216 BRADLEY M, PATTEN 



all Loeb's conclusions is a matter of small importance beside the 

 tremendous advance in clear thinking which has resulted from the 

 analytical attitude he has from the first maintained. 



In his earliest work bearing on the orientation of animals to 

 light ('88), he cites the reactions of fly larvae in support of his 

 explanation of orientation. The main interest of this work was 

 theoretical and but few facts were added to the fairly complete 

 account already published by Pouchet. Perhaps the most sig- 

 nificant of these were his establishment of the restriction of the 

 sensitive region to the anterior end of the larva and the balanced 

 reaction to equal lights acting on opposite sides of the body. 

 Loeb concluded that the factor of prime importance in orientation 

 was the direction in which the rays of light penetrated the 

 tissue, as Sachs had believed for plants. This view, however, 

 he has since abandoned, as may be seen from the following 

 quotation ('06, p. 130), 



We started with the assumption that the heliotropic reactions are 

 caused by a chemical effect of light; in all such reactions, time plays a 

 role. We assume, furthermore, that if light strikes two sides of a 

 symmetrical organism with unequal intensity, the velocity or the char- 

 acter of the chemical reactions in the photosensitive elements of both 

 sides of the body is different; that in consequence of this difference the 

 muscles, or contractile elements on one side of the organism are in a 

 higher state of tension than their antagonists. The consequence is a 

 curvature or bending of the head. This is followed by a turning of the 

 body, kept up until the stimulus acts equally on the bilaterally located 

 sensitive areas. When such a condition of balance is attained, the ani- 

 mal no longer deviates toward either side, but pursues a direct path 

 toward or away from the source of light. 



If it be true that the immediate effect of light in causing the helio- 

 tropic reactions is of a chemical nature, we should expect that it must be 

 possible by use of chemicals to control the precision and sense of helio- 

 tropic reactions ('06, p. 131). 



The striking results which Loeb ('93, '04, '06) obtained in 

 his experiments on the chemical control of heliotropism are the 

 strongest sort of evidence for his interpretation that light reac- 

 tions depend fundamentally on a chemical reaction, the extent of 

 which is dependent on the intensity of the light. 



Loeb thus opened a new field of tremendous interest. The 

 mechanical explanation of orientation to light became the ob- 



