REACTIONS TO LIGHT 253 



sity. The principal reason for thinking it is due to the 

 effect of constant intensity is the fact that there is no 

 immediate response when the intensity is changed. But 

 this retardation and slowness in reaction may be due to the 

 general character of the animals. As a matter of fact we 

 have as yet presented no conclusive experimental evidence 

 showing that reactions are dependent upon any action of 

 light other than that due to changes of intensity, although 

 we have several times intimated that in all probability the 

 activity of many organisms is affected by constant inten- 

 sity. The strongest evidence we have in support of this 

 is found in connection with observations on the change in 

 sense of reaction. 



However this may be, there is in this group no evidence 

 of a reaction to a sign. The reaction is undoubtedly a 

 direct response to the light itself. 



In organisms with image-forming eyes the reactions are 

 preeminently responses to signs, at least in so far as the eyes 

 function in the responses. These animals are not primarily 

 interested in light as light, but in what may follow a given 

 light condition, e.g., an image on the retina. At first 

 thought it seems as though here were a clear case of stimu- 

 lation due to the action of light through constant intensity, 

 for objects can be seen for some time without changing the 

 light configuration on the retina. No new image is how- 

 ever formed on the retina without change of intensity, and 

 it is consequently evident that here, too, the stimulation 

 may be due to changes of intensity rather than to constant 

 intensity. 



5. Classification of Reactions to Light — Phototropism, 



Photopathy 



Reactions to light have ordinarily been classified as 

 phototropic (phototropism) or phototactic (phototaxis) and 

 photopathic (photopathy). In some instances '' helio " has 

 been substituted for " photo." Organisms which orient and 



