ess genera; but almost all of them are of a beautiful green
colour, sometimes more or less adorned with white or dusky bands and
spots. There can be little doubt that this colour is doubly useful to
them, since it will tend to conceal them from their enemies, and will
lead their prey to approach them unconscious of danger. Dr. Gunther
informs me that there is only one genus of true arboreal snakes (Dipsas)
whose colours are rarely green, but are of various shades of black,
brown, and olive, and these are all nocturnal reptiles, and there can be
little doubt conceal themselves during the day in holes, so that the
green protective tint would be useless to them, and they accordingly
retain the more usual reptilian hues.

Fishes present similar instances. Many flat fish, as for example the
flounder and the skate, are exactly the colour of the gravel or sand on
which they habitually rest. Among the marine flower gardens of an
Eastern coral reef the fishes present every variety of gorgeous colour,
while the river fish even of the tropics rarely if ever have gay or
conspicuous markings. A very curious case of this kind of adaptation
occurs in the sea-horses (Hippocampus) of Australia, some of which bear
long foliaceous appendages resembling seaweed, and are of a brilliant
red colour; and they are known to live among seaweed of the same hue, so
that when at rest they must be quite invisible. There are now in the
aquarium of the Zoological Society some slender green pipe-fish which
fasten themselves to any object at the bottom by their prehensile
tails, and float about with the current, looking exactly like some
simple cylindrical algae.

It is, however, in the insect world that this principle of the
adaptation of animals to their environment is most fully and strikingly
developed. In order to understand how general this is, it is necessary
to enter somewhat into details, as we shall thereby be better able to
appreciate the significance of the still more remarkable phenomena we
shall presently have to discuss. It seems to be in proportion to their
sluggish motions or the absence of other means of defence, that insects
possess the protective colouring. In the tropics there are thousands of
species of insects which rest during the day clinging to the bark of
dead or fallen trees; and the greater portion of these are delicately
mottled with gray and brown tints, which though symmetrically disposed
and infinitely varied, yet blend