THE WAYS OF LIFE 193 



invisible, they did not move in any definite direction, 

 and the control experiments showed that their behaviour 

 was not affected by the sound or smell of the sea. 



Differential Sensitiveness. — Associated with tro- 

 pism, is the phenomenon of ' differential sensitiveness ', to 

 which Loeb and Bohn have attached great importance. 

 An animal which is moving towards the Ught comes to a 

 shadow ; it may cross it, it may come to a standstill, it 

 may recoil, but usually it tends to rotate through 180° and 

 to proceed for a time in the opposite direction. The same 

 phenomenon is observed in relation to gravity and 

 chemicals diffusing in the water. 



It is remarkable to see a tube-inhabiting worm in an 

 aquarium instantaneously draw in its head and tentacles 

 when one simply puts one's hand between it and the hght. 

 In the cells of these beautifully expanded filaments chemical 

 processes were going on briskly and at a certain rate ; by 

 making a sudden shadow one makes a sudden change in the 

 rate. The disturbance stimulates the sensory nerves, and 

 a message travelhng outwards again commands the muscles 

 to contract. But it all happens so quickly — ^before one 

 has time to say, ' Look at that ! ' 



Differential sensitiveness is often mixed up in actual 

 life with a tropism-reaction. Thus Bohn's experiments on 

 starfishes led him to conclude that these brainless creatures 

 are the slaves of diverse impulses, whose combination may 

 be recognized in their behaviour. There is the impulse 

 due to the immediately preceding state, there is the 

 tropism-impulse, and there is the rotatory or oscillatory 

 impulse. The result is an organic (not a deliberate) 

 compromise, which Bohn says may be almost certainly 

 predicted in given conditions, It cannot, however, be 







