HELIOTROPISM IN EUGLENA 387 



tropism theory object to. Parker ('07, p. 548) has objected to 

 it, but Jennings thinks that he is not a good judge of what the 

 general opinion on the question is ('08, p. 70). Loeb has objected 

 to it ('07, p. 156, footnote), but Jennings thought that he referred 

 to something entirely different ('08, p. 700, footnote). It is diffi- 

 cult to see how Jennings can have thought that such a man as 

 Loeb, who entered the field of tropisms as a brain physiologist 

 and worked mainly on the tropisms of animals with eyes, ever 

 thought that ''orientation is produced by the direct action of the 

 stimulating agent on the motor organs of that side of the body on 

 which it impinges" ('06, p. 266). Loeb certainly never thought 

 anything of the kind, and has never written anything of the kind 

 so far as I have been able to discover. He certainly does not 

 make any such statement in any of the passages that Jennings 

 quotes or refers to by page. Thus the passage from Loeb COO, 

 p. 7) that Jennings quotes on two occasions ('04, p. 92 and '06, 

 p. 266) reads: 



The explanation of them [the tropisms] depends first upon the speci- 

 fic irritability of certain elements of the body-surface, and, second, upon 

 the relations of symmetry of the body. Symmetrical elements at the 

 surface of the body have the same irritability; unsymmetrical elements 

 have a different irritability. Those nearer the oral pole possess an irri- 

 tability greater than those near the aboral pole. These circumstances 

 force an animal to orient itself toward a source of stimulus in such a way 

 that symmetrical points on the surface of the body are stimulated 

 equally. In this way the animals are led without will of their own either 

 toward the source of stimulus or away from it. 



In this and many other^ similar cases Jennings seems to think 

 that 'certain elements of the body surface' means motor organs; 

 though why he should think so I am at a loss to understand. 

 Not only does Loeb refrain from supporting the 'local action 



^In Jennings's ('06, p. 26S) quotation from Davenport the meaning is less clear 

 for Davenport speaks of darkened and illuminated muscles, though he does not 

 say that it is the illumination of the muscles that directly causes their relaxation. 

 But when the passage in Loeb ('93, p. 86) from which Davenport has taken his 

 exposition is looked up we find: "Trifft das Licht eine Seite des Tieres, so gehen 

 zunachst uns einstweilen unbekannte Veranderungen in den durchleuchteten Teilen 

 vor sich. Die Folge ist eine \erdnderung in der Spannung der Muskeln." So 

 in this case "unbekannte Veranderungen" are taken by Jennings to mean direct 



