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.