Page:Popular Science Monthly Volume 28.djvu/107

Rh Mr. Nicholas Wagner, using an exceedingly sensitive galvanometer, has discovered fixed currents in the wings of butterflies; and, with the aid of electric currents, has succeeded in producing changes in the color and disposition of their pigments. What part electricity may play in this matter is still, however, unsettled.

In regard to the effects of feeding, Darwin cites cases of complete changes in the color of birds brought about by modifications of their alimentation. Bullfinches, fed with hemp-seed, turned black. The common green paroquet, fed with the fat of certain fishes, became striped with red and yellow.

Volumes have been written on the influence of natural selection upon color, and have elucidated the subject so fully that we need not dwell on it at length. The principal aspect in which the influence asserts itself is that in which the prevailing color among animals gives them a kind of resemblance to the ground on which or the medium in which they live, or to the objects by which they are surrounded, so that they are more readily hidden from their enemies. In other cases they are made conspicuous in color or to resemble disagreeable objects, so that their enemies, mistaking them for something else, shall avoid them. Such cases belong to the classes of phenomena which Mr. Wallace has grouped under the designation of protective mimicry. In other cases, certain colors may be associated with peculiarities that render the animal more capable of resisting peculiar conditions to which it may be exposed; when natural selection, aided by selection by the breeder, may contribute to preserve this color to the exclusion of others. Thus, according to Darwin, in Virginia, black hogs alone can endure a course of feeding consisting largely of the roots of Lachnantes tinctoria; so a race of black hogs became established in that country.

Much more might be said on this subject. We might consider the phenomena of sexual selection to which male birds largely owe their bright plumage; the heredity of colors, correlative variations, and the complex and obscure action of domestication; the action of moisture and of some secreted principles; and the distribution of colors as related to geographical regions. What I have said has been really only introductory to the subject, and for the purpose of reminding investigators what a full field of work they might find in exhaustively following it up.—Translated for the Popular Science Monthly from the Revue Scientifique.