298 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 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 in- 

 dividual variation, have been modified in innumerable Avays 

 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 poAvers of attack. Most frec[uent 

 of all has been the specialisation of each distinct form by some 

 tint or marking for j^iu'poses of easy recognition, especiall}' in 

 the case of gregarious animals AA'hose safety largely depends 

 upon association and mutual defence. 



5. As a general rule the colours of the tAvo 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 excitability. In many groups in Avhich 

 this superabundant A'itality is at a maximum, the dcA'elop- 

 ment of dermal appendages and brilliant colours has gone on 

 increasing till it has resulted in a great diA^ersity betAveen the 

 sexes ; and in most of these cases there is eAadence to shoAv 

 that natural selection has caused the female to retain the 

 primitiA'e and more sober colours of the group for purposes of 

 protection. 



