CHAPTER XI 



TROPISMS AND INSTINCTS AS ADAPTATIONS 



Of 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 

 r61e 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 



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