Page:Popular Science Monthly Volume 77.djvu/133

Rh responses are about as uniform and predictable as those of an electric bell. Remove the blind and naked cedarbird from its nest, and the complex food-reaction is given as regularly and as continuously as before (Fig. 26). By the second or third day, however, all this has changed, and it is difficult to get any food-response if the bird be out of its nest with which association has become established. If the young are not removed, however, the feeding reaction is usually regularly given, unless checked by satiety or the rise of the instinct of fear. Association in the early life of young birds thus tends, as we have seen,

 blind and naked; but little over twenty-four hours old: a, typical prone position, when at rest; b, typical food-reaction, or reflex response to sound or contact-stimulus.

to cut out a lot of useless reactions, and to limit their responses to those which count.

Growth in Relation to the Development of the Instincts and Intelligence.—We have used the term "instinct" as synonymous with compound reflexes, that is, as reflexes involving relatively complex coordinations of the muscles and other organs. Although the sign or manifestation of an instinct may be suddenly given, the instinct itself, like every other power, seems to be unfolded gradually, and in correlation with the organs upon which its action depends.

In many precocious birds, which run, swim or fly at birth or shortly after, some of the instincts are relatively perfect at the moment of emergence from the shell, or according to certain observers even before this event, as when the young, which remain for hours with the shell chipped, are thought to respond instinctively to the warning cries of their parents. In rare cases, as in certain megapodes, they are born masters of their own destiny, and receive no care from parents which they never see. At the other extreme stand the common altrices, like the robin or cedarbird, which are blind at birth, and so helpless