156 HARRY BEAL TORRE Y 



VI 



The crux of the matter, however, for the physiologist, lies in 

 the third question. Here the problem appears under a purely 

 physiological guise. It is formulated in physiological terms. 

 It invites attack from the side of physics and chemistry. It 

 suggests a measure of energy transformations. 



If there is a definite relation between the application of energy 

 to an organism and its behavior, it might be expected to appear 

 in an examination of the evidence for constant stimulation. For 

 the sake of brevity but a few of the recent contributions to this 

 end will be considered, including certain results as yet unpub- 

 lished. They have been chosen so as to outline the gradual 

 emergence of demonstrable fact from a crowd of observations 

 whose real significance could only be surmised in the absence 

 of later experimentation. We may begin with the phototropic 

 behavior of Euglena. 



It has frequently been observed that when Euglena is exposed 

 to light from two sources the direction of its locomotion may 

 be most delicately adjusted to their intensity as well as to their 

 position, and experimentally controlled with remarkable preci- 

 sion by modifying these two factors. These facts have been 

 interpreted in various ways. To some they strongly suggest, 

 though they do not demonstrate, the presence of constant 

 stimulation. 



Two years ago, experiments that involved the most careful 

 and detailed observations of the behavior of Euglena that have 

 ever been published, led Bancroft to the conclusion that the 

 reactions of this organism to light are not only of two distinct 

 types but governed by two distinct mechanisms. Of especial 

 significance was the observation, which has been confirmed in 

 my own laboratory, that when Euglena swims directly toward 

 the source of light in its characteristic spiral path, the width 

 of the spiral changes with the distance of the organism from 

 the light. In this orientation, the photosensitive area is pre- 

 sented constantly to the source of light, not intermittently as 

 would be the case were the light falling on the organism from 

 one side. Constant illumination produces a constant response. 

 The variation of the width of the spiral with the intensity of 



