have aided in such differentiation and that the pairing-place should ultimately expand into a "bower" would be a result brought about by the high and gradually increasing æsthetic faculties of the birds constructing it. One can understand, too, that as the thalamum passed into a bower, and as the bower became more and more elaborate and complicated, its original purpose might be gradually obscured, superseded, and more or less lost sight of. Such, indeed, has been the case with our own houses and gardens, which in the manifold wants, tastes, and pleasures that they now minister to, have become something very different to their rude originals—beginning with the mere cave—amongst primitive savages. There has, too, been the process of differentiation as between the bedroom and sitting-room or bower. What was the original cave but a sleeping place?
I believe that the key to the unlocking of many of the wonderchambers of bird doings is to be sought in the highly nervous and excitable organization which birds, as a class, possess, and, especially, in the extraordinary development of this during the breeding and rearing time. This nervous sexual or parental excitation produces all sorts of extravagant motions and antics which are at first quite useless, but on the raw material of which both natural and sexual selection have seized and are constantly seizing. By these two powers they have been or are being directed into various useful channels, such as nest-building, ruses to decoy enemies from the young, displays of plumage by one sex to the other, and so forth. On this view the fact of many bird (or other) antics not being attributable to sexual selection should not be used (as it has been used) to throw discredit on that hypothesis. By what agency the raw material has been shaped in any one case is a question of the evidence in and relating to such case. And as the exercise of intelligence in all these matters would be an advantage, intelligence, as I believe, has, by the same means, through memory, been gradually worked and woven into them, giving to some or all species a special intelligence in some special directions, which, though much above the general level of its capacity, yet reacts upon this and tends to raise it. I believe, too, that, if closely watched, many actions of birds which seem now to be altogether intelligent and purposive (and, no doubt, are so to a very large extent) will be found to betray traces of a nervous and non-purposive origin.