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Show 228 DARWINISM CIIAP. or dead leaves on or among which they habitun.lly live. Then we have the innumerable special adaptn.tions to the tints ancl forms of len.ves, or twigs, or flowers; to bn.rk or moss; to rock or pebble; by which such vast numbers of the in~ect tribes obtn.in protection; and we have seen thn.t these vanous forms of colomtion n.re eqnn.lly prevalent in the waters of the sea. n.nd oceans, n.nd are thus coextensive with the domain of lifo upon the earth. The comparatively small numbers which possess "terrifying" or "alluring" coloration may be cbs. eel under the gcneml hcn.d of the protectively coloured. But under the next hen.d-colour for recognition-we have n. totally distinct cn.tcgory, to some extent antagoni.tic or complementn.ry to the ln.st, since its essentia,l principle ]:-; visibility rather thn.n conccn.lment. Yet it has been shown, I think, thn.t this mode of coloration is almost equally important, since it not only aids in the preservation of exi ting species and in the perpetuation of pure races, hut 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 geneml permanence of type ; and its range of action has been perhaps equally extcnnivc with that of coloration for concealment. Influence of Locality or of Climate on ColoU?·. Certain relations between locality and coloration have long been noticed. Mr. Gould observed that birds from inland or continental localities were more brightly coloured th,m 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. 1 Many American natural ists have observed similar facts, and they assert that the intensity of the colours of birds and mammals increases from north 1o south, and also with the increase of humidity. This ch;_wgc is imputed by Mr. J. A. Allen to the direct n.ction of tho environment. He says: "In respect to the correlation of in tcw:;it.y 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 1 See Origin of Species, p. 107. VIII ORIGIN AND USES OF COLOUR IN ANIMALS 229 a decrease of humidity, the paleness evidently resulting from exposure and the blanching efl'ect of intense sunlight, and a dry, often intensely heated atmosphere. With the decrease. of the aqueous precipitation the forest growth and the protect10n afforded by arborescent vegetation gradually also decreases, as of course docs also the protection afforded by clouds, the excessively humid regions being also regions of extreme cloudiness, while the dry regions arc comparatively cloudless districts." 1 Almost identical changes occur in birds, and arc 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 colom that is prevalent in arid districts is in harmony with the aeneral 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 clue 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 ; hut now we are told that cloudy skies and a, humid atmosphere intensify colour. In my 1'r·opical Natu1·e (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 butterflies and birds of islands. 1 The" Geographicrrl Vari ation of North Ameri crrn Squirreb," J>roc. Bost" Soc. of Nat.Jlist., 1874, p. 284; aud .illammctl.s and Winter B irds of FlO'rida,pp· 233-241. |
