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another property of these various upon the molecular structure of the radiations, that they are un qually body. Chemical action almost always refracted or bent in passing obliquely implies change of molecular structure, through transparent bodies, the longer hence chemical action is the most waves being least refracted, the shorter potent cause of change of colour. most. Hence it becomes possible to Sometimes simple solution in water analyse white or any other light into effects a marvellous change, as in the its component rays; a small ray of case of the well-known aniline dyes; the sunlight, for example, which would magenta and violet dyes exhibiting, produce a round white spot on a wall, when in the solid form,various shades of if passed through a prism is lengthened golden or bronzy metallic green. Heat out into a band of coloured light ex- again often produces change of colour, actly corresponding to the colours of and this without effecting any chemical the rainbow Any one colour can change. Mr. Ackroyd has recently thus be isolated and separately ex- investigated this subject, and has amined, and by means of reflecting shown that a large number of bodies mirrors the separate colours can be are changed by heat, returning to their again compounded in various ways, normal colour when cooled, and that and the resulting colours observed. this change is almost always in the This band of coloured light is called direction of the less refrangible rays a spectrum, and the instrument by or longer wave-lengths; and he conwhich the spectra of various kinds of nects the change with molecular exlight are examined is called a spectro- pansion caused by heat. As examples scope.
This branch of the subject has, may be mentioned mercuric oxide, however, no direct bearing on the which is orange-yellow, but which mode in which the colours of living changes to orange, red, and brown things are produced, and it has only when heated; chromic-oxide, which is been alluded to in order to complete green, and changes to yellow; cinnabar, our sketch of the nature of colour. which is scarlet, and changes to puce;
The colours which we perceive in and metaborate of copper, which is material substances are produced either blue, and changes to green and greenish by the absorption or by the inter- yellow. The colouring matters of ference of some of the rays which form animals are very varied.
Copper has white light. Pigmental or absorption- been found in the red of the wing of colours are the most frequent, com- the turaco, and Mr. Sorby has detected prising all the opaque tints of flowers no less than seven distinct colouring and insects, and all the colours of dyes matters in birds' eggs, several of which and pigments. They are caused by are chemically related to those of blood rays of certain wave-lengths being and bile. The same colours are often absorbed, while the remaining rays are produced by quite different substances reflected and give rise to the sensation in different groups, as shown by the of colour. When all the colour-pro- red of the wings of the burnet-moth ducing rays are reflected in due pro- changing to yellow with muriatic acid, portion the colour of the object is while the red of the red-admiral-butwhite, when all are absorbed the colour terfly undergoes no such change. is black. If blue rays only are absorbed These pigmental colours have a difthe resulting colour is orange-red ; and ferent character in animals according to generally, whatever colour an object their position in the integument. Folappears to us, it is because the com- lowing Dr. Hagen's classification, epiplementary colours are absorbed by it. dermal colours are those which exist in The reason why rays of only certain the external chitinised skin of insects, refrangibilities are reflected and the in the hairs of mammals, and, partially, rest of the incident light absorbed by 1 "Metachromatism, or Colour-Change," each substance, is supposed to depend Chemical News, August, 1876.
in the feathers of birds. They are object must be reflected in the same often very deep and rich, and do not proportions as they exist in solar light, fade after death. The hypodermal whereas if rays of any one or more colours are those which are situated kinds are absorbed or neutralised the in the inferior soft layer of the skin. resultant reflected light will be coloured, These are often of lighter and more and that this colour may be infinitely vivid tints, and usually fade after death. varied according to the proportions in Many of the reds and yellows of but- which different rays are reflected or terflies and birds belong to this class, absorbed, we should expect that white as well as the intensely vivid hues of would be, as it really is, comparatively the naked skin about the heads of rare and exceptional in nature. The many birds. These colours sometimes same observation will apply to black, exude through the pores, forming an which arises from the absorption of all evanescent bloom on the surface. the different rays.
