Harper, Behavior of Corethra. ^.^J 



The apparent desire to be rid of the directive influence of exter- 

 nal stimuh and exalt the animal's self-regulative pov^er, lands one 

 in the difficulty of seeming to diminish the animal's self-regulative 

 capacity, by explanations which fortunately do not affect the 

 modus Vivendi of the animals under discussion. 



The principle of trial and error in its application to the behavior 

 of some of the lowest forms, has been applied with greatest plausi- 

 bility to animals whose movements are of an undifferentiated 

 type, as in many Protozoa. The fact that behavior may be of 

 various degrees of differentiation has an important bearing on 

 these problems. In the undifferentiated activities of Euglena or 

 Paramcecium the same movement comprises both internal and 

 external factors and can be interpreted only by analysis into its 

 components. Thus the protozoan, while continually swerving 

 aborally along its spiral path, and thus coming into contact with 

 an ever-changing environment, may broaden the spiral by swerv- 

 ing still further aborally in answer to an external stimulus. So 

 in the same movements are blended spontaneous trial movements 

 and directed ones; locomotion of a peculiar, adaptive type, food- 

 taking, and it may be, an avoiding reaction. But in any form 

 having a differentiated type of behavior, the different forms of 

 reaction occur separately. 



It is manifest that an undifferentiated movement cannot be 

 regarded as a pure tropism, for the latter is defined as the result of 

 an external stimulus alone. The pure tropism can only be found 

 in a differentiated type of behavior and is to be regarded as a 

 product of evolution from behavior of the undifferentiated sort. 

 The difficulty in harmonizing the behavior of Paramcecium with 

 the accepted and simple type of the tropism found in more special- 

 ized animals, upon which the "local action" theory of tropisms has 

 been based, is the point here emphasized, that the movements 

 of Paramcecium do not show the tropism in pure form, but an 

 undifferentiated type of activity. Moreover the apparent sim- 

 plicity of tropisms should give no comfort to the other school who 

 regard movements as directly produced by the environment, when 

 they are reminded of the probable phylogeny of the tropism. 

 Simplicity and directness are the outcome of an evolutionary 

 process, not signs that the organism is acted upon as iron filings 

 are by a magnet. 



It is scarcely necessary to point out this similarity between exter- 



