58 PLANT-ANIMALS [CH. 



tropistic responses to light and gravity. A stimulus 

 may however induce responses in an organism of a 

 very different kind. It may give rise, not to a change 

 of place, but to a change of state. Such effects of 

 stimulation are called tonic effects, and the organism 

 which responds to them is said to be in a state of 

 tone or tonus. Certain peculiar effects of this kind 

 are well known among human beings and may serve 

 us as illustrations. People who work all day by 

 artificial light, specially by unmitigated electric or 

 incandescent gas light become irritable and depressed. 

 Professional photographers, Avho spend long hours in 

 "dark rooms" developing photographs in red light, 

 suffer mentally in a similar way. The change of 

 state induced by such abnormal conditions we may 

 describe as a change of tone. We will assume that 

 light is indispensable to the human race, that men's 

 bodies are attuned to light and that this harmony 

 is maintained by an unceasing sequence of light- 

 stimuli which contribute to the well-being of the 

 nervous system. Then, if we adopt this view, it will 

 be easy to imagine that a cessation of the rain of 

 stimuli may prejudice the well-being of the nervous 

 system and be the origin of disorders of more or 

 less severity. 



How far the normal nervous state of human 

 beings is determined by the tonic effect of light it 

 is not possible to say ; but there is no doubt that 



