93 THE BEHAVIOR OF LOWER ORGANISMS. 



ESSENTIAL POINTS IN THE THEORY OF TROPISMS. 



The two essential features of the theory of tropisms are apparently 

 the following: (i) The movements of organisms toward certain 

 regions and their avoidance of others are due to orientation; i. e., to 

 a certain position which the organism is forced by the external stimulus 

 to take, and which leads the organism toward (or away from) the 

 source of stimulus, without any will or desire of the organism, if we 

 may so express it, to approach or avoid this region. (2) The external 

 agent by which the movement is controlled produces its characteristic 

 eflect directly on that part of the body upon which it impinges. It 

 thus brings about direct changes in the state of contraction of the 

 motor organs of that part of the body affected as compared with the 

 remainder of the body, and to these direct changes are due the changes 

 shown in the movements of the organism. This is brought out clearly 

 in the quotation from Verworn given on page 8. Loeb (1900, p. 7) 

 sums up the theory of tropisms as follows : 



The explanation of them [the tropisms] depends first upon the specific irrita- 

 bility of certain elements of the body surface, and, second, upon the relations 

 of symmetry of the body. Symmetrical elements at the surface of the body 

 have the same irritability; unsymmetrical elements have a different irritability. 

 Those nearer the oral pole possess an irritability greater than that of those 

 near the aboral pole. These circumstances force an animal to orient itself toward 

 a source of stimulus in such a way that symmetrical points on the surface of the 

 body are stimulated equally. In this way the animals are led without will of 

 their own either toward the source of stimulus or away from it. 



Holt & Lee (1901) again bring out our second point, as applied 

 to reactions to light, with especial clearness : 



The phenomena that have led to such an assumption can be satisfactorily 

 explained on the simpler theory that every ray of light impinging on an organism 

 stimulates at the point oh -which it falls * and in proportion to its intensity. * * * 

 The light operates, naturally, on the part of the animal which it reaches. The 

 intensity of the light determines the sense of the response, whether contractile 

 or expansive, and the place of the response, the part of the body stimulated, 

 determines the ultimate orientation of the animal." (Holt & Lee, 1901, pp. 

 479-480.) 



The theory of tropisms as above set forth depends upon the reflex 

 contractility of the motor organs when affected by certain stimuli. An 

 attempt has been made to give it a still simpler form in a recent paper 

 by Ostwald (1903). Ostwald would omit even the factor of reflex 

 irritability, holding that the turning which brings about orientation is 

 a mechanical result of differences in the internal friction of the water or 



♦Original not italicized. 



