Doubtless in many cases habits react upon and modify the structure of parts; and in this instance the modified structure has apparently reacted on and modified the habits. In seeking for and taking food, the body is required to perform certain definite motions and assume repeatedly the same attitudes; this is most frequently the case in birds of aquatic habits. A facility for assuming at all times, and an involuntary falling into, these peculiar attitudes and gestures appears to become hereditary; and the species in which they are the most noticeable seem incapable of throwing the habit or manner off, even when placed in situations where it is useless or even detrimental. _Tringæ_ rapidly peck and probe the mud as they advance; Plovers peck and run, peck and run again. Now I have noticed scores of times that these birds cannot possibly lay aside this habit of pecking as they advance; for even a wounded Plover running from his pursuer over dry barren ground goes through the form of eating by pausing for a moment every yard or so, pecking the ground, then running on again. The Paraguay Snipe, and probably other true Snipes, possesses the singular habit of striking its beak on the ground when taking flight. In this instance has not the probing motion, performed instinctively as the bird moves, been utilized to assist it in rising? Grebes on land walk erect like Penguins and have a slow awkward gait; and whenever they wish to accelerate their progress they throw themselves forward on the breast and strike out the feet as in swimming. The Glossy Ibis feeds in shallow water, thrusting its great sickle beak into the weeds at the bottom at every step. When walking on land it observes these motions, and seems incapable of progressing without plunging its beak downwards into imaginary water at every stride. The Spoonbill wades up to its knees and advances with beak always immersed, and swaying itself from side to side, so that at each lateral movement of the body the beak describes a great semicircle in the water; a flock of these birds feeding reminds one of a line of mowers mowing grass. On dry ground the Spoonbill seems unable to walk directly forward like other birds, but stoops, keeping the body in a horizontal position, and, turning from side to side, sweeps the air with its beak, as if still feeding in the water. In the foregoing instances (and I could greatly multiply them), in which certain gestures and movements accompany progressive motion, it is difficult to see how the structure can be in any way modified by them; but the preying attitude of the Heron, the waiting motionless in perpetual readiness to strike, has doubtless given the neck its very peculiar form. Two interesting traits of the Heron (and they have a necessary connexion) are its tireless watchfulness and its insatiable voracity; for these characters have not, I think, been exaggerated even by the most sensational of ornithologists.