1 8 THE BEHAVIOR OF LOWER ORGANISMS. 



begins, and thinks this is due to " individual differences, or to ill- 

 defined internal causes, or perhaps rather to the heterogeneity of the 

 medium in which they find themselves" (Mendelssohn, 1902, c, p. 

 493), and that it has nothing to do with thermotaxis proper. This is 

 typical of many of the statements made concerning the behavior of the 

 lower organisms ; the movements, so long as they do not agree with 

 the preconceived schema, are cast aside as disordered, and attention is 

 called only to the movements that do not conflict with the theory. 

 Thus Mendelssohn says that this disordered movement u ceases im- 

 mediately as soon as the thermotactic action manifests itself" (/. c., p. 

 492). This is true merely because the thermotactic action is conceived 

 to begin only after the organism has, through the movements above 

 described, gotten itself into such a position that it moves away from 

 the heated region. Of course if all movements except those after 

 orientation has occurred are thrown out of consideration, the orientation 

 can be accounted for in any way desired. 



In Paramecium, for which alone Mendelssohn attempts to give an 

 account, based on observation, of the mechanism of the thermotactic 

 response, the exact character of the movements is undoubtedly difficult 

 to observe. This animal is nearly cylindrical in section ; the oral side 

 is very slightly marked, the movements are rapid, and the animal con- 

 tinually revolves rapidly on its long axis, so that observation of the 

 relation of the direction of turning to the differentiations of the body is 

 very difficult. In Oxytricha and other Hypotricha these difficulties 

 are almost absent ; the body is markedly differentiated ; the movements 

 are less rapid, and, most important of all, there is usually no revolu- 

 tion on the long axis. It is unfortunate therefore that Mendelssohn 

 included none of the Hypotricha among the organisms which he 

 studied. With careful observation of the movements of individuals 

 the mechanism of the reactions is in these animals absolutely clear. 



A crucial test of the theory of direct orientation as maintained by 

 Mendelssohn is given by observation of the direction in which the 

 animals turn in becoming oriented. Mendelssohn (1902, c, p. 492) 

 says that after the disordered movements " the movements executed to 

 place the body in orientation are rather movements of rotation." This 

 could hardly be otherwise, but the important question for deciding as 

 to the nature of the reaction is, How does the rotation take place? Is 

 it determined by the direction from which the heat comes, as required 

 by Mendelssohn's theory, or is it determined by the differentiations of 

 the animal's body ? This point is a decisive one for interpreting the 

 nature of the reaction. Suppose we have an Oxytricha in the position 

 a-a, Fig. 8, and heat is applied in such a way as to reach the organism 



