ACTIVITY AND ENVIRONMENT 29 



Reactions to light are called phototaxis or heliotropism 



" temperature are called thermotaxis " thermotropism 



" moisture are called hydrotaxis " hydrotropism 



" gravitation are called geotaxis " geotropism 



" chemicals are called chemotaxis " chemotropism 



" contact are called thigmotaxis " stereotropism 



" pressure are called barotaxis " barotropism 



electric currents galvanotaxis " galvanotropism 



current in medium rheotaxis " rheotropism 



If we place a number of common pond snails in a dish which is dark 

 at one end and grades to sunlight at the other, we find that most of the 

 snails are found after a time in faint light. The explanation of this 

 phenomenon is that the snails are stimulated by intense light and by 

 very weak light, i.e., either of these conditions of illumination interferes 

 with some of the internal processes of the animal, and the random 

 movements which result bring the animal into various conditions, one of 

 which (faint light) relieves the disturbance. The animal then ceases to 

 move at random, because its internal processes are no longer interfered 

 with by the stimulus. The snail's activity is lessened, or it turns back 

 from regions of either too strong or too weak light; accordingly, most of 

 the snails are found in faint light. The internal processes have been 

 adjusted or regulated. The snails are said to be negatively phototactic to 

 strong light and positively phototactic to weak light. 



The animal lives in an environment which is constantly changing. 

 Its spontaneous movements are constantly bringing it into different 

 conditions. It tends to regulate its internal processes by selecting the 

 point in the environment in which its internal processes -are not dis- 

 turbed. The writer has observed snails in ponds. They move into their 

 optimum light, i.e., the light which does not disturb them. On dark 

 days they are found in the light. On sunny days they are found in the 

 shade of the vegetation. They shift their position according to condi- 

 tions and their distribution at any given time is a better index of 

 conditions than the distribution of plants in the same pond. 



c) Modifiability of behavior and different physiological states. We all 

 know that our actions may be modified by experience. There are but 

 few people who have not been greatly frightened by some accident 

 accompanied by a characteristic noise. For days afterward, one starts 

 at the slightest unexpected noise. His response has been modified. 

 It is a well-known fact in animal training that an animal may be 

 "spoiled." A horse may be ruined for some purposes by an accident 



