HABITAT PREFERENCE 31 
b) Seasonal changes—These involve great changes in the physio- 
logical states. Inactivity is the rule in winter; growth and activity in 
the other seasons. The plants and animals of a locality do not all reach 
sexual maturity or the greatest growth activity at the same time during 
the growing season, but different species succeed each other as the season 
advances (47). The food and enemies of a given species, which is present 
in an animal community for a large part of the growing season, differ 
from time to time. 
c) Weather changes.—These constitute fluctuations of conditions 
calling forth special types of behavior. Some animals hide when the 
wind begins to blow; some burrow into the ground on cool and cloudy 
days. 
4. HABITAT PREFERENCES 
By virtue of being unlike or possessing different properties, the various 
animal species require different conditions for the best adjustment 
of their internal processes. For example, the carp lives in shallow and 
muddy ponds and rivers, while the brook trout lives only in clear swift 
streams. These two organisms are able to move about and find places 
to which they are suited. The differences between them are clearly 
indicated by the differences in the habitats which they prefer. 
By observation and by experimentation it has been shown that 
animals select their habitats. By this we do not mean that the animal 
reasons, but that selection results from regulatory behavior (p. 29). 
The animal usually tries a number of situations as a result of random 
movements, and stays in the set of conditions in which its physiological 
processes are least interfered with. This process is called selection by 
trial and error. If animals are placed in situations where a number of 
conditions are equally available, they will almost always be found living 
in or staying most of the time in one of the places. The only reason to 
be assigned for this unequal or local distribution of the animals is that 
they are not in physiological equilibrium in all the places. However, 
some animals move about so much that it is with some difficulty that we 
determine what their true habitats are. 
5. THE MOST IMPORTANT ACTIVITIES OF ANIMALS 
Animal activities are classified as feeding, breeding, hiding, sleeping, 
etc. The strength of a chain is the strength of its weakest link; the 
activity which determines the range of conditions under which a species 
will be successful is the activity which takes place within the narrowest 
limits. This is usually the breeding activity. The breeding instincts 
