v COLOURS OF ANIMALS 391 
however, that this would apply to white-coloured butterflies ; 
and this may be a reason why the effect of an insular habitat 
is more marked in these insects than in birds or mammals.) 
It is even possible that this relation of sense-acuteness 
with colour may have had some influence on the development 
of the higher human races. If light tints of the skin were 
generally accompanied by some deficiency in the senses of 
smell, hearing, and vision, the white could never compete 
with the darker races so long as man was in a very low or 
savage condition, and wholly dependent for existence on the 
acuteness of his senses. But as the mental faculties became 
more fully developed and more important to his welfare than 
mere sense-acuteness, the lighter tints of skin and hair and 
eyes would cease to be disadvantageous whenever they were 
accompanied by superior brain-power. Such variations would 
then be preserved ; and thus may have arisen the Xantho- 
chroic race of mankind, in which we find a high development 
of intellect accompanied by a slight deficiency in the acuteness 
of the senses as compared with the darker forms. 
Summary on Colour-development in Animals 
Let us now sum up the conclusions at which we have 
arrived as to the various modes in which colour is produced 
or modified in the animal kingdom. 
The various causes of colour in the animal world are, 
molecular and chemical change of the substance of their 
integuments, or the action on it of heat, light, or moisture. 
It is also produced by interference of light in superposed 
transparent lamelle, or by excessively fine surface-striz. 
These elementary conditions for the production of colour are 
found everywhere in the surface-structures of animals, so that 
its presence must be looked upon as normal, its absence as 
exceptional. 
Colours are fixed or modified in animals by natural 
selection for various purposes; obscure or imitative colours 
for concealment; gaudy colours as a warning; and special 
markings, either for easy recognition by strayed individuals, 
females, or young, or to divert attack from a vital part, as in 
1 In Darwinism, pp. 229, 230, I have suggested an explanation of most of 
the facts of colour in islands as due to the lesser need of protection. 
