THE TROPISM THEORY OF JACQUES LOEB 349 



The changing of one kind of movement into another, however, 

 is always correlated with the application of some new stimulus, 9 

 by means of which the physiological condition of the animal is 

 so changed that the same optical stimulus now produces the 

 opposite reaction. A good and clear example of this is furnished 

 by certain other Mysidae (see footnote 12) which become posi- 

 tively heliotropic after lighting, negatively heliotropic after 

 shading the field. But the change of movement observed in 

 Hemimysis takes place without any alteration in the physiological 

 stimulation, and in the starfish we see even that one and the 

 same animal may be brought to either a positive or a negative 

 reaction, according to the desire of the experimenter. 



Now, according to Loeb's theory, we see in an animal a defi- 

 nite system of chemo-physical forces. When energy of definite 

 constant amount acts upon such a system, the reaction must 

 necessarily be certain and definite, i.e., with similar stimuli and 

 similar physiological conditions in the animal, the movement 

 must always occur in the same way. The case of Hemimysis 

 is therefore not to be explained by the tropism theory. 



Conversely: If two separate and distinct amounts of energy 

 each compel a definite system of forces to make the same move- 

 ment, by logic these two amounts of energy should be equally 

 great. 



This principle is overthrown by the case of the starfish, which, 

 in the same physiological condition, is attracted by light as well 

 as by shadow. It therefore follows that, either the starfish is 

 not such a system of forces as Loeb assumes, or that the energy 

 acts on the eye in an entirely different manner. What the 

 meaning of the phenomenon is does not concern us here, we 

 are content with the assertion that the cases of changeable 

 heliotropism cannot be explained by the tropism theory. 



The impossibility of explaining the turning around the hori- 

 zontal cross axis. The turning around a horizontal cross axis 

 is a phenomenon which nearly all Metazoa exhibit in their 

 heliotropic and geotropic movements, and it will be recognized 

 as a third example of tropisms which take place in a manner 



9 This need not always be an external stimulus. It is also conceivable that in 

 the course of individual development the physiological condition may be altered 

 by the internal changes that are going on. The behavior of many larvae which, 

 without outward cause, change their heliotropism after a certain age, serves as 

 an example. 



