3o6 BEHAVIOR OF THE LOWER ORGANISMS 



F, Localization of Reactions 



A second factor that is of great importance in making the move- 

 ments effective lies in the proper localization of the reactions. An 

 organism that moves directly away from an unfavorable agent (or di- 

 rectly toward a favorable one) has a great advantage over an organism 

 whose movements are not thus accurately directed. There are great 

 differences in different organisms in this respect ; some react very pre- 

 cisely with reference to the position of the stimulating agent, while others 

 do not. 



How is the relation of the reaction to the localization of the stimulus 

 brought about, and what is the cause of the differences between differ- 

 ent organisms in this respect ? 



In answering this question, we can distinguish three different classes 

 of phenomena. These are the following : — 



(i) First we have the simple phenomenon that when a portion of 

 an organism is stimulated this portion may respond by contraction, ex- 

 tension, or other change of movement. If the remainder of the body 

 does not respond, or responds in a different way, this gives at once a 

 reaction localized in a certain way with reference to the place of stimu- 

 lation. Such local responses we find in Amoeba, where the part strongly 

 stimulated contracts, or if stimulated by a food body it extends. The 

 same phenomenon is found in Hydra, in the bending of the body when 

 one side is powerfully stimulated, in the bending of the tentacles of 

 Sagartia toward the point stimulated, and in the local contractions of 

 the medusa and of stimulated points on the body of the flatworm and 

 many other soft-bodied animals. The same thing is seen even in man 

 when the electrode of a battery is applied directly over a muscle ; this 

 muscle now contracts. This seems a simple and primitive phenomenon, 

 and as such has been seized upon by the "tropism theory" and made 

 the chief factor in the behavior of lower organisms, and particularly in 

 all directed reactions. As we have shown in our chapter on that theory, 

 this factor plays by no means the extensive part assumed by the theory, 

 and is quite inadequate to account for most of the behavior of lower 

 organisms. Even in the behavior of the organisms mentioned above, 

 where it clearly does play a part, this part is a subordinate one (see 

 Chapter XIV). In many organisms, such as the free infusoria and 

 some rotifers, it is hard to detect any part of the effective behavior that 

 is due to local reaction at the point stimulated. The fact that such 

 local reactions may and do occur in organisms is of course a fact of 

 much importance, but taken by itself it is utterly inadequate as a general 

 explanation of directed reactions. 



