612 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



from the environment, whether living or non-living, determine the action of 

 animals. As a rule, these sense organs also undergo an increasing differen- 

 tiation with phylogenetic evolution, although some of them may be developed 

 to a higher degree in more primitive organisms than in man. In general, the 

 mechanistic basis of primitive animal behavior is clearly discernible, but with 

 further phylogenetic advancement and - with increasing structural and func- 

 tional complexities new and more complex processes arise, which may render 

 difficult the analysis of the behavior of the organism as a whole. 



The mechanistic character of the behavior of animals was recognized by 

 Jacques Loeb, and it was with particular clearness discernible in the tropistic 

 reactions of animals as highly developed as certain insects. Especially sug- 

 gestive was the observation of this investigator that slight physico-chemical 

 changes in the environment were able to cause a reversion in the direction of 

 the tropistic reaction. 



Simple non-conditioned reflexes represent the functional elements which 

 seem to underlie animal reactions. However, it appears that ganglia of the 

 central nervous system give off, also automatically, stimuli which are trans- 

 mitted to the peripheral nerves and to the recipient end-organs. This would 

 mean that a specific stimulation by afferent nerves of the reflex arc or by 

 hormones is not required for the function of these ganglia, but that they 

 may discharge stimuli under the influence of non-specific substances or 

 mechanisms which reach them. Yet, it is probable that the important func- 

 tions of the central nervous system, which determine the behavior of higher 

 organisms, are essentially of a reflex nature. The most important complica- 

 tion which next arose in animal behavior consists in the conditioned reflexes 

 discovered by Pavlov. Even at a very early phylogenetic stage, former actions 

 of the environment may modify the subsequent behavior by means of condi- 

 tioned reflexes. Thus, learning is made possible. But the importance of these 

 processes is, on the whole, limited in invertebrates, although they seem to be 

 widely distributed. Thus, conditioned reflexes have been shown to exist in 

 polyclad flatworms. It has been maintained that they can be demonstrated 

 also in protozoa ; however, A. R. Moore has pointed out that in the latter 

 one may have to deal with a condition analogous to the hysteresis of metals 

 and colloids, in which a longer lasting after-effect of certain treatments can 

 be shown to exist, comparable to certain fatigue phenomena rather than to 

 true conditioned reflexes. The latter exist, however, in larvae of lower 

 vertebrates, as, for instance, in Ambly stoma (A. R. Moore). Even in verte- 

 brates they bear a definite relation to the inherited structural and reflex 

 constitution of the various animals and, as stated, represent an addition to 

 these latter. The more varied the simple reflex activities of the organisms have 

 become, the more varied may be the conditioned reflexes which are added to 

 them. 



Therefore, behavior at first is rigid and fixed, corresponding to the origi- 

 nally relatively simple structure of the organisms and especially to their 

 nervous system and sense organs. There are no indications that individual 

 differences are very significant in the behavior in the more primitive animals. 



