impulse of the female? If we admit any differential influence, and if this influence takes effect in the sexual union to which courtship is preparatory, we so far admit the efficacy of that which Darwin termed sexual selection. Let us, however, before proceeding with general considerations, present one or two examples of the facts which observation has furnished with regard to specialized modes of behaviour at the time of pairing. Speaking of American night-hawks, Audubon says, “Their manner of flying is a good deal modified at the love season. The male employs the most wonderful evolutions to give expression to his feelings, conducting them with the greatest rapidity and agility in sight of his chosen mate, or to put to rout a rival. He often rises to a height of a hundred metres and more, and his cries become louder and louder as he mounts; then he plunges downward with a slanting direction, with wings half open, and so rapidly that it seems inevitable that he should be dashed to pieces on the ground. But at the right moment, sometimes when only a few inches from it, he spreads his wings and tail, and, turning, soars upward once more.”[137] Mr. Strange, quoted by Darwin[138] says of the satin bower-bird, “at times the male will chase the female all over the aviary, then go to the bower, pick up a gay feather or a large leaf, utter a curious kind of note, set all his feathers erect, run round the bower and become so excited that his eyes appear ready to start from his head; he continues opening first one wing, and then the other, uttering a low, whistling note, and, like the domestic cock, seems to be picking up something from the ground, until at last the female goes quietly towards him.”