The æsthetic theory of Darwin has its chief interest in the fact that it can be applied to the activities of animals as well as to those of men. Darwin himself has chosen the majority of his examples from bird life, and his critics have generally restricted the discussion to the zoological application of his thesis. As the data of animal psychology are less complicated than the corresponding facts in the mental life of man, it is in every respect advantageous to begin with this supposed animal art. When the illustrations are chosen from bird life the argumentation can, moreover, be handled with a freedom which would be impossible in discussing the delicate questions of human erotics and sexual life. The theory of an appeal to some primordial æsthetic appreciation in the hen birds is one which, however well it may account for the beautiful plumage of some species and for the melodious singing of some others, necessarily must arouse objections from the point of view of comparative psychology. Æsthetic judgment presupposes a certain development not only of intellectual qualities, but also of moral self-restraint. In other words, it might be said that attention to beauty, whether manifested in forms or colours or sounds, always is what Ribot calls an “attention volontaire.”[263] It is hard to believe that the hen really has reached such a state of spiritual freedom that, when looking at the finery and the antics of her rival suitors, she could be able to bestow her attention upon the æsthetic qualities in the display. One has only to work out into all its details and consequences the idea of a bird who approves or disapproves the performance, and who, after balancing the merits of the various competitors, awards her prize to the one nearest her ideal of beauty, in order to realise the improbability, not to say the absurdity, of this avian connoisseurship. M. Espinas, who describes in detail the dilemma of the anxiously hesitating arbiter, has added nothing to the theory of æsthetic selection. But it may safely be said that his illustration of the thesis acts as a caricature of it.[264] The improbability of an æsthetic judgment is yet more palpable when one considers the state of mind in which the hen is likely to be on the occasion. It has indeed been contended with regard to some species—as, for instance, the satyr bird—that the hen behaves with a perfect calm and indifference during the display of the cock.[265] But it would be undoubtedly too rash to form any general conclusion on the ground of her outward show in some single instances. Amongst the wood-birds, on the other hand, the hens are known to be so excited during the “Balz” that they can easily be caught with the hands.[266]