WAYS AND MEANS OF LIVING 



sound, heat, light, and gravity. Most of these things stim- 

 ulate the nerve centers indirectly through nerves connected 

 with the skin or with specialized parts of the skin called 

 sense organs. An animal can respond, therefore, only to 

 those stimuli, or to the degrees of a particular stimulus, to 

 which it is sensitive. If, for example, an animal has no re- 

 ceptive apparatus for sound waves, it will not be affected 

 by sound; if it is not sensitized to certain wave lengths of 

 light, the corresponding colors will not stimulate it. There 

 are few kinds of natural activities in the environment that 

 animals do not perceive; but even our own perceptive 

 powers fall far short of registering all the degrees of any 

 activity that are known to exist and which the physicist 

 can measure. 



Insects respond to most of the kinds of stimuli that we 

 perceive by our senses; but if we say that they see, hear, 

 smell, taste, or touch we make the implication that insects 

 have consciousness. It is most likely that their reactions 

 to external stimuli are for the most part performed un- 

 consciously, and that their behavior under the effect of a 

 stimulus is an automatic action entirely comparable to 

 our reflex actions. Behavioristic acts that result from 

 reflexes the biologist calls tropisms. Coordinated groups 

 of tropisms constitute an instinct, though, as we have 

 seen, an instinct may depend also on internal stimuli. It 

 can not be said that consciousness does not play a small 

 part in determining the activities of some insects, es- 

 pecially of those species in which memory, i.e., stored 

 impressions, appears to give a power of choice between 

 different conditions presented. The subject of insect 

 psychology, however, is too intricate to be discussed 

 here. 



The phases of life thus far described, the complexity 

 of physical organization, the response to stimuli, the 

 phenomena of consciousness from their lowest to their 

 highest manifestations, all pertain to the soma. Yet, 

 somehow, the plan of the edifice is carried along in the 



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