HISTORICAL REVIEW 25 



zen, und nicht durch die Unterschiede in der Lichtinten- 

 sitat auf den verschiedenen Seiten des Thieres." It is 

 evident from this quotation that Loeb at this time held 

 that the direction of the rays through the tissue is the con- 

 trolHng factor in orientation of animals; that is, that orienta- 

 tion in animals takes place just as Sachs had said it does 

 in plants; that it is not due to difference of intensity on 

 different parts of the organism, but to the direction in 

 which the directive rays pass through the tissue. 



The results recorded in the second paper, dated 1889, are 

 in all essentials like those found in the first. The principal 

 points established are (i) that positive animals will pro- 

 ceed toward the window under conditions such that they 

 continually get into weaker light; (2) that only the more 

 refrangible rays are active in causing reactions. From 

 these results Loeb concludes as follows (1905, p. 3; first 

 edition, 1889): " The conditions which control the movements 

 of animals toward light are identical, point for point, with 

 those which have been shown to be of paramount influence in 

 plants.'' Five conditions are considered: (i) ray direction; 

 (2) wave length; (3) constancy of intensity; (4) limits of 

 intensity; (5) temperature. Two of them, the first and 

 the third, are of special interest to *is at present. I shall 

 therefore quote Loeb's words with reference to them* (1905, 

 p. 2), "So far as the light is concerned, the circumstance 

 which controls the orientation of the animal and the direc- 

 tion of its movements is the direction of the rays falling 

 upon the animal. The condition which is of importance 

 on the part of the animal is the symmetrical shape of the 

 body." It consequently appears that he, at this time, 

 no longer considered the direction in which the rays pass 

 through the tissue of the organism of special importance 

 but that he still regarded the direction in which they fall 

 upon it of importance. At the same time, however, he 

 accepted Sachs' theory as giving an adequate explanation 

 of orientation in plants and claimed that this theory also 

 holds for animals, for he says (1905, p. 89), "I showed 



