250 LIGHT AND THE BEHAVIOR OF ORGANISMS 



it is in the preceding group. In both, the reaction has to 

 do with the well-being of the individuals responding. But 

 in the one it has to do with protection against mechanical 

 injury, while in the other it has to do with the process of 

 procuring food. The important point is that the shadow 

 in itself is of no particular importance in either case, but 

 what follows may be. It is a response to a sign just as 

 truly as is the reaction of a dog to which Brooks refers in 

 the following characteristically convincing words (1907, 

 P- 53) » " The kick is a sign of something which may follow, 

 and the actions which do follow are not the effect of the 

 kick, for they are directed or adjusted, either consciously 

 or unconsciously, to an event of which it is only the fore- 

 runner." 



3. Reactions to Sudden Increase of Light Intensity 



There are reactions to changes of intensity which appear 

 to be more directly concerned with the effect of light than 

 are those just referred to. They are mostly responses to 

 an increase of intensity and may be due to the effect of the 

 change of intensity or to the effect of the absolute illumina- 

 tion. An earthworm, e.g., jerks back into its burrow when 

 light is flashed upon it, as definitely as Hydroides does when 

 it is suddenly shaded. If however the intensity is gradually 

 increased it may not react at all. This is therefore evi- 

 dently a reaction which is primarily dependent upon the 

 time rate of change of light intensity. A sudden decrease 

 of intensity produces no such reaction. I have frequently 

 observed similar reactions in Stentor coeruleus, as did also 

 Bronn in the actinia, Edwardsia and Cerianthus; Nagel 

 (1896) in the sea squirt, Ciona, and in several different 

 mollusks, and Parker (1908, p. 419) in Amphioxus. Parker 

 says: " In all the tests I carried out, I never observed a 

 reaction to a rapid diminution of light, and the reactions 

 to light that did occur were always the result of a rapid 

 increase of intensity. When an animal was resting quietly 