Many of the Interference colours are less frequent complex substances which exist in in the organic world. They are caused animals and plants are subject to in two ways: either by reflection from changes of colour under the influence the two surfaces of transparent films, of light, heat, or chemical change, and as seen in the soap-bubble and in thin we know that chemical changes are films of oil on water; or by fine striæ continually occurring during the phywhich produce colours either by re- siological processes of development and flected or transmitted light, as seen in growth. We also find that every exmother-of-pearl and in finely-ruled ternal character is subject to minute metallic surfaces. In both cases colour changes, which are generally perceptible is produced by light of one wave-length to us in closely allied species; and we being neutralised, owing to one set of can therefore have no doubt that the such waves being caused to be half a extension and thickness of the transwave length behind the other set, as parent lamellæ, and the fineness of the may be found explained in any treatise striæ or rugosities of the integuments, on physical optics. The result is, that must be undergoing constant minute the complementary colour of that neu- changes; and these changes will very tralised is seen; and as the thickness frequently produce changes of colour. of the film or the fineness of the striæ These considerations render it probable undergo slight changes almost any that colour is a normal and even necescolour can be produced. This is be- sary result of the complex structure of lieved to be the origin of many of the animals and plants, and that those glossy or metallic tints of insects, as parts of an organism which are underwell as of those of the feathers of some going continual development and adapbirds. The iridescent colours of the tation to new conditions, and are also wings of dragon-flies are caused by the continually subject to the action of superposition of two or more trans- light and heat, will be the parts in parent lamellæ ; while the shining blue which changes of colour will most of the Purple-Emperor and other but- frequently appear. Now there is little terflies, and the intensely metallic doubt that the external changes of colours of humming birds are probably animals and plants in adaptation to the due to fine striæ.
environment are much more numerous This outline sketch of the nature of than the internal changes, as seen in colour in the animal world, however the varied character of the integuimperfect, will at least serve to show ments and appendages of animals us how numerous and varied are the hair, horns, scales, feathers, &c. &c.— causes which perpetually tend to the and in plants, the leaves, bark, flowers, production of colour in animal tissues. and fruit, with their various appendIf we consider, that in order to produce ages,-compared with the comparative white all the rays which fall upon an uniformity of the texture and composition of their internal tissues; and or more kinds of rays. This is well this accords with the uniformity of the exemplified by the comparative diffitints of blood, muscle, nerve, and bone culty of producing definite pure tints throughout extensive groups, as com- by the mixture of two or more pigpared with the great diversity of colour ments, while a hap-hazard mixture of of their external organs.
It seems a a number of these will be almost sure fair conclusion that colour per se may to produce browns, olives, or other be considered to be normal, and to neutral or dirty colours. An indefineed no special accounting for, while nite or irregular absorption of some the absence of colour (that is, either rays and reflection of others would, white or black), or the prevalence of therefore, produce obscure tints ; while certain colours to the constant exclu- pure and vivid colours would require sion of others, must be traced, like a perfectly definite absorption of one other modifications in the economy of portion of the coloured rays, leaving living things, to the needs of the the remainder to produce the true species. Or, looking at it in another complementary colour. This being the aspect, we may say, that amid the
case we may expect these brown tints constant variations of animals and to occur when the need of protection plants colour is ever tending to vary is very slight or even when it does not and to appear where it is absent, and exist at all, always supposing that that natural selection is constantly bright colours are not in any way eliminating such tints as are injuri- useful to the species. But whenever ous to the species, or preserving and a pure colour is protective, as green in intensifying such as are useful. tropical forests or white among arctic
This view is in accordance with the snows, there is no difficulty in prowell-known fact, of colours which rarely ducing it, by natural selection acting or never appear in the species in a on the innumerable slight variations state of nature continually occurring of tint which are ever occurring. Such among domesticated animals and culti- variations may, as we have seen, be vated plants; showing us that the produced in a great variety of ways ; capacity to develop colour is ever either by chemical changes in the present, so that almost any required secretions or by molecular changes in tint can be produced which may, under surface structure, and may be brought changed conditions, be useful, in how- about by change of food, by the photoever small a degree.
