INSTINCTS 163 



nature of the leaf starts the eating reactions, but "once 

 the eating reaction be set into play, it tends to continue, 

 so that the larva may then be induced to eat substances 

 which it would never have commenced to eat in the first 

 instance." 351 



These few examples may suffice to show that the theory 

 of tropisms is at the same time the theory of instincts if 

 due consideration is given to the role of hormones in 

 producing certain tropisms and suppressing others. A 

 systematic analysis of instinctive reactions from the view- 

 point of the theory of tropisms and hormones will prob- 

 ably yield rich returns. As an example we may quote the 

 fact that diurnal depth migrations of aquatic animals, 

 consisting in an upward motion during the night and a 

 downward motion during the day, are in all probability 

 determined by a periodic change in the sense of 

 heliotropism. 183 ' 30 



