98 THE BEHAVIOR OF LOWER ORGANISMS. 



do for mechanical stimuli. This* is owing to the fact that the organism 

 determines for itself the region in which it shall be stimulated by a 

 chemical in solution, as well as the side toward which it shall turn. 

 Now, it appears that the side on which, by its own activities, it is, as a 

 rule, first stimulated by a chemical, is (usually, at least) opposite that 

 toward which it turns. (For details, see Jennings, 1902, a.) It could be 

 contended, therefore, that the direction of turning, in the case of chemi- 

 cal stimuli, is a result of the direct action of the stimulating agent on 

 the side stimulated. Such a contention would have little general 

 significance, however, in view of the fact that the same reaction occurs 

 as a response to various other stimuli, where this explanation is quite 

 impossible. 



In certain higher organisms, researches which were made under the 

 direction of the writer and are soon to appear will show (i) that chemi- 

 cal stimuli may produce local contractions in the part of the body with 

 which the chemical comes in contact ; (2) that these local contractions 

 have little to do with the characteristic behavior of the animals when 

 subjected to chemicals. 



REACTIONS TO HEAT AND COLD. 



Reactions to heat and cold have been fully discussed in the first of 

 these contributions. It is only necessary, therefore, to point out that 

 the results are in almost every detail parallel with those for reactions 

 to chemicals, and in the same way and to the same degree inconsistent 

 with the theory of tropisms. In organisms higher than the Infusoria 

 and Rotifera, the reactions to heat and cold have been very little studied 

 from the present point of view. 



REACTIONS TO CHANGES IN OSMOTIC PRESSURE.* 



In the ciliate infusoria the reactions to differences in osmotic pressure 

 are identical with those to chemicals, save that the organisms are much 

 less sensitive to osmotic changes. (Details in Jennings, 1897 and 1899.) 

 The bearing of these reactions on the theory of tropisms is, therefore, 

 the same as was brought out above in the discussion of the reactions to 

 chemicals. 



REACTIONS TO LIGHT. 



The phenomena shown in the reactions of organisms to light have per- 

 haps formed the chief basis for the theory of tropisms. There is usually 

 a definite orientation shown by the organisms ; they move with the axis 

 of the body parallel with the light rays either to or from the source of 

 light. The existence of such orientation forms the basis of the theory 

 of tropisms, and has been considered sufficient in itself as a proof of the 



* "Tonotaxis," Massart; " Osmotaxis," Rothert. 



