BEHAVIOR IN LOWER METAZOA 237 



tive and negative reactions of the flatworm. Reactions of this charac- 

 ter are commonly spoken of as tropisms. In the higher animals and 

 man behavior is, of course, largely determined by the same factors. As 

 we have seen in previous chapters and shall find in the following sec- 

 tions of the present one, in neither lower nor higher animals are the 

 reactions with reference to the general forces of nature of a completely 

 fixed and invariable character. 1 



In more complex animals than those considered in the present vol- 

 ume, definite reaction forms are often combined into complex trains of 

 action which are known as instincts. Recent work has shown that in 

 these instincts there is by no means that absolute fixity of behavior that 

 was formerly assumed to exist. A detailed treatment of this matter 

 would take us outside the field of the present work. 



In the highest animals and man, definite reaction forms, which may 

 take place in certain organs independently of the rest of the body, are 

 of course found as abundantly as in lower organisms. Such reactions 

 are seen in the reflexes of muscles, etc., which persist even after the 

 muscle has been removed from the body. There is no difference in 

 principle along this line between higher and lower animals. The 

 former possess a much larger number of such definite types of move- 

 ment, and these doubtless make up fully as large a portion of behavior 

 as in the lower animals. 



There are some accounts of behavior in various lower animals in 

 w r hich only these definite reaction forms are described and only those 

 conditions are dealt with in which these appear in the typical way. 

 Such accounts have given rise to a widespread impression that behavior 

 in the lower animals differs from that of higher forms in that it is of a 

 fixed, stereotyped character, occurring invariably in the same way under 

 the same external conditions. This impression is in a high degree 

 erroneous. These definable reaction forms are usually in themselves 

 variable within wide limits, as exemplified in the avoiding reaction of 

 infusoria. But even if this were not true, the criteria for judging as to 

 the fixity or modifiability of behavior are to be derived from the study 

 of the conditions that induce reaction, that determine which of several 

 possible reactions shall occur, and that determine the order and combina- 

 tion of reactions. Such a study shows that in lower as well as in higher 

 animals varied internal conditions and changes are of the greatest im- 

 portance in determining behavior, the animal by no means behaving 

 always in the same way under the same external conditions. With this 

 aspect of the matter we shall deal in the two following sections. 



1 See, for example, the section on reactions to gravity in ccelenterates, Chapter XI. 



