HISTORICAL REVIEW 27 



organism which regulates the movement. Consequently if 

 Loeb's explanation holds for animals and Sachs' for plants, 

 it is clear that the orientation in animals is not necessarily 

 regulated in the same way as in plants. 



Sachs opposed the idea of De Candolle that difference 

 of intensity on opposite sides of the reacting organism con- 

 trols orienting reactions; while Loeb at this time opposed 

 the idea of Bert and Graber that difference of intensity in 

 the field determines the place of aggregation, and that 

 animals are " unterschiedsempfindlich." Sachs argued in 

 favor of ray direction through the tissue of the reacting 

 organ, Loeb in favor of ray direction in general. Failure 

 to recognize the difference between these views has led to 

 much confusion. It is on this account that the problem 

 has generally been so loosely stated in the terms ''Is it 

 ray direction or intensity difference? " — a question which 

 evidently cannot be answ^ered without an explicit state- 

 ment of the sense in which these terms are used. 



Do Loeb's experimental results prove the absence of 

 sensations as factors in animal behavior as he assumes? 

 The experiments on which he bases his conclusions are 

 similar to those of Strasburger on swarm spores referred 

 to on p. 15. Loeb found that positive animals very gen- 

 erally move toward a source of light even if in so doing 

 they pass into regions of lower light intensity. He con- 

 cluded from this result correctly that this cannot be due 

 to variation in the intensity of light in the space, but 

 incorrectly that this disproves the existence of sensation, 

 for the animals with which he worked are more sensitive 

 to light at the anterior than at the posterior end. If they 

 enjoy light one would expect them to continue to face its 

 source even if the general illumination is decreased, be- 

 cause, if they should turn, the sensitive anterior end would 

 become shaded and this would cause a decrease in the 

 pleasant effect of light. The experimental results just 

 cited, therefore, do not prove the absence of sensation as a 

 controlling factor in the behavior of animals; neither do 



