THE THEORY OF TROPISMS. 103 



two halves of the body becomes much more powerful, because the 

 number of cilia affected in this way is much greater. The result is that 

 it almost alone determines the nature of the reaction. The direction of 

 turning is, therefore, determined by the way in which the current strikes 

 the body, as required by the theory of tropisms. But it should be 

 recognized that this is by no means universal among the infusoria ; in 

 doubtless fully as many cases the direction of turning is determined, 

 even under the electric stimulus, by an internal factor. 



This peculiar dual character of the reaction to the electric current 

 one strong factor being due to the inherent tendency of the organism 

 to turn in a certain definite way, without regard to the way in which 

 the stimulating agent impinges upon it has been studied in detail in 

 recent contributions by Pearl (1900), Putter (1900), and Wallengren 

 (1902 and 1903). We may perhaps compare it, without indicating any 

 similarity in details, to the behavior of a person who has taken hold of 

 the electrodes from a powerful induction coil. He attempts in various 

 ways to free himself from the electrodes. This may be compared with 

 the attempt of the infusorian to perform its usual reaction to strong 

 stimuli. He also undergoes involuntary contortions, due to the action 

 of the electricity on his muscles ; these may be compared with the pecu- 

 liar effect of the electric current on the cilia of the infusoria, causing 

 them to strike in opposite directions on the two halves of the body. 



Putting all together, we are not justified in taking the reaction of the 

 infusoria to the electric current as a general type for the reactions of 

 the lower organisms, because in its characteristic features it differs 

 from all the other known reactions. Yet it is exactly these unique 

 features that bring it into (partial) agreement with the tropism schema. 



RESUME AND DISCUSSION. 



We have thus passed in review the reactions of a large number of 

 lower organisms to the commoner stimuli, so far as they are known 

 from exact observations. We have found that as a rule they do not fit 

 into the tropism schema. In the reactions to mechanical stimuli, to 

 heat and cold, to chemicals, to changes in osmotic pressure, orientation 

 is not a primary or striking factor of the reaction ; when a common 

 orientation of a large number of organisms occurs, it is a secondary 

 result, due to the fact that the organisms are prevented from swimming 

 in any other direction. In the reaction to light orientation is a strik- 

 ing feature ; but the orientation, in certain precisely investigated cases 

 at least, is brought about in a manner which is inconsistent with the 

 tropism schema. In the reaction to gravity the precise reaction method 

 has not been worked out. Only in the reaction to the constant electric 

 current do we have in some organisms a partial agreement in principle 



