HISTORICAL REVIEW 35 



idea that orientation is controlled by the direction in which 

 the rays strike the surface, or the angle they make with the 

 surface. His statements from 1906 to 1909 indicate that 

 he thinks that orientation is regulated by the relative inten- 

 sity of light on symmetrically located sensitive structures on 

 opposite sides of the organism, a view which Sachs strenuously 

 opposed. 



(7) Loeb's theory of orientation with reference to plants 

 implies that the external agent acts on the motor apparatus 

 directly, and with reference to animals that it acts either on 

 the motor apparatus directly or through a direct reflex arc. 



(8) He thinks that movements in plants and animals are 

 controlled unequivocally by external agents and that they 

 are not fundamentally adaptive. " Eine 'Auswahl' einer 

 passenden Beleuchtungsintensitat habe ich nie beobachtet" 



(1909, p. 35). 



(9) Reactions to light may be heliotropic or photokinetic. 

 The former are never due to change in light intensity, they 

 " are a function of the constant intensity; (the latter) a 

 function of the quotient of the change of intensity over 

 time," i.e., rate of change of intensity. There is a perfect 

 analogy between the effect of light and the effect of a 

 constant electric current. 



(10) Aggregation in some forms is due to photokinetic 

 reactions. 



(11) Loeb considers his theory applicable to the reactions 

 of the infusoria as well as to those of higher animals and 

 plants. 



(12) He stands for an objective explanation of the be- 

 havior of animals in all his work, but he cannot be con- 

 sidered as the originator of this idea. Nor was he the first 

 to attempt to put it on an experimental basis. 



Verworn was one of the first investigators in comparative 

 physiology in its broadest sense. He was of the opinion 

 that the fundamental physiological and psychological pro- 

 cesses are common to all animals and that they can be 

 solved in the simple forms more readily than in the more 



