenger_, and Dr. E.S.

Morse in _Proc. of Bost. Soc. of Nat. Hist._, vol. xiv. 1871.]

[Footnote 90: See _Origin of Species_, p. 107.]

[Footnote 91: The "Geographical Variation of North American Squirrels,"

_Proc. Bost. Soc. of Nat. Hist._, 1874, p. 284; and _Mammals and Winter Birds of Florida_, pp. 233-241.]

CHAPTER IX

WARNING COLORATION AND MIMICRY