THE TROPISM THEORY 271 



current we find certain features which agree with the local action tropism 

 theory. These features are so striking and so utterly at variance with 

 everything found in the remainder of the behavior of these organisms 

 that they throw into strong relief the contrast between the usual behavior 

 and the requirements of this tropism schema. Owing to the remarkable 

 cathodic reversal of the cilia (a phenomenon not paralleled under any 

 other conditions), the m6tor organs of opposite sides or ends of the 

 ciliate infusorian act under the electric current in different ways. The 

 result is behavior partly in accordance with the tropism schema. This 

 furnishes us with a picture of what behavior would be if this schema 

 held throughout. The unity and coordination that are so striking in 

 the remainder of the behavior are here quite lost. Different parts of 

 the motor organs urge the organism in different directions at the same 

 time. The animal seems to be trying to do two opposed things at once 

 (see p. 89). Nothing more ineffective and unpurposive can be imag- 

 ined than such behavior. But in producing these local effects the elec- 

 tric current is unique among stimuli, and the reaction is as far from 

 the typical behavior of these organisms as can be imagined. The elec- 

 tric current may be used for producing local contractions in man as 

 well as in Paramecium, but such contractions cannot be considered an 

 adequate type of the behavior of mankind. The electric current never 

 acts effectively on the organisms under the natural conditions, so that 

 normally they never show the pecuhar behavior produced by it. To 

 all the natural conditions of existence they react in a totally different 

 manner — a manner quite at variance with this tropism schema. 



In the bacteria as in the infusoria th6 behavior is not in accordance 

 with the above-discussed theory of tropisms. The details of the re- 

 actions are not so completely known as in the infusoria. But what we 

 know shows that the behavior of these organisms so far as involved in 

 the directed reactions is as follows: When stimulated the bacterium 

 changes its course, moving in some other direction, — a direction de- 

 termined by its own body structure, and not by the position of the 

 stimulating agent. 



Thus we find in the unicellular organisms very little in the behavior 

 that can be interpreted in accordance with this local action theory of 

 tropisms. The latter does not by any means express the fundamental 

 nature of their behavior in directed reactions. These are based chiefly 

 on the performance under stimulation of varied movements, with selec- 

 tion from the resulting conditions, — the "method of trial." 



In the symmetrical Metazoa we of course find many cases in which 

 the animal turns directly toward or away from a source of stimulation, 

 without anything in the nature of preliminary trial movements. This 