graphic action of light, or by the norLet us now see how these principles mal process of generative variation. will enable us to understand and ex- Protective colours therefore, however plain the varied phenomena of colour curious and complex they may be in in nature, taking them in the order of certain cases, offer no real difficulties. our functional classification of colours Theory of Warning Colours. These (p. 389).
differ greatly from the last class, inTheory of Protective Colours. We asmuch as they present us with & have seen that obscure or protective variety of brilliant hues, often of the tints in their infinitely varied degrees greatest purity, and combined in are present in every part of the animal striking contrasts and conspicuous kingdom, whole families or genera patterns. Their use depends upon being often thus coloured. Now the their boldness and visibility, not on various brown, earthy, ashy, and other the presence of any one colour; hence neutral tints are those which would be we find among these groups some of most readily produced, because they are the most exquisitely-coloured objects due to an irregular mixture of many in nature.
Many of the uneatable kinds of rays; while pure tints require caterpillars are strikingly beautiful; either rays of one kind only, or definite while the Danaidæ, Heliconidæ, and mixtures in proper proportions of two protected groups of Papilionidæ comprise a series of butterflies of the most be sure they are of vast antiquity and brilliant and contrasted colours. The have undergone great modification. A bright colours of many of the sea- large number of them, however, are anemones and sea-slugs will probably still of comparatively plain colours, be found to be in this sense protective, often rendered extremely elegant by serving as a warning of their uneat- the delicate transparency of the wingableness. On our theory none of these membrane, but otherwise not at all colours offer any difficulty. Conspicu- conspicuous. Many have only dusky ousness being useful, every variation or purplish bands or spots, others have tending to brighter and purer colours patches of reddish or yellowish brown was selected, the result being the -perhaps the commonest colour among beautiful variety and contrast we find. butterflies; while a considerable num
But when we come to those groups ber are tinged or spotted with yellow, which gain protection solely by being also a very common colour, and one mistaken for some of these brilliantly especially characteristic of the Pieridæ, coloured but uneatable creatures, a the family to which Leptalis belongs. difficulty really exists, and to many We may therefore reasonably suppose minds is so great as to be insuperable. that in the early stages of the developIt will be well therefore to endeavour ment of the Danaidæ, when they first to explain how the resemblance in began to acquire those nauseous secrequestion may have been brought about. tions which are now their protection, The most difficult case, which may be their colours were somewhat plain, taken as a type of the whole, is either dusky with paler bands and that of the genus Leptalis (a group spots, or yellowish with dark borders, of South American butterflies allied to and sometimes with reddish bands or our common white and yellow kinds), spots. At this time they had probably many of the larger species of which shorter wings and a more rapid flight, are still white or yellow, and which just like the other unprotected families are all eatable by birds and other of butterflies. But as soon as they insectivorous creatures. But there are became decidedly unpalatable to any also a number of species of Leptalis, of their enemies, it would be an which are brilliantly red, yellow, and advantage to them to be readily disblack, and which, band for band and tinguished from all the eatable kinds; spot for spot, resemble some one of the and as butterflies were no doubt already Danaidæ or Heliconidae which inhabit very varied in colour, while all probthe same district and which are nau- ably had wings adapted for pretty seous and uneatable. Now the common rapid or jerking flight, the best disobjection is, that a slight approach to tinction might have been found in one of these protected butterflies would outline and habits; whence would be of no use, while a greater sudden arise the preservation of those varievariation is not admissible on the ties whose longer wings, bodies, and theory of gradual change by indefinite antennæ, and slower fight rendered slight variations. This objection de- them noticeable, — characters which pends almost wholly on the supposition now distinguish the whole
