ANIMAL BEHAVIOR 849 



vironment, they are objectionable. They are seldom used in the same 

 sense by different writers in the classification of types of behavior. 



Tropistic Behavior 



The concept of tropism has its origin in plant physiology where it 

 was applied to the positive or negative orientation of plant to light, 

 gravity, or other external stimuli. The term was later used by Loeb 

 to cover similar growth responses in lower organisms and simple re- 

 sponses of the reflex type as well. In time, Loeb extended the concept 

 to include practically all reflex and instinctive actions. Loeb used the 

 tropistic theory as a definite explanatory principle. He assumed that 

 symmetrical parts of the body surface of an organism possess identical 

 irritability value. Orientation behavior is due to a forced turning of 

 the organism so that the corresponding sides are equally stimulated. 

 This simple explanation ignores the factor of internal regulation. This 

 has been pointed out by Jennings, Mast, Buddenbrock, and others. 

 Tlie tropistic theory is useful, however, in describing the behavior of 

 Protozoa. Since these forms are composed of but a single cell, 

 any response is likely to involve the whole body. But even in 

 Protozoa it has been asserted that the metabolic or internal activities 

 influence the tropisms. Thus the chemotropic response might be en- 

 tirely different toward a substance that was food to Protozoa, if 

 the animal was hungry rather than satiated. Also it has been found 

 that the reactions of an animal to a strong stimulus may be the op- 

 posite of its reaction to a weak stimulus. Some insects are negatively 

 phototropic to daylight while they are positively phototropic to weak 

 artificial light. And again it has been demonstrated that tropistic re- 

 actions may be modified through experience. Paramecium is a free- 

 swimming organism, and when it finds its progress in one direction 

 hindered by some obstacle, such as a strand of thread, it reverses its 

 course and heads in a new direction. If in the next trial, the obstacle 

 is not avoided, the little animal continues its trials until it does 

 succeed in avoiding the obstacle. The analysis of the behavior 

 of many organisms shows the trial and error method to be a very im- 

 portant factor in their behavior. Many revisions have been made in 

 our concepts of the tropisms during the past few years, but it has not 

 altered the significance of the theory as a means of describing the be- 

 havior of Protozoa and lower vertebrates. Various tropisms are 

 described by the response animals make to these stimuli. Thus we 



