THOPHIC ACTION OF LIGHT. 



101 



' 



Extrinsic stimuli may be (1) im- 

 mediate in action, as when a body 

 acts by mechanical contact ; (2) me- 

 diate, as when the sight of an object 

 produces a visual stimulus which 

 passes to the subject from the object 

 seen. These mediate influences have 

 sometimes been called sympathies. 

 In the infant at birth, respiration 

 is a mediate reflex act. Air ex- 

 cites the movement in a normally 

 constructed child. Sucking is a re- 

 flex act due to immediate mechanical 

 irritation, and is dependent upon the 

 medulla oblongata. Such actions are 

 called " instinctive." In this sense 

 movements in plants are "instinc- 

 tive," that is, due to their essential 

 construction. 



As to the more distinctly trophic 

 effects of light. Pigmentation of the 

 skin occurs in the tropical regions 

 when light is strong. 



Charles Darwin * gives an account 

 of certain cave-living animals whose 

 eyes have, in the course of successive 

 generations, been lost owing to dis- 

 use. "It is well known that several 

 animals, belonging to the most 

 different classes, which inhabit the 

 caves of Carinola and of Kentucky, 

 * " Origin of Species," p. 110. 



