HISTORICAL REVIEW 33 



tropism covers only those cases where the turning to the 

 Hght is compulsory and irresistible, and is brought about 

 automatically or mechanically by the light itself. On the 

 other hand, there are compulsory and mechanical reactions 

 to light which are not cases of heliotropism; namely, the 

 reaction to sudden changes in the intensity of light." Orien- 

 tation is therefore, according to Loeb, never due to change 

 in light intensity. ''At a constant intensity light acts as 

 a continuous source of stimulation." When animals are 

 not oriented both sides are continuously stimulated but 

 one is stimulated more than the other. This causes one 

 side to move faster than the other " until symmetrically 

 situated points on the body of the animal are struck at the 

 same angle by equally strong rays of light." 



In a recent paper (1907) Loeb again emphasizes this 

 difference between " heliotropism " and '' Vnterschieds- 

 empfindlichkeit.'' It is therefore evident that he was well 

 aware of the fact that certain animals respond to changes 

 in light intensity. This, however, is an old idea. As a 

 matter of fact it was the fundamental postulate of all who 

 thought that reactions are controlled by psychic phenomena. 

 And in his earlier work Loeb attempted to prove the ab- 

 sence of such phenomena, by showing that aggregation of 

 animals in a given light intensity is not due to difference of 

 intensity, i.e., that the animals are not "unterschiedsemp- 

 findlich.'' Later, however, he found that planarians collect 

 in regions of lowest intensity because they are " unter- 

 schiedsempfindlich "; (1907), '' Both forms of reaction may 

 occur in the same animal (e.g., Spirographis) , but this is 

 neither necessary nor the rule." 



Loeb did not study the reactions of unicellular organisms 

 to light and it has been frequently stated that he did not 

 apply his theory to their reactions. Such statements, how- 

 ever, are erroneous as the following quotations will show: 

 (1905, p. 73), " Experiments on infusoria are already suffi- 

 ciently complete to show that Sachs's laws of heliotropism 

 also hold good for them. . . . Trembley's experiments on 



