246 THE BEHAVIOR OF LOWER ORGANISMS. 



work of Lloyd Morgan, Thorndike. and others. With the details of its 

 manifestations in higher animals we need not concern ourselves here. 



Let us now return to the method of trial and error in the infusoria. 

 Here it is most strongly marked, and certain general problems which 

 arise from it are sharply defined. It is possible to formulate the reac- 

 tion method of the infusoria as follows : When effectively stimulated 

 by agents of almost any sort the organism moves backward, and turns 

 toward a structurally defined side A', while at the same time it may 

 continue to revolve on its long axis. As thus stated, the reaction 

 method seems exceedingly simple and stereotyped, and as such I pre- 

 sented the behavior of these organisms in a former general paper.* If 

 we limit ourselves to a consideration of the reaction itself, this seems 

 inevitable, although certain additional reactions have been described 

 since that paper was written. But when we study the relation of this 

 reaction method to the environmental conditions, the results are most 

 remarkable, and a totally new set of problems appears. Whether the 

 behavior is to be called simple and stereotyped, or complicated and 

 flexible, is not so easy to decide. The relations of the reaction of the 

 environmental conditions are, perhaps, the really essential point in 

 animal behavior. What are the relations which we find in the organ- 

 isms reacting in the way set forth above? 



In general terms we find that through this reaction by trial and error 

 the organisms are kept in conditions favorable to their existence, and 

 prevented from entering unfavorable regions. Through it they keep 

 out of hot and cold regions and collect in regions of moderate temper- 

 ature. Through it they tend to keep out of strong or injurious chemi- 

 cals and out of regions where the osmotic pressure is much above or 

 below that to which they are accustomed. Through it they gather in 

 regions containing small amounts of certain chemicals, not leaving 

 them for reo-ions where there is either more or less of these chemicals. 



O 



When oxygen is needed they collect through this reaction in regions 

 containing oxygen ; when the oxygen pressure is high, they do not 

 react with reference to oxygen, or through this reaction they avoid 

 regions containing much oxygen. Through this reaction organisms 

 which contain chlorophyll, and therefore need light, gather in lighted 

 regions or move toward the source of light ; through the same reaction 

 the same organisms avoid very powerful light. f In all these cases, 



* Psychology of a Protozoan, Amer. Journ. Psychol., vol. 10, 1899, pp. 503-515. 



t For details, see the author's Studies, already referred to, and the preceding 

 contributions. In papers by Engelmann (1882, a) and Rothert (1901) the reac- 

 tion method involved is also described for certain organisms, though these 

 writers, like the present author in his earlier papers, did not bring out the rela- 

 tions to a general method of trial and error. Engelmann, however, character- 

 ized the reaction of Euglena to light directly as a " Probiren" a " trial." 



