Observe the animal which, leading a life demanding solitary habits, has an adapted organization, and it gives no sign of need for the presence of its kind. Observe, conversely, a gregarious animal separated from the herd, and you see marks of unhappiness while the separation continues, and equally distinct marks of joy on joining its companions. In the one case there is no nervous structure which finds its sphere of action in the gregarious state, and in the other case such a structure exists. As was implied by instances cited in the last chapter for another purpose, animals leading lives involving particular kinds of activities, have become so constituted that pursuance of those activities, exercising the correlative structures, yields the associated pleasures. Beasts of prey confined in dens, show us by their pacings from side to side the endeavor to obtain, as well as they can, the satisfactions that accompany roaming about in their natural habitats, and that gratification in the expenditure of their locomotive energies, shown us by porpoises playing round a vessel, is shown us by the similarly unceasing excursions from end to end of its cell which a captured porpoise makes. The perpetual hoppings of the canary from bar to bar of its cage, and the ceaseless use of claws and bill in climbing about its perch by the parrot, are other activities which, severally related to the needs of the species, have severally themselves become sources of agreeable feelings. Still more clearly are we shown by the efforts which a caged beaver makes to build with such sticks and pieces of wood as are at hand, how dominant in its nature has become the building instinct, and how, apart from any advantage gained, it gets gratification by repeating, as well as it can, the processes of construction it is organized to carry on. The cat which, lacking something to tear with her claws, pulls at the mat with them, the confined giraffe which, in default of branches to lay hold of wears out the upper angles of the doors to its house by continually grasping them with its prehensile tongue, the rhinoceros which, having no enemy to fight, plows up the ground with his horn--all yield us analogous evidence. Clearly, these various actions performed by these various creatures are not intrinsically pleasurable, for they differ more or less in each species and are often utterly unlike. The pleasurableness is simply in the exercise of nervo-muscular structures adapted to the performance of the actions.