164 THE STUDY OF ANIMAL LIFE CHAP. 



others ; they are independent of individual experience 

 and do not require control on the part of the central 

 nervous system ; they depend on inborn structural 

 linkages between particular sensory-cells which receive 

 signal-messages, and particular motor-cells which issue 

 commands to the muscles or to glandular organs. 



There is no doubt that these reflex actions are suffi- 

 cient to cope with many of the problems of life, especially 

 among the lower animals. But there is much more. In 

 difficult circumstances the creature tries a number of 

 answers and persists along the line that pays, as we see 

 when a starfish turned upside down tries to right itself. 

 Or again, a customary reflex may be inhibited by what 

 involves, or seems to involve, some appreciation of cir- 

 cumstances, as when a sea-anemone repeatedly cheated 

 with false food ceases to exhibit the usual reflex response. 

 The answer-back often varies with the physiological con- 

 dition of the organism at the time, and this includes the 

 registered results of past experiences. 



6. Tropisms. - - Another relatively simple kind of 

 activity is seen in what are called " tropisms ' -obliga- 

 tory movements which tend to secure physiological 

 equilibrium or comfort in reference to particular stimuli. 

 When a moth, flying about, comes within the sphere of 

 influence of a candle, and has one of its eyes much more 

 illumined than the other owing to the way in which it 

 happens to be flying, chemical processes are set up in 

 the illumined eye which are different from (e.g. quicker 

 than) those in the other. It follows that the nerves and 

 muscles of the two sides of the animal are differently 

 affected, and the moth tends to move so as to secure 

 that the two eyes are similarly affected. Thus it may 

 circle round and round the flame with a narrowing 

 radius, and eventually into it. This, briefly stated, is 

 an illustration of Locb's theory of tropisms. 



Tropistic movements are illustrated in reference to 

 light, warmth, gravity, pressure and currents, odours and 

 diffusing chemical substances, and they are of great im- 

 portance in the lower reaches of life. Some believe that 

 the " way-finding ' or " homing ' of migrating birds is 



