THE TROPISM THEORY 275 



that of lower organisms; in this sense the botanist Pfeffer (1904, p. 587) 

 consistently remarks that a man who bends toward a lighted window 

 shows phototropism as does a plant. The use of the word in this purely- 

 descriptive sense is often convenient, but we need to keep in mind the 

 fact that the word thus used involves no explanation, and includes phe- 

 nomena of the most heterogeneous character. 



By some writers the word " tropism " is restricted to the bending or 

 inclination of a fixed organism, while the movements of free organisms 

 under the influence of external agents are called taxis. This distinc- 

 tion is a purely descriptive one. 



Some writers reserve the term " tropism " (or taxis) for those reactions 

 in which the organism takes up a well-defined orientation with relation 

 to the line of action of some external agent. Other reactions, in which 

 orientation is not a feature, are variously designated as kinesis (Engel- 

 mann, 1882 a; Rothert, 1901; Garrey, 1900), as -pathy (Davenport, 

 1897 ; Yerkes, 1903 b ; and others), as -metry (Strasburger, 1878 ; Olt- 

 manns, 1892), and by various other names, depending on the method 

 by which the author in question considers them to be brought about. 

 On this basis the reactions of infusoria to water currents, gravity, the 

 electric current, and to light coming from one side would be called 

 tropisms or taxis; while the reactions to chemicals, osmotic pressure, 

 heat and cold, and mechanical stimuli would be designated by some 

 other term. 



An immense number of technical terms have been devised for appli- 

 cation to the phenomena of behavior in the lower organisms. A system- 

 atic exposition of a very complete set of such terms will be found in the 

 paper of Massart (1901). The "Plant Physiology" of Pfeffer (1904) 

 likewise deals extensively with this matter. A proposed new terminol- 

 ogy applying to many of the features of behavior is set forth by Beer, 

 Bethe, and v. UexkiiU (1899). A number of other references to this 

 matter will be found in the literature hst at the end of the present chapter. 



As to the value of giving technical names to every distinguishable act 

 that an organism performs, opinions will differ. So far as the names 

 are purely descriptive, expressing nothing more than some observed 

 action of the organism, it is difficult to see any very great advantage in 

 their use. To say that an organism shows phobism (Massart), is merely 

 to say that it moves backward; to say that it reacts by dorsoclinism 

 (Massart), is the same as to say that it reacts by turning toward the dorsal 

 side. To most readers the latter expressions are more intelligible than 

 the former, and they are equally accurate and complete. Such purely 

 descriptive terms embody no results of scientific analysis. Their use is 

 therefore merely a question of convenience or taste on the part of the 



