WAYS AND MEANS OF LIVING
sound, heat, light, and gravity. Most of these things stimulate 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 receptive 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 out 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 unconsciously, and that their behavior under the effect of a stimulus is an automatic action entirely comparable to out 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, especially 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
[121]
