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
Chamaeleon) 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 larvae, and pupae, which undergo
chan