At the present day nobody seriously questions the statement that the
action of light upon organisms is primarily one of a chemical character.
While this chemical action is of the utmost importance for organisms,
the nutrition of which depends upon the action of chlorophyll, it
becomes of less importance for organisms devoid of chlorophyll.
Nevertheless, we find animals in which the formation of organs by
regeneration is not possible unless they are exposed to light. An
observation made by the writer on the regeneration of polyps in a
hydroid, Eudendrium racemosum, at Woods Hole, may be mentioned as an
instance of this. If the stem of this hydroid, which is usually covered
with polyps, is put into an aquarium the polyps soon fall off. If the
stems are kept in an aquarium where light strikes them during the day, a
regeneration of numerous polyps takes place in a few days. If, however,
the stems of Eudendrium are kept permanently in the dark, no polyps are
formed even after an interval of some weeks; but they are formed in a
few days after the same stems have been transferred from the dark to
the light. Diffused daylight suffices for this effect. Goldfarb, who
repeated these experiments, states that an exposure of comparatively
short duration is sufficient for this effect, it is possible that the
light favours the formation of substances which are a prerequisite for
the origin of polyps and their growth.
Of much greater significance than this observation are the facts which
show that a large number of animals assume, to some extent, the
colour of the ground on which they are placed. Pouchet found through
experiments upon crustaceans and fish that this influence of the ground
on the colour of animals is produced through the medium of the eyes.
If the eyes are removed or the animals made blind in another way these
phenomena cease. The second general fact found by Pouchet was that the
variation in the colour of the animal is brought about through an action
of the nerves on the pigment-cells of the skin; the nerve-action being
induced through the agency of the eye.
The mechanism and the conditions for the change in colouration were made
clear through the beautiful investigations of Keeble and Gamble, on
the colour-change in crustaceans. According to these authors the
pigment-cells can, as a rule, be considered as consisting of a central
body from which a system of more or less complicated ramifications or
processes spreads out in all directions. As a rule, the centre of the
cell contains one or more different pigments which under the influence
of nerves can spread out separately or together into the ramifications.
These phenomena of spreading and retraction of the pigments into or from
the ramifications of the pigment-cells form on the whole the basis for
the colour changes under the influence of environment. Thus Keeble
and Gamble observed that Macromysis flexuosa appears transparent and
colourless or grey on sandy ground. On a dark ground their colour
becomes darker. These animals have two pigments in their chromatophores,
a brown pigment and a whitish or yellow pigment; the former is much more
plentiful than the latter. When the animal appears transparent all the
pigment is contained in the centre of the cells, while the ramifications
are free from pigment. When the animal appears brown both pigments are
spread out into the ramifications. In the condition of maximal spreading
the animals appear black.
This is a comparatively simple case. Much more complicated conditions
were found by Keeble and Gamble in other crustaceans, e.g. in Hippolyte
cranchii, but the influence of the surroundings upon the colouration of
this form was also satisfactorily analysed by these authors.
While many animals show transitory changes in colour under the influence
of their surroundings, in a few cases permanent changes can be produced.
The best examples of this are those which were observed by Poulton
in the chrysalids of various butterflies, especially the small
tortoise-shell. These experiments are so well known that a short
reference to them will suffice. Poulton (Poulton, E.B., "Colours of
Animals" (The International Scientific Series), London, 1890, page 121.)
found that in gilt or white surroundings the pupae became light coloured
and there was often an immense development of the golden spots, "so that
in many cases the whole surface of the pupae glittered with an apparent
metallic lustre. So remarkable was the appearance that a physicist to
whom I showed the chrysalids, suggested that I had played a trick and
had covered them with goldleaf." When black surroundings were used "the
pupae were as a rule extremely dark, with only the smallest trace, and
often no trace at all, of the golden spots which are so conspicuous in
the lighter form." The susceptibility of the animal to this influence of
its surroundings was found to be greatest during a definite period when
the caterpillar undergoes the metamorphosis into the chrysalis stage.
As far as the writer is aware, no physico-chemical explanation, except
possibly Wiener's suggestion of colour-photography by mechanical colour
adaptation, has ever been offered for the results of the type of those
observed by Poulton.