392 TROPICAL NATURE v 
the large brilliantly-marked wings of some butterflies and 
moths. 
Colours are produced or intensified by processes of develop- 
ment, either where ‘the integument or its appendages undergo 
great extension or modification, or where there is a surplus of 
vital energy, as in male animals generally, and more especially 
at the breeding season. 
Colours are also more or less influenced by a variety of 
causes, such as the nature of the food, the photographic or 
physiological action of light, and also by some unknown local 
action, probably dependent on chemical peculiarities in the soil 
or vegetation. 
These various causes have acted and reacted in a variety 
of ways, and have been modified by conditions dependent on 
age or on sex, on competition with new forms, or on geo- 
graphical or climatic changes. In so complex a subject, for 
which experiment and systematic inquiry have done so little, 
we cannot expect to explain every individual case, or solve 
every difficulty ; but it is believed that all the great features of 
animal coloration and many of the details become explicable 
on the principles we have endeavoured to lay down. 
It will perhaps be considered presumptuous to put forth 
this sketch of the subject of colour in animals as a substitute 
for one of Mr. Darwin’s most highly elaborated theories— 
that of voluntary or perceptive sexual selection; yet I ven- 
ture to think that it is more in accordance with the whole of 
the facts, and with the theory of natural selection itself; and 
I would ask such of my readers as may be sufficiently in- 
terested in the subject, to read again chapters xi. to xvi. of 
the Descent of Man, and consider the whole subject from the 
point of view here laid down. The explanation of almost all 
the ornaments and colours of birds and insects as having been 
produced by the perceptions and choice of the females, has, 
I believe, staggered many evolutionists, but has been pro- 
visionally accepted because it was the only theory that even 
attempted to explain the facts. It may perhaps be a relief 
to some of them, as it has been to myself, to find that the 
phenomena can be conceived as dependent on the general 
laws of development, and on the action of “natural selection,” 
which theory will, I venture to think, be relieved from an 
