REACTIONS TO LIGHT IN CILIATES AND FLAGELLATES. 7I 



What we desire in the study of animal behavior is (i) a correct 

 description of what occurs ; (2) an understanding of the relation of 

 what occurs here to other phenomena, this constituting their " explana- 

 tion" so far as an explanation is possible. Whether the phenomena 

 when correctly described and unders'tood are found to fall under some- 

 one's definition of a tropism is comparatively unimportant ; it is only 

 after such correct description and understanding that final definitions 

 can be made. I question much if there has not been undue haste in 

 framing precise definitions for the phenomena of animal behavior, when 

 we know so little about the phenomena in any thorough way. Radl, 

 I believe, makes a fundamental error in attempting to separate " Pho- 

 totropismus" rigidly from otlier reactions to light. Thus, he repeat- 

 edly cites Euglena as an example of an organism that shows undoubted 

 phototropism. On page 114 he further cites the motor reaction of 

 Euglena when suddenly shaded * as a reaction that has nothing to do 

 with phototropism. As I have shown above, the two are really 

 closely bound up together ; the orientation in the "phototropism" is 

 produced through this motor reaction. When the reactions of organ- 

 isms to light are known in detail, I believe that many other reactions 

 which Radl (p. 1 14) attempts to separate sharply from " phototropism " 

 will be found closely connected with the reactions that go under that 

 name. I had occasion to point out, in the paper preceding this, on 

 the reactions to heat, that if everything which the organisms do, except 

 the orientation itself, is left out of consideration, the orientation can be 

 accounted for by any theory desired. A thorough study of precisely this 

 point — the relation of "phototropism" to the phenomena supposedly 

 unconnected with it — would, I believe, have saved Radl from marring 

 his otherwise most excellent and useful contribution to the study of 

 light reactions by the proposal of so fantastic a theory to account for 

 the reactions to light ; a theory that fairly produces a shock in the mind 

 of the reader when it is reached, coming as it does after Radl's thorough 

 and valuable objective study of many of the phenomena and his exceed- 

 ingly sane, if somewhat sharp, criticism of other theories. Definition 

 and precise classification are of course valuable at a certain stage of 

 knowledge, but when carried out without a thorough knowledge of the 

 phenomena dealt with they may be a hindrance rather than a help. 

 The thorough knowledge of the phenomena of animal behavior required 

 for this is far from existing at present. 



* Radl says when '' beleuchtet " ; this is evidently a slip of the pen. 



