THE WORLD OF LIVING ORGANISMS 133 



stimulus are the bacteria of putrefaction, which form the 

 creature's food. Beside these the restless infusorian takes 

 anchor. When it touches these (second indication), even ever 

 so slightly, it stops the movements of its locomotor cilia, and, 

 by means of the cilia round the mouth, drives the food into 

 its fluid interior. 



The enemy-circle has a special indication, for the in- 

 fusorian, when attacked by its enemy — i.e. Didinium nasutum 

 — fires off a sort of gelatinous discharge. There is also an 

 indication belonging to the sexual circle, which initiates 

 copulation. 



For further examples, I refer the reader to my book 

 Umwelt und Innenwelt der Tiere. I shall mention only the 

 mollusc, Pecfen jacohcBiis, which has eyes that receive as 

 indication the slow movement of all sorts of objects, but 

 gives no active response to any of these pictorial impressions. 



STIMULUS — INDICATION — QUALITY 



From the examples quoted, it must be clear that stimulus 

 and indication are not identical concepts, although they are 

 applied to the same qualities. 



Only if we bear in mind wherein the three modes of in- 

 vestigation — i.e. the physiological, the biological, and the 

 psychological — differ from one another, shall we understand 

 how to apply these three elementary concepts correctly in 

 each case. 



In the process of investigation, the physiologist and the 

 biologist take up the same attitude, for they consider them- 

 selves as observers external to what they observe. They 

 have before them an object and an animal; they both \/ 

 study the influence which the object exerts on the animal. 

 But the physiologist investigates the causal connection, and 

 the biologist its conformity with plan. As a consequence, 



