whites and yellows, and the Satyridae or ringlets-we shall find no great disproportion in colour between those of temperate and tropical regions.

The various facts which have now briefly been noticed are sufficient to indicate that the light and heat of the sun are not the direct causes of the colours of animals, although they may favour the production of colour when, as in tropical regions, the persistent high temperature favours the development of the maximum of life. We will now consider the next suggestion, that light reflected from surrounding coloured objects tends to produce corresponding colours in the animal world.

This theory is founded on a number of very curious facts which prove, that such a change does sometimes occur and is directly dependent on the colours of surrounding objects; but these facts are comparatively rare and exceptional in their nature, and the same theory will certainly not apply to the infinitely varied colours of the higher animals, many of which are exposed to a constantly varying amount of light and colour during their active existence. A brief sketch of these dependent changes of colour may, however, be advantageously given here.

Variable Protective Colouring.

There are two distinct kinds of change of colour in animals due to the colouring of the environment. In one case the change is caused by reflex action set up by the animal seeing the colour to be imitated, and the change produced can be altered or repeated as the animal changes its position. In the other case the change occurs but once, and is probably not due to any conscious or sense action, but to some direct influence on the surface tissues while the creature is undergoing a moult or change to the pupa form.

The most striking example of the first class is that of the chameleon, which changes to white, brown, yellowish, or green, according to the colour of the object on which it rests. This change is brought about by means of two layers of pigment cells, deeply seated in the skin, and of bluish and yellowish colours. By suitable muscles these cells can be forced upwards so as to modify the colour of the skin, which,

when they are not brought into action, is a dirty white. These animals are excessively 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 flatfish are also capable of changing their colour according to the colour of the bottom they rest on; and frogs have a similar power to a limited extent. Some crustacea also change colour, and the power is much developed in the Chameleon shrimp (Mysis Chamæleon) which is gray when on sand, but brown or green when among brown or green seaweed. It has been proved by experiment that when this animal 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 larvæ, and pupa, which undergo changes of colour when exposed to differently coloured surroundings. This subject has been carefully investigated by Mr. E. B. Poulton, who has communicated the results of his experiments to the Royal Society.1 It had been noticed that some species of larvæ 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 Protective Colouring" (Proc. 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 correspondence 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 Theory of Descent, Professor Meldola seemed disposed to think that the variations of colour of some of the species might be 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 Royal Society, vol. clxxviii. B. pp. 311-441.

This was shown by feeding two sets of larvæ on the same plant but exposed to differently coloured surroundings, obtained 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 larvæ, confirming the experiments on different individuals of the same batch of larvæ which had been supplied with different food-plants or exposed to a different coloured light.

An even more interesting series of experiments was made on the colours of pupa, 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 pupæ of the common cabbage butterflies (Pieris brassica and P. rape) were either light, or dark, or green, according to the 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 pupæ 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 elaborate series of experiments with the larvae 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 a suitable place to undergo its transformation. When this is found it rests quietly for a day or two, spinning the web from which it is to suspend itself; and it is during this period of quiescence, and perhaps also the first hour or two after its suspension, that the action of the surrounding coloured surfaces determines, to a considerable 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 butterfly from nearly black to pale, or to a brilliant golden; and that of Pieris rapæ

from dusky through pinkish to pale green. It is interesting to note, that the colours produced were in all cases such only as assimilated with the surroundings usually occupied by the species, and also, that colours which did not occur in such surroundings, as dark red or blue, only produced the same effects as dusky or black.

Careful experiments were made to ascertain whether the effect was produced through the sight of the caterpillar. The ocelli were covered with black varnish, but neither this, nor cutting off the spines of the tortoise-shell larva to ascertain whether they might be sense-organs, produced any effect on the resulting colour. Mr. Poulton concludes, therefore, that the colour-action probably occurs over the whole surface of the body, setting up physiological processes which result in the corresponding colour-change of the pupa. Such changes are, however, by no means universal, or even common, in protectively coloured pupæ, since in Papilio machaon and some others which have been experimented on, both in this country and abroad, no change can be produced on the pupa by any amount of exposure to differently coloured surroundings. It is a curious point that, with the small tortoise-shell larva, exposure to light from gilded surfaces produced pupa with a brilliant golden lustre; and the explanation is supposed to be that mica abounded in the original habitat of the species, and that the pupa thus obtained protection when suspended against micaceous rock. Looking, however, at the wide range of the species and the comparatively limited area in which micaceous rocks occur, this seems a rather improbable explanation, and the occurrence of this metallic appearance is still a difficulty. It does not, however, commonly occur in this country in a natural state.

The two classes of variable colouring here discussed are evidently exceptional, and can have little if any relation to the colours of those more active creatures which are continually changing their position with regard to surrounding objects, and whose colours and markings are nearly constant throughout the life of the individual, and (with the exception of sexual differences) in all the individuals of the species. We will now briefly pass in review the various characteristics and uses of the colours which more generally prevail in nature;

and having already discussed those protective colours which serve to harmonise animals with their general environment, we have to consider only those cases in which the colour resemblance is more local or special in its character.

Special or Local Colour Adaptations.

This form of colour adaptation is generally manifested by markings rather than by colour alone, and is extremely prevalent both among insects and vertebrates, so that we shall be able to notice only a few illustrative cases. Among our native birds we have the snipe and woodcock, whose markings and tints strikingly accord with the dead marsh vegetation among which they live; the ptarmigan in its summer dress is mottled and tinted exactly like the lichens which cover the stones of the higher mountains; while young unfledged plovers are spotted so as exactly to resemble the beach pebbles among which they crouch for protection, as beautifully exhibited in one of the cases of British birds in the Natural History Museum at South Kensington.

In mammalia, we notice the frequency of rounded spots on forest or tree haunting animals of large size, as the forest. deer and the forest cats; while those that frequent reedy or grassy places are striped vertically, as the marsh antelopes and the tiger. I had long been of opinion that the brilliant yellow and black stripes of the tiger were adaptive, but have only recently obtained proof that it is so. An experienced tiger-hunter, Major Walford, states in a letter, that the haunts of the tiger are invariably full of the long grass, dry and pale yellow for at least nine months of the year, which covers the ground wherever there is water in the rainy season, and he adds: “I once, while following up a wounded tiger, failed for at least a minute to see him under a tree in grass at a distance of about twenty yards-jungle open-but the natives saw him, and I eventually made him out well enough to shoot him, but even then I could not see at what part of him I was aiming. There can be no doubt whatever that the colour of both the tiger and the panther renders them almost invisible, especially in a strong blaze of light, when among grass, and one does not seem to notice stripes or spots till they are dead." It is the black shadows of the vegetation that