Spatially Determined Reactions 199 



A similar suggestion that orientation may occur either by a 

 definite reflex or as the outcome of random movements, 

 according to the animal's physiological condition, is to be 

 found as early as the work of Pouchet on fly larvae. He 

 noted that the courses taken by the larvae were either 

 straight, "or they present to right and left indentations 

 due to the wavering movements which the animal makes . . . 

 in a certain number of cases, as if to take at each instant a 

 new direction." These individual differences might have 

 been accounted for, says Pouchet, by differing degrees of 

 hunger in the larvae (614). Herms (296) reports that to 

 low intensities sarcophagid flies orient by random move- 

 ments : while to high intensities they orient directly. 

 Bittner, Johnson, and Torrey (58) find that the earthworm 

 orients to light without any random movements. Hadley 

 (274) finds the same true of larval lobsters, Crozier (159) 

 of a holothurian, and Bancroft (16) reports of Euglena, a 

 protozoon which has the spiral method of swimming char- 

 acteristic of so many animals in this group, that "there is 

 nothing of trial and error here : the organism orients as 

 definitely as its spiral locomotion will allow." 



When the " direction theory " of the tropism was receiving 

 more attention than at present, evidence that an animal 

 oriented in response to the direction of the light rather than 

 to the comparative intensity of stimulation on symmetrical 

 points was taken as arguing against Jennings J s view of the 

 tropism as a response to changes in light intensity produced 

 by random movements. Attempts were made to demon- 

 strate the direction theory experimentally. A typical 

 experiment of this type was that of Strasburger (695), 

 made long before Jennings's views were in the field, upon 

 the swarm spores of certain plants. He placed over the 

 vessel containing them an India ink screen, thicker at one 



