7O THE BEHAVIOR OF LOWER ORGANISMS. 



p. 397) has called particular attention to this as a possible factor in the 

 so-called phototropism. Radl also refers (p. 99) to Exner's view that in 

 Copilia the movements of the eyes are in the nature of a trial (" abtasten ") 

 of the surroundings. Radl's statement, quoted above, can then hardly 

 be considered strictly accurate, even leaving out of consideration the 

 results set forth in the present paper. In many organisms, doubtless, 

 the reaction to light is of that direct character assumed to be general by 

 Radl. But it may be strongly doubted whether this is what we may 

 call a primitive condition ; in other words, whether it does not involve 

 more complicated internal processes than the reaction by " trial and 

 error." In any case, I am convinced that a similar reaction to light by 

 the method of " trial and error " will be shown to exist in many other 

 organisms ; it is demonstrated, for example, in Rotifera, in the paper 

 which follows the present one. 



Recourse will doubtless be taken to the usual refuge when a sharp 

 concept has been defined to which the phenomena are not found to 

 correspond ; the reactions of the ciliates and flagellates will be simply 

 excluded from the tropisms and the definition of the latter maintained 

 in all its pristine purity. Indeed, it may be questioned whether the 

 reactions of infusoria (and Rotatoria) to light are not excluded from 

 phototropism through the definition given by Radl on p. 140, what- 

 ever the method by which they are produced. Radl says " Unter 

 phototropischer Orientierung ist die Fahigkeit der Organismen zu 

 verstehen, eine feste Einstellung der Achsen des gesamten Korpers in 

 dem Lichtfelde einzunehmen." Since the ciliate or flagellate (or 

 rotifer) revolves continually on its long axis, and swerves continually 

 toward a certain side, it can hardly be said that the body axes have a 

 " feste Einstellung" with reference to the light. In an explanatory 

 paragraph Radl says that in orientation " immer geht dann der 

 Lichtstrahl durch die (morphologische) Symmetrieebene des Korpers" 

 (/. c., p. 140) . This is certainly not true for the ciliate or flagellate (or 

 rotifer), even leaving out of consideration the fact that in the former 

 two groups the animals are usually unsymmetrical. If it be proposed, 

 then, to exclude the light reactions of ciliates, flagellates, and rotifers 

 from the concept of " Phototropismus," one can only agree that this is 

 necessary, in view of the definitions of that concept. 



But what is the value of a definition which excludes some of the chief 

 phenomena on which the concept that we are attempting to define is 

 based ? And what is the value of a theory that depends on such a 

 definition and that can only be correct so long as we hold to this 

 definition? The phenomena themselves are, after all, the final refer- 

 ence for testing the correctness of any definition or theory ; it is the 

 observed phenomena that we are attempting to formulate and explain. 



