rest. There is a North American frog found on lichen-covered
rocks and walls, which is so coloured as exactly to resemble them, and
as long as it remains quiet would certainly escape detection. Some of
the geckos which cling motionless on the trunks of trees in the tropics,
are of such curiously marbled colours as to match exactly with the bark
they rest upon.

In every part of the tropics there are tree snakes that twist among
boughs and shrubs, or lie coiled up in the dense masses of foliage.
These are of many distinct groups, and comprise both venomous and
harmless 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. Gunthner 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-horse (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 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