The majority of birds live to-day in constant danger from predatory species, and that this danger was still greater in bygone ages there can be but little doubt. A curious mode of behaviour of the Curlew, Whimbrel, and Godwit demonstrates this, for it must be the outcome of the necessity for constant watchfulness. Whilst resting with its head turned back and its beak buried in the feathers of the mantle, the bird constantly moves the axis of its body, so that an observer, if placed in a direct line behind it, sees at one moment the right eye and at another the left. No movement of the feet or of the legs is perceptible, and the shifting of the body continues whether the eyes are open or closed. This body movement enables the bird to survey a much larger area of ground than it would otherwise be capable of doing, and thus adds to its security. As far as my experience goes, the movement is less evident amongst the members of a flock than when an individual is resting alone, or even with a few companions, which may be due to the fact that since some members are always awake and watchful, a bird of prey would have more difficulty in approaching a flock unawares than it would have in approaching a single individual. With the greatest ease a Sparrow-Hawk can pick up a Thrush as it feeds on the meadow by itself, but if it attempts to seize one of a flock, the chances are that its approach is signalled and that its prey escapes. And not only do the different members give warning one to another of the approach of danger, but they also combine to harass or even to drive away an enemy. So that there can be no doubt that the gregarious instinct is serviceable in promoting the welfare of the race, and has, as its end, the preservation of the individual in order that it may take its share at the appropriate time in procreating its kind.