Part 2 — THE CHANGES OF COLOUR IN THE CHAMELEON.

From very ancient times the curious changes of colour which take place in the chameleon, and its supposed power of living on air, have been the wonder of the uninformed, and have furnished philosophers and poets with abundant material for metaphor. The belief that the animal can live on air has been exploded long ago, and was no doubt due to its power of long fasting and to its peculiar manner of breathing. It is only quite lately, however, that any satisfactory explanation has been given of the apparently capricious changes which take place in the colour of the chameleon; the latest researches on the subject being those of M. Paul Bert, the French naturalist, which have been described in a recent paper by M. E. Oustalet. As most of our readers are no doubt familiar with the appearance and figure of this curious reptile, and as descriptions of it may be found in any encyclopædia or elementary work on natural history, we do not consider it necessary to repeat them here.

Many and various theories have been proposed to explain the changes of colour which chameleons undergo; changes the importance of which have been greatly exaggerated. It is generally believed that these animals have the power of assuming in a few seconds the colour of any neighbouring object, and that they intentionally make use of this trick to escape more easily from the sight of their enemies. But this opinion is erroneous; and experiments conducted with the greatest care have proved that chameleons are incapable of modifying their external appearance in anything like so rapid and complete a manner.

The first probably to give any rational account of the causes of the puzzling changes of colour in these reptiles was the celebrated French naturalist, Milne-Edwards, about forty years ago. After a patient and minute examination, he discovered that the colouring matters of the skin, the pigments, are not confined as in mammals and birds, to the deep layer of the epidermis, but are partly distributed on the surface of the dermis or true skin, partly located more deeply, and stored in a series of little cells or bags of very peculiar formation. These colour-cells are capable of being shifted in position. When they are brought close to the surface of the outer skin, they cause a definite hue or hues to become apparent; but by depressing the cells and causing them to disappear, the hues can be rendered paler, or may be altogether dispersed. It is noteworthy that the cuttlefishes change colour in a similar manner.

Underneath the colour-bags (or chromoblasts as they are called) of Milne-Edwards, Pouchet, a recent inquirer, has discovered a remarkable layer, which he calls cærulescent, and which possesses the singular property of appearing yellow on a clear, and blue on an opaque background.

M. Paul Bert, within the last two years, has by his researches thrown still further light upon these curious changes, and upon the mechanism by which they appear to be accomplished. He endorses most of the results of Milne-Edwards and subsequent inquirers, but has carried his observations much further. It would be out of place here to give a detailed account of the methods by which M. Bert has arrived at his conclusions. Suffice it to say, that by a series of careful experiments, he has discovered that these changes of colour seem to be entirely under the control of the nervous system, and that the chameleon can no more help them taking place than a toad can help twitching its leg when pinched. By acting in various ways upon the spinal marrow and the brain, the operator can send the colour to or withdraw it from any part of the body he pleases. Indeed a previous observer was able to cause a change of colour in a piece of the skin of the animal by acting upon it with electricity; and M. Bert has proved that even in the absence of the brain the usual changes can be produced by exciting the animal in any way; thus shewing that they are due to that class of nervous action which physiologists name reflex, and of which sneezing is a good example. M. Bert has also made some interesting experiments on the animal while under the influence of anæsthetics and during sleep. It was formerly known that in the latter case, and also after death, the chameleon assumed a yellowish colour, which under the influence of light became more or less dark. M. Bert has found that exactly the same effects are produced during anæsthesia as during natural sleep, and that light influences not only dead and sleeping chameleons, but that it modifies in a very curious fashion the coloration of the animal when wide awake. The same result is produced when the light is transmitted through glass of a deep blue colour, but ceases completely when red or yellow glass is used. To render these results more decisive, M. Bert contrived to throw the light of a powerful lamp upon a sleeping chameleon, taking care to keep in the shade a part of the animal's back, by means of a perforated screen. The result was curious: the head, the neck, the legs, the abdomen, and the tail became of a very dark green; while the back appeared as if covered with a light brown saddle of irregular outline, with two brown spots corresponding to the holes in the screen. Again, by placing another animal, quite awake, in full sunlight, but with the fore-part of its body behind a piece of red glass, and the hind-part underneath blue glass, M. Bert divided the body into two quite distinct parts—one of a clear green with a few reddish spots, and the other of a dark green with very prominent spots.

From his researches as a whole, M. Bert concludes: 1. The colours and the various tints which chameleons assume are due to changes in the position of the coloured corpuscles, which sometimes, by sinking underneath the skin, form an opaque background underneath the cærulescent layer of Pouchet; sometimes, by spreading themselves out in superficial ramifications, leave to the skin its yellow colour, or make it appear green and black. 2. The movements of these colour-bags or chromoblasts are regulated by two groups of nerves, one of which causes them to rise from below to the surface, while the other produces the opposite effect.

As to the effects produced by coloured glass, they no doubt result from the fact that the coloured corpuscles, like certain chemical substances, are not equally influenced by all the rays of the spectrum, the rays belonging to the violet part having alone the power of causing the colour-bags to move and drawing them close to the surface of the skin. This exciting action of light on a surface capable of contraction, an action which hitherto has only been recognised in the case of heat and electricity, is one of the most unexpected and curious facts which in recent times have transpired in the domain of physiology. Hence M. Paul Bert's researches are likely to prove of far more value than merely to explain the changes of colour which take place in the chameleon. He hopes especially in carrying out his researches to discover the reason of the favourable influence on health which is exerted by the direct action of light on the skin of children and of persons of a lymphatic temperament; and this may lead to some very important practical results in the treatment of disease. In the meantime he has done much to clear up a very puzzling and very interesting fact.