REACTIONS TO LIGHT IN VANESSA ANTIOPA 365 



Loeb ('06, p. 140) attempts to refute any notion of a pleasure- 

 pain tj^pe of behavior in lower organisms, and accepts the phe- 

 nomenon of circus movements as a fact in support of his theory 

 in explanation of the orientation of animals. This is discussed 

 fully in the Mechanistic Conception of Life. ('12, p. 35-62.) 

 He holds that the orientation of animals is controlled unequivo- 

 cally by external agents, and that in orientation to light, there 

 are two essential factors, the continuous action of light and the 

 symmetrical structure of the organisms. According to his view, 

 which may be called the 'continuous action theory,' the tension 

 of the muscles of the appendages on the two sides of the body is 

 controlled through direct reflex arcs by the photochemical changes 

 produced by light in the two retinas. He says ('12, p. 39): 

 ''When two retinae (or other points of symmetry) are illuminated 

 with unequal intensity, chemical processes, also of unequal in- 

 tensity, take place in the two optic nerves (or in the sensory nerves 

 of the two illuminated points). This inequality of chemical 

 processes passes from the sensory to the motor nerves and even- 

 tually to the muscles connected with them. We conclude from 

 this that with equal illumination of both retinae the symmetrical 

 groups of muscles on both halves of the body will receive equal 

 chemical stimuli and thus reach equal states of contraction, while 

 when the rate of reaction is unequal, the symmetrical muscles 

 on one side of the body come into stronger action than those on 

 the other side. The result of such an inequality of the action 

 of symmetrical muscles of the two sides of the body is a change 

 in the direction of movement on the part of the animal." 



It is clear that in this theory it is assumed that light is effective 

 in orientation through its continuous action, that after orienta- 

 tion has occurred, light continues to stimulate the photosensitive 

 areas, and through direct reflex arcs, continues to affect the 

 muscles of the appendages on the two sides of the body. These 

 assumptions, as stated above, are, according to Loeb, supported 

 by the behavior of animals with the sense organs functional only 

 on one side. He quotes Parker as follows ('06, p. 140): "Loeb 

 has pointed out that the orientation of an organism in light is 

 dependent upon the equal stimulation of symmetrical points on 



