1877. ] The Colors of Animals and Plants. 127 
vital energy, as in male animals icici and more especially 
at the breeding-season. 
Colors are also more or less influenced by a variety of causes, 
such as the nature of the food, the photographic 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 geographical 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 
endeavored to lay down. 
It will perhaps be considered A cus to put forth this 
sketch of the subject of color in animals as a substitute for one 
of Mr. Darwin’s most highly elaborated theories, — that of volun- 
tary or perceptive sexual selection, — yet I venture 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 interested in the subject to 
read again chapters xi. to xvi. of the Descent of Man, and 
consider the whole theory from the point of view here laid down. 
The explanation of almost all the ornaments and colors 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 provisionally accepted, because it was the only 
theory that even attempted to explain the facts. It may per- . 
haps be a relief to some of them, as it has been to myself, to find 
that the phenomena can be shown to depend on the general laws 
of development and on the action of “ natural selection,” which 
theory will, I venture to think, be relieved from an abnormal 
excrescence, and gain additional vitality by the adoption of my 
view of the subject. 
Although we have arrived at the conclusion that tropical light 
and heat can in no sense be considered the cause of color, there — 
remains to be explained the undoubted fact that all the more in- 
tense and gorgeous tints are manifested by the animal life of the 
_ tropics, while in some groups, such as butterflies and birds, there 
is a marked preponderance of highly colored species. This is 
probably due to a variety of causes, some of which we can indi- 
