158 HARRY BEAL TORREY 



stimulation. According to Patten there is evidence of it in the 

 behavior of blowflies. And one of my students, G. P. Hays, 

 appears to have demonstrated that the phototropism of the 

 fruit fly Drosophila conforms to it also. This case will be 

 briefly sketched. 



The imagoes of Drosophila are positively phototropic. The 

 method of experimentation consisted essentially in subjecting a 

 given number confined in a large test tube, to diffused light 

 from a source whose area could be readily controlled by dia- 

 phragms. The flies were first aggregated at the end of the tube 

 away from the light, and then exposed to it, from darkness, 

 for a given number of seconds. The number of flies entering 

 during that time the fifth of the tube which lay nearest the 

 light was adopted as the criterion of the effect of the light. 

 Drosophila is so sensitive to light that it was found necessary 

 to use very low intensities in order that the responses obtained 

 at different exposures might be determined with a sufficient 

 degree of accuracy. 



Five intensities, multiples of 1, were used, with exposures, 

 varying from 16 to 80 seconds, calculated to deliver the same 

 quantity of photic energy in all five cases. Every day ten 

 trials w r ere made at each intensity, and the average number of 

 flies affected was computed. This was repeated on five succes- 

 sive days with five different sets of flies and the results aver- 

 aged. These averages agree excellently with Blaauw's figures, 

 varying from 1.44 to 2.1. Under the conditions, this is a satis- 

 factorily constant result. I do not see how it can be inter- 

 preted on any theory that neglects the quantity of photic energy 

 involved. 



If it appears from the facts that have been thus hastily sum- 

 marized that tropisms are not interpretable in terms of random 

 movement and selection processes, must we then conclude that 

 they possess no fundamental characteristics in common with 

 reactions of the non-directive type ? By no means. The essen- 

 tial relationship of these superficially different kinds of reflexes 

 is to be found in the physiological processes that they represent, 

 processes that are evident also in the phenomena of growth, 

 development, differentiation, and all other manifestations of life. 

 As physiological reflexes, tropisms exercise their fascination for 



