654 The Colors of Animals and Plants. [ November, 
small parrot above referred to. For fuller details of the varieties 
of sexual coloration we refer our readers to Mr. Darwin’s De- 
scent of Man, chapters x. to xviii., and to chapters iii., iv., and 
vii., of my Contributions to the Theory of Natural Selection. 
The fourth group — of typically-colored animals — includes all 
species which are brilliantly or conspicuously colored in both 
sexes, and for whose particular colors we can assign no function 
or use. Itcomprises an immense number of showy birds, such as 
 kingfishers, barbets, toucans, lories, tits, and starlings ; among in- 
sects most of the largest and handsomest butterflies, innumerable 
bright-colored beetles, locusts, dragon-flies, and hymenoptera; a 
few mammalia, as the zebras; a great number of marine fishes; 
thousands of striped and spotted caterpillars; and abundance of 
mollusea, star-fish, and other marine animals, Among these we 
have included some which, like the gaudy caterpillars, have warn- 
ing colors; but as that theory does not explain the particular 
colors or the varied patterns with which they are adorned, it is 
best to include them also in this class. It is a suggestive fact 
that all the brightly colored birds mentioned above build in holes 
or form covered nests, so that the females do not need that pro- 
tection during the breeding season, which I believe to be one of 
the chief causes of the dull color of female birds when their part- 
ners are gayly colored. This subject is fully argued in my Con- 
tributions, etc., chapter vii. | 
As the colors of plants and flowers are very different from those 
of animals, both in their distribution and functions, it will be 
well to treat them separately: we will therefore now consider 
how the general facts of color here sketched out can be explained. 
We have first to inquire what is color, and how it is produced ; 
what is known of the causes of change of color ; and what theory 
best accords with the whole assemblage of facts. ee 
The sensation of color is caused by vibrations or undulations © 
the ethereal medium of different lengths and velocities. The 
whole body of vibrations caused by the sun is termed pene 
and consists of sets of waves which vary considerably in their . 
mensions and their rate of vibration, but of which the middle 
portion only is capable of exciting in us sensations of light si 
color. Beginning with the largest and slowest rays OT ad 
brations, we have first those which produce heat sensations ys 
as they get smaller and quicker, we perceive a dull-red- z ak e 
and as the waves increase in rapidity of vibration and dimint” 
in size, we get successively sensations of orange, yellow, gree™ 