in that when the first steps towards mi- every part of the world. Now it micry occurred, the South American would be at this stage that some of Danaidæ were what they are now, the weaker-flying Pieridæ which hapwhile the ancestors of the Leptalides pened to resemble some of the Danwere like the ordinary white or yellow aidæ around them in their yellow Pieridæ to which they are allied. and dusky tints and in the general But the danaioid butterflies of South outline of their wings, would be America are so immensely numerous sometimes mistaken for them by the and so greatly varied, not only in common enemy, and would thus gain colour but in structure, that we may an advantage in the struggle for exist
ence. Admitting this one step to be Theory of Sexual Colours.-In Mr. made, and all the rest must inevitably Darwin's celebrated work, The Descent follow from simple variation and sur- of Man and Selection in Relation to Sex, vival of the fittest. So soon as the he has treated of sexual colour in nauseous butterfly varied in form or combination with other sexual characcolour to such an extent that the cor- ters, and has arrived at the conclusion responding eatable butterfly no longer that all or almost all the colours of closely resembled it, the latter would the higher animals (including among be exposed to attacks, and only those these insects and all vertebrates) are variations would be preserved which due to voluntary sexual selection; and kept up the resemblance. At the same that diversity of colour in the sexes is time we may well suppose the enemies due, primarily, to the transmission of to become more acute and able to colour-variations either to one sex only detect smaller differences than at first. or to both sexes, the difference dependThis would lead to the destruction of ing on some unknown law, and not all adverse variations, and thus keep being due to natural selection. up in continually increasing complexity I have long held this portion of Mr. the outward mimicry which now so Darwin's theory to be erroneous, and
During the long ages in have argued that the primary cause of which this process
has been going on, sexual diversity of colour was the need many a Leptalis may have become of protection, repressing in the female extinct from not varying sufficiently those bright colours which are normally in the right direction and at the right produced in both sexes by general time to keep up a protective resem- laws; and I have attempted to explain blance to its neighbour; and this will many of the more difficult cases on accord with the comparatively small this principle (“ A Theory of Birds' number of cases of true mimicry as
Nests," in Contributions, &c., p. 231). compared with the frequency of those As I have since given much thought protective resemblances to vegetable to this subject, and have arrived at or inorganic objects whose forms are some views which appear to me to be less definite and colours less change- of considerable importance, it will be able. About a dozen other genera
well to sketch briefly the theory I now of butterflies and moths mimic the hold, and afterwards show its applicaDanaidæ in various parts of the world, tion to some of the detailed cases adand exactly the same explanation will duced in Mr. Darwin's work. apply to all of them. They represent The very frequent superiority of the those species of each group which at
male bird or insect in brightness or the time when the Danaidæ first intensity of colour, even when the acquired their protective secretions general tints and coloration are the happened outwardly to resemble some same, now seem to me to be due to the of them, and have by concurrent vari- greater vigour and activity and the ation, aided by a rigid selection, been higher vitality of the male. The able to keep up that resemblance to colours of an animal usually fade the present day.1
during disease or weakness, while
robust health and vigour adds to their 1 For fuller information on this subject the intensity. This intensity of colorareader should consult Mr. Bates's original paper, tion is most manifest in the male “ Contributions to an Insect-fauna of the Amazon Valley," in Transactions of the Linnean
during the breeding season, when the Society, vol. xxiii. p. 495; Mr. Trimen's paper
vitality is at a maximum. It is also in vol. xxvi. p. 497 ; the author's essay on very manifest in those cases in which “Mimicry,” &c., already referred to ; and, in the male is smaller than the female, as the absence of collections of butterflies, the plates of Heliconidæ and Leptalidæ, in
in the hawks and in most butterflies Hewitson's Erotic Butterflies, and Felder's
and moths. The same phenomena occur, Voyage of the “Novara,” may be examined. though in a less marked degree, among