to
neutralise each other, leaving the remainder to produce the effects of
colour. Such are the colours of soap-bubbles, or of steel or glass on
which extremely fine lines have been ruled; and these colours often
produce the effect of metallic lustre, and are the cause of most of the
metallic hues of birds and insects.

As colour thus depends on molecular or chemical constitution or on the
minute surface texture of bodies, and, as the matter of which organic
beings are composed consists of chemical compounds of great complexity
and extreme instability, and is also subject to innumerable changes
during growth and development, we might naturally expect the phenomena
of colour to be more varied here than in less complex and more stable
compounds. Yet even in the inorganic world we find abundant and varied
colours; in the earth and in the water; in metals, gems, and minerals;
in the sky and in the ocean; in sunset clouds and in the many-tinted
rainbow. Here we can have no question of _use_ to the coloured object,
and almost as little perhaps in the vivid red of blood, in the brilliant
colours of red snow and other low algae and fungi, or even in the
universal mantle of green which clothes so large a portion of the
earth's surface. The presence of some colour, or even of many brilliant
colours, in animals and plants would require no other explanation than
does that of the sky or the ocean, of the ruby or the emerald--that is,
it would require a purely physical explanation only. It is the wonderful
individuality of the colours of animals and plants that attracts our
attention--the fact that the colours are localised in definite patterns,
sometimes in accordance with structural characters, sometimes altogether
independent of them; while often differing in the most striking and
fantastic manner in allied species. We are thus compelled to look upon
colour not merely as a physical but also as a biological characteristic,
which has been differentiated and specialised by natural selection, and
must, therefore, find its explanation in the principle of adaptation or
utility.

_The Constancy of Animal Colour indicates Utility._

That the colours and markings of animals have been acquired under the
fundamental law of utility is indicated by a general fact which has
received very little attention. As a rule, colour and marking are
constant in each species of wild animal, while, in almost every
domesticated animal, there arises grea