HISTORICAL REVIEW 4 1 



Represent the worm by an arrow whose head Indicates the 

 head end [Fig. 3, ^]. Let solar rays 55 fall upon it 

 horizontally and perpendicularly to its axis. Then the 



S ii 



Low Light Attunetnent 



A <- . 



Low Light Attunem.ent 



Fig. 3. Diagram representing sunlight (SS) falling upon an elongated, bilateral 

 organism (represented by the arrow) whose head is at ^. After Davenport (1897, 

 p. 209). 



impinging ray strikes it laterally, or, in other words, it is 

 illuminated on one side and not on the other. Since, now, 

 the protoplasm of both sides is attuned to an equal intensity 

 of light, that which is the less illuminated is nearer Its 

 optimum intensity. Its protoplasm is in a phototonic con- 

 dition. That which is strongly Illuminated has lost its 

 phototonic condition. Only the darkened muscles, then, 

 are capable of normal contraction; the brightly illuminated 

 ones are relaxed. Under these conditions the organism 

 curves towards the darker side; and since its head region 

 is the most sensitive, response begins there. Owing to a 

 continuance of the causes, the organism will continue to 

 turn from the light until both sides are equally illumi- 

 nated ; i.e. until it is in the light ray. Subsequent 

 locomotion will carry the organism in a straight line, since 

 the muscles of the two sides now act similarly. Thus 

 orientation of the organism is effected. The same ex- 

 planation . . . will account, mutatis mutandis, for positive 

 phototaxis." 



It Is evident that this theory assumes a direct effect of 

 the stimulating agent on the locomotor organs. Daven- 

 port thus claims that orientation may be brought about in 

 two ways: " Light acts directly either through difference 

 in intensity on the two sides of the organism, or by the 



