182 REPORT OF COMMISSIONERS OF INLAND FISHERIES. 



I. The Reactions. 



Reactions to Light. — In the discussion of this problem it will be found 

 of advantage not to separate our considerations of photopathy and 

 phototaxis. It is still a problem among students of animal behavior, 

 whether these two kinds of response are actually different, or are 

 •one and the same form of reaction. Many facts demonstrate that 

 they are really different, and that, although the phototactic reactions 

 do depend upon a certain optimum intensity of light, still the re- 

 action of organisms may be brought about sometimes by one and 

 sometimes by the other of these two factors. Indeed we shall find 

 in the case of the larval lobsters, that they may be either photo- 

 pathic or phototactic; moreover, that positive photopathy is not 

 necessarily associated with positive phototaxis, nor negative photo- 

 pathy with negative phototaxis, but that one form of reaction may 

 be positive, while the other is at the same time negative, and vice 

 versa. 



Yet, in view of the differences of opinion regarding what consti- 

 tutes a phototactic and what a photopathic reaction, it is perhaps 

 advisable to state more definitely just what meaning is given these 

 terms as they are used in the following pages. In other words, how 

 shall we differentiate between the intensity of light and the directive 

 influence of the light rays. It is hardly to be doubted that the 

 direction, per se, of the rays has no influence upon the movements of 

 organisms, other than that it determines what side of the organism 

 shall receive light of greater intensity. For this reason in the present 

 article, the term directive influence of light has no other meaning than 

 that the light coming from a certain direction strikes the eyes of the 

 organism and, by virtue of the unequal illumination, brings about a 

 constant form of response manifested by the turning of the organism 

 in one direction or another. In the following pages, then, a photo- 

 tactic response (which necessarily involves both a body orientation 

 and a progressive orientation) will be conceived of as a reaction in 

 which the organism tends to 'place the longitudinal axis of the body in a 



