1877. ] The Colors of Animals and Plants. 725 
as much variety and beauty as among insects and birds. Yet 
here, not only is sexual selection out of the question, but the need 
for recognition and identification by others of the same species 
seems equally unnecessary. We can then only impute this variety 
to the normal production of color in organic forms, when fully 
exposed to light and air and undergoing great and rapid devel- 
opmental modification. Among more perfect animals, where the 
need for recognition has been added, we find intensity and vari- 
ety of color at its highest pitch among the South American but- 
terflies of the families Heliconide and Danaidæ, as well as 
among the Nymphalidz and Erycinide, many of which obtain 
the necessary protection in other ways. Among birds, also, 
wherever the habits are such that no special protection is needed 
for the females, and where the species frequent the depths of 
tropical forests, and are thus naturally protected from the swoop 
of birds of prey, we find almost equally intense coloration, as in 
the trogons, barbets, and gapers. 
Of the mode of action of the general principles of color devel- 
opment among animals, we have an excellent example in the 
humming-birds. Of all birds these are at once the smallest, 
the most active, and the fullest of vital energy. When poised 
in the air, their wings are invisible, owing to the rapidity of 
their motion, and when startled they dart away with the rapid- 
ity of a flash of light. Such active creatures would not be an 
easy prey to any rapacious bird; and if one at length was cap- 
tured, the morsel obtained would hardly repay the labor. We 
may be sure, therefore, that they are practically unmolested. 
The immense variety they exhibit in structure, plumage, and 
color indicates a high antiquity for the race, while their general 
abundance in individuals shows that they are a dominant group, 
well adapted to all the conditions of their existence. Here we find 
everything necessary for the development of color and accessory 
plumes. The surplus vital energy shown in their combats and 
excessive activity has expended itself in ever-increasing develop- 
ments of plumage and greater and greater intensity of color, 
= regulated only by the need for specific identification, which would 
___ be especially required in such small and mobile creatures. Thus 
= May be explained those remarkable differences of color between 
closely-allied species, one having a crest like the topaz, while in 
another it resembles the sapphire. The more vivid colors and 
= more developed plumage of the males, I am now inclined to 
_ think, may be wholly due to their greater vital energy and to 
