334 THE BIOLOGY OF BIRDS 



we see the beginning of experiment — simple tentatives, 

 as when an Amoeba hunts another Amoeba. 



(3) A third level is illustrated on its reaction side by 

 simple reflex actions, as when a sea-anemone grapples an 

 animal that has touched its tentacles, or when an earth- 

 worm jerks itself into its hole when the earth vibrates 

 under the blackbird's light tread. These responses depend 

 on inborn, pre-established linkages between certain nerve- 

 cells and certain muscle-cells. On the experimental side 

 may be ranked those cases where an animal, faced by some 

 " problem," tries its repertory of reactions, one after the 

 other, until, perhaps, it finds one which offers a solution. 

 The Trumpet-Animalcule (Stentor) on which the experi- 

 menter showers microscopic dust will try three reactions 

 before it comes to that of loosening itself and swimming 

 away. A Brittle-star entangled in little hoops of metal 

 over its gymnastic limbs will try a variety of movements 

 before it gets free, but as it becomes more experienced it 

 reduces the number of useless attempts, thus striking a 

 new note — the beginning of learning. 



(4, 5, 6) On a higher level must be ranked compound 

 reflex actions, tropisms, and enregistered rhythms. When 

 the nestling opens its mouth at the touch of the food in its 

 mother's bill, and then proceeds to swallow, the behaviour 

 is reflex. But the reflex actions that are involved are more 

 complicated than those of the sea-anemone closing its 

 tentacles. 



By a tropism is meant an engrained constitutional 

 obligation to adjust the body so that the two sides — it may 

 be the two eyes, ears, nostrils — are equally stimulated. 

 It is an automatic means of securing physiological equili- 

 brium. When young eels are ascending the river they 

 automatically adjust themselves so that both sides of the 

 body are equally aff"ected by the pressure of the stream. 

 Thus they tend to swim straight up-stream. When a moth 

 flies past a candle and has its right eye much more illumined 

 than its left, it automatically adjusts its body so that equi- 

 librium of stimulus is attained. Thus it often flies into the 



