of sexual selection suggests that the accepted mate is the one which adequately evokes the pairing impulse. Here Dr. Groos makes an interesting and important contribution to the subject. He lays stress on the coyness and reluctance of the female, and illustrates it by examples derived from observation. “Thus the female cuckoo answers the call of her mate with an alluring laugh that excites him to the utmost, but it is long before she gives herself up to him. A mad chase through tree-tops ensues, during which she constantly incites him with that mocking call, till the poor fellow is fairly driven crazy. The female kingfisher often torments her devoted lover for half a day, coming and calling him, and then taking to flight. But she never lets him out of her sight the while, looking back as she flies and measuring her speed, and wheeling back when he suddenly gives up the pursuit. The bower-bird leads her mate a chase up and down their skilfully built pleasure-house, and many other birds behave in a similar way. The male must exercise all his arts before her reluctance is overcome. She leads him on from limb to limb, from tree to tree, until it seems that the tantalizing change from allurement to resistance must include an element of mischievous playfulness.”[148] Professor Groos regards the instinctive coyness of the female as the most efficient means of preventing the too early and too frequent yielding to sexual impulse.[149] He thinks it probable that “in order to preserve the species the discharge of the sexual function must be rendered difficult, since the impulse to it is so powerful that, without some such arrest, it might easily become prejudicial to that end. This same strength of impulse is,” he adds,[150] “itself necessary to the preservation of the species; but, on the other hand, dams must be opposed to the impetuous stream, lest the impulse expend itself before it is made effectual, or the mothers of the race be robbed of their strength, to the detriment of their offspring.” It has its origin in the general fact that, before any important motor discharge takes place, there is apt to be a preparatory and gradually increasing excitement. But this is specially emphasized in association with the sexual impulse. As Professor Zeigler wrote, in a private communication to Dr. Groos,[151] “Among all animals a highly excited condition of the nervous system is necessary for the act of pairing, and consequently we find an exciting playful prelude very generally indulged in.”