596 WOLFGANG F. EWALD 



mals in being oriented by the light rays follow a line strictly 

 defined by the parallelogram of forces. An animal swimming 

 on one side of a trough covered by the light of a spectrum, say, 

 in the violet, will be directed both by the violet rays coming 

 from the source of light and by the blue, green and yellow-green 

 light dispersed by the front pane of the trough, as can be seen 

 with the naked eye. The animal may consequently be observed 

 to direct its course between the two, arriving at the front pane, 

 say, in the blue. It now continues to work along against the 

 pane, but directed obliquely against the green part, till it comes 

 to rest in the green light itself and is now oriented chiefly by 

 the direct light. Even a very slight excess of stimulation on 

 one side by the dispersed green rays will suffice to bring animals 

 utlimately into the green. Loeb is therefore perfectly justified 

 in assuming that the phototactic animal moves in the direction 

 of the light rays and statements to the contrary made by differ- 

 ent authors since the beginning of experiments on animal tro- 

 pisms, lastly by Hess^ as recently as 1911, must be based either 

 on insufficient observation or on inaccurate reasoning. It is 

 necessary to come to a plain understanding on this question 

 after so many years of experiments. 



Having ascertained these facts it was important to know 

 whether the green rays possessing the strongest effect on pho- 

 totactic motor reflexes would also be most important for the 

 process of making positive animals negative. This question 

 cannot by any means be answered in the affirmative a priori, as 

 previous authors have tacitly done. If it is probable that the 



2 Hess's assumption that phototactic animals are not forced to move along 

 the line of the light rays but choose their way to the field of strongest illumina- 

 tion — at right angles to the direction of the light rays if necessary — is due to his 

 overlooking the facts mentioned above and to a faulty interpretation of the 

 tropism theory. The tropism theory does not assume the animals to move to- 

 wards 'the' source of light, unless there is really one source of light only. If 

 light can strike the animal from several points, even if the excess of light on one 

 side be very slight, it follows a course defined by the parallelogram of forces, 

 as has been pointed out by Loeb from the very beginning. It is Loeb's merit, 

 to have pointed out the purely machine-like and stereotyped character of photo- 

 tactic reactions which differ from orientation in higher vertebrates by the very 

 fact that there never can be anv 'choice' in the direction of locomotion. 



