avour their concealment
in the daytime.

An additional illustration of general assimilation of colour to the
surroundings of animals, is furnished by the inhabitants of the deep
oceans. Professor Moseley of the Challenger Expedition, in his British
Association lecture on this subject, says: "Most characteristic of
pelagic animals is the almost crystalline transparency of their bodies.
So perfect is this transparency that very many of them are rendered
almost entirely invisible when floating in the water, while some, even
when caught and held up in a glass globe, are hardly to be seen. The
skin, nerves, muscles, and other organs are absolutely hyaline and
transparent, but the liver and digestive tract often remain opaque and
of a yellow or brown colour, and exactly resemble when seen in the water
small pieces of floating seaweed." Such marine organisms, however, as
are of larger size, and either occasionally or habitually float on the
surface, are beautifully tinged with blue above, thus harmonising with
the colour of the sea as seen by hovering birds; while they are white
below, and are thus invisible against the wave-foam and clouds as seen
by enemies beneath the surface. Such are the tints of the beautiful
nudibranchiate mollusc, Glaucus atlanticus, and many others.

_General Theories of Animal Colour._

We are now in a position to test the general theories, or, to speak more
correctly, the popular notions, as to the origin of animal coloration,
before proceeding to apply the principle of utility to the explanation
of some among the many extraordinary manifestations of colour in the
animal world. The most generally received theory undoubtedly is, that
brilliancy and variety of colour are due to the direct action of light
and heat; a theory no doubt derived from the abundance of
bright-coloured birds, insects, and flowers which are brought from
tropical regions. There are, however, two strong arguments against this
theory. We have already seen how generally bright coloration is wanting
in desert animals, yet here heat and light are both at a maximum, and if
these alone were the agents in the production of colour, desert animals
should be the most brilliant. Again, all naturalists who have lived in
tropical regions know that the proportion of bright to dull coloured
species is little if any greater there than in the temperate zone, while
there are many tropical groups in which bright colours are almost
entirely unk