HISTORICAL REVIEW 39 



believed that the direction of the rays is more responsible 

 for the manifestation of phototactic phenomena than are 

 differences in intensity. It is difficult to conceive this, for, 

 since the assumption of an axial direction is possible only 

 when differences exist at two different points of the surface 

 of the body, it is wholly mystical how the direction of the 

 rays, which is the same upon all sides of the body, can pro- 

 duce such an effect." Loeb is here classified with Sachs 

 where he claimed to belong. His experimental results and 

 conclusions are, however, from the beginning, more nearly 

 in harmony with the theory of Verworn than they are with 

 that of Sachs. 



Verworn considers his theory applicable to orienting 

 reactions in unicellular forms induced by stimuli of various 

 kinds. He says (1899, P- 5^3) > " Thus the phenomena of 

 positive and negative chemotaxis, barotaxis, thermotaxis, 

 phototaxis and galvanotaxis which are so highly interesting 

 and important in all organic life, follow with mechanical 

 necessity as the simple results of differences in biotonus, 

 which are produced by the action of stimuli at two different 

 poles of the free-living cell." 



In 1892 Oltmanns attempted to settle the dispute as to 

 the relative effect of ray direction and intensity difference 

 by studying the reactions of Volvox in an aquarium in 

 which the light became more intense gradually from one 

 end to the other. Such a distribution of light was pro- 

 duced by placing a hollow prism filled with a mixture of 

 India ink and glycerine-gelatine between the source of 

 light and the aquarium. The India-ink mixture of course 

 absorbed only a little light at the thin end of the prism, but 

 gradually more toward the thicker end. Under these con- 

 ditions the Volvox colonies collected in light of a given 

 intensity. Oltmanns says (1892, p. 195) that when the 

 prism was put between the source of light and a vessel 

 containing colonies which had a given direction of motion, 

 they changed their direction of motion almost instantly 

 and moved toward the region of optimum intensity. Olt- 



