But animals have other means of wooing their mates. To this end the male in a much higher degree than the female is provided with certain notes or calls, strong odors, beautiful top-knots, fine plumes, brilliant colors, or similar ornaments. Even with the most pugnacious species of birds, says Darwin, "it is probable that the pairing does not depend exclusively on the mere strength and courage of the male; for such males are generally decorated with various ornaments, which often become more brilliant during the breeding season, and which are sedulously displayed before the females. The males also endeavor to charm their mates by love-notes, songs, and antics; and the courtship is, in many instances, a prolonged affair. Hence it is not probable that the females are indifferent to the charms of the opposite sex, or that they are invariably compelled to yield to the victorious males. It is more probable that the females are excited, either before or after the conflict, by certain males, and thus unconsciously prefer them."[651] Such colors, love-songs, and ornaments belong to what Darwin calls the "secondary sexual characters." For, in the sexual selection, the "struggle is of two kinds; in the one it is between the individuals of the same sex, generally the males, in order to drive away or kill their rivals, the females remaining passive; whilst in the other, the struggle is likewise between the individuals of the same sex, in order to excite or charm those of the opposite sex, generally the females, which no longer remain passive, but select the more agreeable partners."[652] These characters, he thinks, depend upon the æsthetic sense of the females. "Just as a man can give beauty, according to his standard of taste, to his male poultry, or more strictly can modify the beauty originally acquired by the parent species, ... so it appears that female birds in a state of nature, have by a long selection of the more attractive males, added to their beauty or other attractive qualities."[653] Brilliant colors, for instance, have thus been acquired by birds and insects because they are "beautiful or otherwise agreeable, whereas the characters resulting from natural selection have been acquired because they are useful." Hence "far from co-operating with the process of natural selection, sexual selection, as described by Mr. Darwin, produces effects disadvantageous to the species;"[654] for many of the secondary characters are a source of danger.[655] But Wallace, in his well-known criticism of Darwin,[656] has established a probability that their primary purpose is not æsthetic, but utilitarian. "The fundamental or ground colors of animals," he says, "are very largely protective;" and these are extended in the line of the greatest structural and nervous development.[657] They are therefore an evidence of a surplus of nervous energy, which is especially active at the excitable period of courtship. So far