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 
2 chemicals are called chemotaxis ‘“‘ chemotropism 
$ contact are called thigmotaxis “ stereotropism 
. pressure are called barotaxis “* barotropism 
es 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. 
¢) 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 
