29S 
DARWINISM 
CHAP. 
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 Ave 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 Uiav 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 in¬ 
dividual variation, have been modified in innumerable ways 
for the benefit of each species. The most general modifica¬ 
tion has been in such directions as to favour concealment 
when at rest in the usual surroundings of the species, some¬ 
times 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 tAvo sexes arc 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 excitability. In many groups in Avhich 
this superabundant vitality is at a maximum, the develop¬ 
ment of dermal appendages and brilliant colours has gone on 
increasing till it has resulted in a great diversity hctAveen the 
sexes; and in most of these cases there is evidence to sIioav 
that natural selection has caused the female to retain the 
primitive and more sober colours of the group for purposes of 
protection. 
