increases in variety and
intensity as external structures and dermal appendages become more
differentiated and developed. It is on scales, hair, and especially on
the more highly specialised feathers, that colour is most varied and
beautiful; while among insects colour is most fully developed in those
whose wing membranes are most expanded, and, as in the lepidoptera, are
clothed with highly specialised scales. Here, too, we find an additional
mode of colour production in transparent lamellae or in fine surface
striae which, by the laws of interference, produce the wonderful
metallic hues of so many birds and insects.

3. There are indications of a progressive change of colour, perhaps in
some definite order, accompanying the development of tissues or
appendages. Thus spots spread and fuse into bands, and when a lateral or
centrifugal expansion has occurred--as in the termination of the
peacocks' train feathers, the outer web of the secondary quills of the
Argus pheasant, or the broad and rounded wings of many butterflies--into
variously shaded or coloured ocelli. The fact that we find gradations of
colour in many of the more extensive groups, from comparatively dull or
simple to brilliant and varied hues, is an indication of some such law
of development, due probably to progressive local segregation in the
tissues of identical chemical or organic molecules, and dependent on
laws of growth yet to be investigated.

4. The colours thus produced, and subject to much individual variation,
have been modified in innumerable ways for the benefit of each species.
The most general modification has been in such directions as to favour
concealment when at rest in the usual surroundings of the species,
sometimes carried on by successive steps till it has resulted in the
most minute imitation of some inanimate object or exact mimicry of some
other animal. In other cases bright colours or striking contrasts have
been preserved, to serve as a warning of inedibility or of dangerous
powers of attack. Most frequent of all has been the specialisation of
each distinct form by some tint or marking for purposes of easy
recognition, especially in the case of gregarious animals whose safety
largely depends upon association and mutual defence.

5. As a general rule the colours of the two sexes are alike; but in the
higher animals there appears a tendency to deeper or more intense
colouring in the male, due probably to his greater vigour and