270 BEHAVIOR OF THE LOWER ORGANISMS 



the agreement with the theory is less complete ; for while the part that 

 contracts is determined by the region stimulated, the extension and 

 consequent direction of movement are, as a rule, not thus determined. 



But while some important features of the behavior of Amoeba are 

 thus in agreement with the underlying assumptions of the tropism 

 theory, it is certain that for such organisms alone the theory would never 

 have been proposed. The facts for Amoeba can be formulated in a 

 much simpler way than by bringing in the conception of orientation, — 

 a conception derived from organisms with permanent body axes, and 

 fitting only these. 



But when we turn to an examination of the behavior of those uni- 

 cellular organisms having permanent body axes, we find the conditions 

 widely at variance with the assumptions of the local action tropism 

 theory. In the infusoria most of the behavior is quite inconsistent with 

 the theory. The reactions are not determined by the direct action of 

 a localized stimulus in producing greater contraction or extension in 

 that part of the body on which it impinges. The organism responds 

 as a whole, by a reaction involving all parts of the body. It does not 

 necessarily turn directly toward or directly away from the source of 

 stimulation, as would be the case if it reacted in accordance with this 

 tropism theory. The direction of turning is determined by internal 

 factors; the animal turns toward a side which is structurally defined. 

 For inducing directed motion it is not necessary that the stimulus should 

 act differently on different parts of the body. The cause of reaction — 

 that is, of a change in the movements — is usually a change from one 

 condition or intensity to another. Thus the essential point in deter- 

 mining whether reaction shall occur is in most cases the direction of 

 movement — whether this takes the organism (or its most sensitive 

 portion) away from, or toward, the optimum. It is difficult to conceive 

 a type of behavior more completely opposed to the local action theory 

 of tropisms above set forth. 



In some cases this method of reaction produces orientation with 

 relation to the direction of some external force, in other cases it does not. 

 The orientation when it occurs is brought about through continued 

 movements that are varied in direction, with final selection of one of 

 these directions. Whether orientation shall or shall not result depends 

 on whether it must result in order that there shall be a cessation of the 

 stimulation which is producing the varied movements. These relations 

 have been set forth in detail in our account of the behavior of Paramecium 

 (Chapter IV, Section 6), so that it is not necessary to take them up here. 



To almost all the relations set forth in the preceding paragraphs 

 there is one exception. In the reaction of ciliate infusoria to the electric 



