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Show 196 DARWINISM OIIAP. when they are not brought into action, is a dirty white. These animals arc exccssi vcly sluggish and defenceless, and the power of changing their colour to that of their immediate surroundings is no doubt of great service to them. Many of the fla,tfish are also capable of changing their colour according to tho colour of the bottom they rest on; and frogs have :L similar power to a limited extent. Some crustacea al:o change colour, and the power is much developed in the Chameleon shrimp (Mysis Chamooleon) which is gray when 0 11 sand, but brown or green when among brown or green seaweed. It has been proved by experiment that when this anima.] is blinded the change does not occur. In all these cases, therefore, we have some form of reflex or sense action by which the change is produced, probably by means of pigment cells beneath the skin as in the chameleon. The second class consists of certain larvoo, and pupoo, which undergo changes of colour when exposed to differently coloured surroundings. This subject has been carefnlly investigated by Mr. E. B. Poulton, who has communicat('d the results of his experiments to the Royal Socicty.1 It had been noticed that some species of larvro which fed on several different plants had colours more or less corresponding to the particular plant the individual fed on. Numerous cases are given in Professor Meldola's article on "Variable Protccti ve Colouring" (P1·oc. Zool. Soc., 1873, p. 153), and while the general green coloration was attributed to the presence of chlorophyll beneath the skin, the particular change in coJTespondence to each food-plant was attributed to a special function which had been developed by natural selection. Later on, in a note to his translation of Weissmann's Th r'llt'.IJ of Descent, Professor Meldola seemed disposed to think Lhat the variations of colour of some of the species might ],e phytophagic-that is, due to the direct action of the differently coloured leaves on which the insect fed. Mr. Poulton's experiments have thrown much light on this question, since he has conclusively proved that, in the case of the sphinx caterpillar of Smerinthus ocellatus, the change of colour is not due to the food but to the coloured light reflected from the leaves. 1 Proceedings of the Royal Society, No. 243, 1886; Transactions of the Ruyal Society, vol. clxxviii. B. pp. 311-441. VI![ ORIGIN AND USES OF COLOUR IN ANIMALS 197 This was shown by feeding two sets of larvro on the same plant but exposed to differently coloured surroundings, obta.ined by sewing the leaves together, so that in one case only the dark upper surface, in the other the whitish under surface was exposed to view. The result in each case was a corresponding change of colour in the larvoo, confirming the experiments on different individuals of the same batch of larvro which had been supplied with difl'erent food-plants or exposed to a different coloured light. An even more interesting series of experiments was made on the colours of pupre, which in many cases were known to be affected by the material on which they underwent their transformations. The late Mr. T. W. Wood proved, in 1867, that the pupro of the common cabbage butterflies (Pieris brassiere and P. rapoo) were either light, or dark, or green, according to tho coloured boxes they were kept in, or the colours of the fences, walls, etc., against which they were suspended. Mrs. Barber in South Africa. found that the pupoo of Papilio Nireus underwent a similar change, being deep green when attached to orange leaves of the same tint, pale yellowish-green when on a branch of the bottle-brush tree whose half-dried leaves were of this colour, and yellowish when attached to the wooden frame of a box. A few other observers noted similar phenomena, but nothing more was done till Mr. Poulton's elabomte series of experiments with the larvoo of several of our common butterflies were the means of clearing up several important points. He showed that the action of the coloured light did not affect the pupa itself but the larva, and that only for a limited period of time. After a caterpillar has done feeding it wanders about seeking :L suitable place to undergo its transformation. When this is found it rest::; qnictly for a da.y or two, spinning the web from which it is to suspend i tsclf ; :uul it is cluri ng thi. period of quiescence, and perhaps a1so the first hour or two after its susr ension, that the action of the surroumling coloured surfaces determines, to a consiJcrable extent, the colour of the pupa. By the application of various surrounding colours during this period, Mr. Poulton was able to modify the colour of the pupa of the common tortoise-shell hnttcrAy from nearly black to pale, or to a brilliant golden; and that of Pieris rapre |