appeared amongst the males even independently of its influence upon the hen. The hygienic need of an outburst is one reason, and a very strong one indeed, for such motor manifestations. But it seems also as if their purpose ought not to be restricted to this cathartic effect. The “lek” of the wood-birds tends no doubt to _increase_ nervous force at the same time as it relieves the tension. M. Espinas thinks that the males, when collecting in great numbers during the pairing season, are led by an unconscious desire to stimulate their feelings by the view of other equally excited fellow-males.[290] Such an unconscious desire of enhancing nervous force probably lies also behind the inclination for brilliant objects and the trifling with them in the bowers.[291] That a sort of artificial stimulation really is necessary may perhaps be concluded from Brehm’s general remark about the sexual life of birds: “The pairing is often repeated, and still more often ineffectually attempted.”[292] In a private communication to Groos, Professor Ziegler, the eminent zoologist, also refers to the increased nervous activity which is necessary for pairing, and the passionate preludes which precede it in so many animals.[293] By these two principles, viz. the necessity of overcoming the instinctive coyness of the hen and that of stimulating the nervous system of the cock, it is, we believe, possible to account for all the secondary sexual characters and activities which in Darwin’s theory necessitated the hypothesis of an animal æsthetic. When the directly physiological importance of the nuptial preludes is acknowledged one can also, without appealing to the effects of association, explain the occurrence of a “lek” in birds who have already made their choice of mate.[294] And, on the other hand, we need not give up any one of the general results at which Darwin arrived in his researches. Although conscious selection on the part of the hen must be denied, the fact that she consents to couple only with the cock who has been able to stir her feelings better than any other, constitutes a kind of unconscious choice. It may seem unnecessary, therefore, to lay so much stress upon the theoretical inappropriateness of Darwin’s language. The æsthetical terms in _The Descent of Man_ could easily be exchanged for more physiological ones without any important alteration in the main thesis of the book. And it must even be conceded that, owing to the extreme cautiousness which was such a remarkable peculiarity of Darwin, this substitution would not be required in more than a few passages. He himself often speaks of ornamental plumage as a means of “exciting,” or “charming,” or “fascinating” the females;[295] once he even restricts himself to saying that the hens “prefer, or are unconsciously excited by, the more beautiful males.”[296] From the biological point of view it would therefore be pretentious to claim any importance for this reformulation of the Darwinian theory.