In neither of the two first cases is there any scope for the idea of purpose; in the third, purpose must be present immediately before the action. In the two first cases, action is supposed to be brought about by means of an initial arrangement, either of bodily or mental mechanism, purpose being conceived of as existing on a single occasion only—that is to say, in the determination of the initial arrangement. In the third, purpose is conceived as present in every individual instance. Let us proceed to the consideration of these three cases. Instinct is not a mere consequence of bodily organisation; for— (_a_.) Bodies may be alike, yet they may be endowed with different instincts. All spiders have the same spinning apparatus, but one kind weaves radiating webs, another irregular ones, while a third makes none at all, but lives in holes, whose walls it overspins, and whose entrance it closes with a door. Almost all birds have a like organisation for the construction of their nests (a beak and feet), but how infinitely do their nests vary in appearance, mode of construction, attachment to surrounding objects (they stand, are glued on, hang, &c.), selection of site (caves, holes, corners, forks of trees, shrubs, the ground), and excellence of workmanship; how often, too, are they not varied in the species of a single genus, as of _parus_. Many birds, moreover, build no nest at all. The difference in the songs of birds are in like manner independent of the special construction of their voice apparatus, nor do the modes of nest construction that obtain among ants and bees depend upon their bodily organisation. Organisation, as a general rule, only renders the bird capable of singing, as giving it an apparatus with which to sing at all, but it has nothing to do with the specific character of the execution . . . The nursing, defence, and education of offspring cannot be considered as in any way more dependent upon bodily organisation; nor yet the sites which insects choose for the laying of their eggs; nor, again, the selection of deposits of spawn, of their own species, by male fish for impregnation. The rabbit burrows, the hare does not, though both have the same burrowing apparatus. The hare, however, has less need of a subterranean place of refuge by reason of its greater swiftness. Some birds, with excellent powers of flight, are nevertheless stationary in their habits, as the secretary falcon and certain other birds of prey; while even such moderate fliers as quails are sometimes known to make very distant migrations. (_b_.) Like instincts may be found associated with unlike organs.