198 Wild Birds 



reflect them back on the muscles, which were made to contract. 

 It thus implied a complicated mechanism of sense-organs at the 

 surface of the body, central cells in the cord, and conducting 

 nerves. The closing of the eyelid in response to a sudden move- 

 ment of any object towards the eye, and the adjustment of the 

 pupil by which the intensity of the light which falls upon the 

 retina is regulated automatically and with wonderful precision, 

 are typical illustrations of adaptive or useful reflex responses. 

 The tropisms usually involve more or less complicated reflex 

 movements, and between these forms of response no sharp dis- 

 tinctions can be drawn. Moreover, when reflex action is re- 

 duced to its essential qualities, the receipt and dispatch of 

 impulses, we must allow that the same type of response is seen 

 in plants as well as in the simplest unicellular animals, in which 

 the refinements of an elaborate system of conducting nerve- 

 fibers and distributing centers are unknown. In this wider sense 

 the simple, and uniform reflex response may be regarded as the 

 unit, as well as the most primitive type, of activity. 



For a still more complicated kind of behavior metaphysics 

 has given us the venerable term instinct, which has proved a 

 veritable apple of Sodom, and has been the source of more in- 

 effectual discussion than almost any other subject with which 

 modern biology has to deal. It is properly applied to a com- 

 plicated series of actions, which are useful to the individual or 

 to its progeny, but which are performed without any foresight 

 of the ends attained or the choice of means. The performance 

 of the most complex instincts, as in building the nest and caring 

 for the young, calls into play the whole body with all its powers, 

 rather than single parts or organs alone, in this respect agreeing 

 with tropisms but differing from most reflexes in the narrower 

 sense. 



We have already seen many illustrations of the feeding in- 

 stincts displayed at the nests of many wild birds. Old and 

 young seem to move like one being, so nicely adjusted are the 

 give-and-take reactions between mother and child. Such a bird 

 as the Cedar Waxwing, though blind at birth, possesses the 

 senses of hearing and touch. When stimulated by hunger, 



