Very probably not only fish, but animals and some birds hear as much by the vibration of the earth as by the sound travelling in the atmosphere, and depend as much upon their immediate perception of the slightest tremor of the earth as upon recognition by the ear in the manner familiar to ourselves. When rabbits, for instance, are out feeding in the grass, it is often possible to get quite close to them by walking in this way, extremely slowly, and carefully placing the foot by slow degrees upon the ground. The earth is then merely pressed, and not stepped upon at all, so that there is no jar. By doing this I have often moved up within gunshot of rabbits without the least aid from cover. Once now and then I have walked across a field straight at them. Something, however, depends on the direction of the wind, for then the question of scent comes in. To some degree it is the same with hares. It is certainly the case with birds, as wood-pigeons, a flock of them, will remain feeding only just the other side of the hedge; but, if you stamp the earth, will rise instantly. So will rooks, though they will not fly far if you are not armed. Partridges certainly secure themselves by their attention to the faint tremor of the ground. Pheasants do so too, and make off, running through the underwood long before any one is in sight. The most sensitive are landrails, and it is difficult to get near them, for this reason. Though the mowing-grass must conceal an approaching person from them as it conceals them from him, these birds change their positions, no matter how quietly he walks. Let him be as cunning as he will, and think to cut off corners and cross the land-rail's retreat, the bird baffles him nine times in ten. That it is advised of the direction the pursuer takes by the vibration of the surface is at least probable. Other birds sit, and hope to escape by remaining still, till they detect the tremor coming direct towards them, when they rise. Rain and dry weather change the susceptibility of the surface to vibrate, and may sometimes in part account for the wildness or apparent tameness of birds and animals. Should any one doubt the existence of such tremors, he has only to lie on the ground with his ear near the surface; but, being unused to the experiment, he will at first only notice the heavier sounds, as of a waggon or a cart-horse. In recent experiments with most delicate instruments devised to show the cosmic vibration of the earth, the movements communicated to it by the tides, or by the "pull" of the sun and moon, it has been found almost impossible as yet to carry out the object, so greatly are these movements obscured by the ceaseless and inexplicable vibrations of the solid earth. There is nothing unreasonable in the supposition that, if an instrument can be constructed to show these, the ears of animals and birds--living organisms, and not iron and steel--should be able to discover the tremors of the surface.