CHAPTER XI 
TROPISMS AND INSTINCTS'AS ADAPTATIONS 
Or the different kinds of adaptation none are more re- 
markable than those connected with the immediate responses 
of organisms to external agents. These responses are usually 
thought of as associated with the nervous system; and while 
in the higher forms the nervous system plays an important 
réle in the reaction, yet in many cases it is little more than 
the shortest path between the point stimulated and the muscles 
that contract ; and in the lower animals, where we find just as 
definite responses, there may be no distinct nervous system, 
as in the protozoa, for instance. 
Many of the so-called instincts of animals have been shown 
in recent years to be little more than direct responses to ex- 
ternal agents. Many of these instincts are for the good of 
the individual, and must be looked upon as adaptations. For 
example: if a frog is placed in a jar of water, and the tem- 
perature of the water lowered, the frog will remain at the 
top until the water reaches 8 degrees C., when it will dive 
down to the bottom of the jar; and, if the temperature is 
further lowered, it will remain there until the water becomes 
warmer again, when it will come to the surface again. It is 
clear that, under the ordinary conditions of life of the frog, 
this reaction is useful to it, since it leads the animal to go to 
the bottom of the pond on the approach of cold weather, 
and thus to avoid being frozen at the surface. 
Another illustration of an instinct that is a simple response 
to light is shown by the earthworm. During the day the 
worm remains in its burrow, but on dark nights it comes out 
382 
