716 The Colors of Animals and Plants, [December, 
able, it may serve as a warning of how impossible it is, without 
exact knowledge of the habits of an animal and a full considera- 
tion of all the circumstances, to decide that any particular color- 
ation cannot be protective or in any way useful. Mr. Darwin 
himself is not free from such assumptions. Thus, he says: 
“The zebra is conspicuously striped, and stripes cannot afford 
any protection on the open plains of South Africa.” But the 
zebra is a very swift animal, and, when in herds, by no means 
void of means of defense. The stripes, therefore, may be of use 
by enabling stragglers to distinguish their fellows at a distance, 
and they may be even protective when the animal is at rest 
among herbage—the only time when it would need protective 
coloring. Until the habits of the zebra have been observed with 
special reference to this point, it is surely somewhat hasty to de- 
clare that the stripes “ cannot afford any protection.” 
The wonderful display and endless variety of color in which 
butterflies and birds so far exceed all other animals seem prima- 
rily due to the excessive development and endless variations of 
the integumentary structures. No insects have such widely-ex- 
panded wings in proportion to their bodies as butterflies and 
moths ; in none do the wings vary so much in size and form, and 
in none are they clothed with such a beautiful and highly-organ- 
ized coating of scales, According to the general principles of the 
production of color already explained, these long-continued ex- 
pansions of membranes and developments of surface-structures 
must have led to numerous color-changes, which have been some- 
times checked, sometimes fixed and utilized, sometimes intensi- 
` 
fied, by natural selection, according to the needs of the animal. . 
In birds, too, we have the wonderful clothing of plumage — the 
most highly-organized, the most varied, and the most expanded 
of all dermal appendages. The endless processes of growth and 
change during the development of feathers, and the enormous ex- 
tent of this delicately-organized surface, must have been highly 
favorable to the production of varied color-effects, which, when 
not injurious, have been merely fixed for purposes of specific 
identification, but have often been modified or suppressed when- 
ever different tints were needed for purposes of protection. 
To voluntary sexual selection, that is, the actual choice by the 
females of the more brilliantly-colored males, I believe very little 
if any effect is directly due. It is undoubtedly proved that m 
birds the females do sometimes éxert a choice; but the evidence 
of this fact collected by Mr. Darwin (Descent of Man, chapter 
