That of the Gardener Bower-bird takes the form of a hut-like structure of twigs, arranged around a central support, commonly a very young sapling. As a rule the thin stems of an orchid (_Dendrobium_) are used in the construction of this curious hut, whose diameter is about three feet. Before the entrance is a carpet of moss, which is kept clear of leaves or debris of any sort, and on this the most vividly coloured fruit, seed-pods, fungi, and flowers are laid, being constantly replaced as they wither. Newton’s Bower-bird, in like manner, forms a hut around a central column: a hut which may attain to a height of as much as six or even eight feet, and the walls of the pyramid thus raised are generally gaily decorated with flowers and fruit. Around the central a number of subsidiary huts are not infrequently found, and in and out of these the birds pursue one another in ecstasies of excitement. We have in these facts some extremely puzzling features, which at present, at any rate, permit of no more than a very rough analysis. Probably the whole of these bower-building instincts have their origin in the habit, which the males of so many birds exhibit, of carrying a leaf in the beak when under the excitement of love-making. This is suggestive of nest-building, and in many species this is actually begun before the arrival of a female in the breeding territory, while others build what are known as “cock-nests” which are never used. Among the Bower-birds these “cock-nests” have taken a new and more elaborate form, and are placed on the ground instead of in the trees, the normal site for the nest in all these birds. Furthermore, stages in the evolution of such strange fabrications can be found. These are furnished by the Tooth-billed Bower-bird (_Scenopaeetes dentirostris_), the Cat-bird (_Aeluredus viridus_) and the gorgeous Lawe’s Bird of Paradise (_Parotid lawesi_)—which is not perhaps a Bird of Paradise. These build no bowers, but are content with clearing a patch of ground, of about ten feet in diameter, on which to disport themselves. But while the “displays” of these birds closely resemble one another, in the matter of coloration and ornament they present the most striking contrasts. CHAPTER VIII SOME “COLD-BLOODED” LOVERS The Courtship of the Crocodile—Amorous Lizards—Horned Chamæleons—A flagellating Terrapin—The Frog that would a-wooing go—Semo musical Frogs—Some marvellous instincts in Newts. The measure of the vitality of animals may be estimated by their response to stimuli; and their behaviour increases in variety and complexity as the nervous system develops. Our interpretation of that behaviour commonly leaves out of account the character of this responsiveness: we are apt to see proof of intelligence in acts which should be read as instinctive. And instinct is to be regarded as a co-ordinated response to stimulus, independent of prior experience.