Mast, Orientation in Euglena with some Remarks on Tropisms. (157 



have also recently (1914) obtained results which seem to indicate 

 that orientation in Eudendrium is, within certain limits of intensity, 

 in accord with the Bunsen-Roscoe law. But this surely does 

 not warrant their conclusion that the process of orientation in 

 plants and animals in general is "identical" and that the Bunsen- 

 Roscoe law holds for all. 



B. Trial and Error. 



Both Bancroft (p. 415) and Torrey (1913, p. 875) maintain 

 that they have proved that orientation in Euglena is direct; that 

 it is not due to the assumption of various axial positions and the 

 retention of that axial position in which the animal is directed 

 toward or from the source of stimulation in accord with the "trial- 

 and error" theory as applied by Jennings. And if I understand 

 Torrey correctly (1913, p. 875) he concludes that this demon- 

 strates that overproduced, random, or trial movements are not 

 primitive reactions from which, as Jennings holds, "the definitely 

 directive reactions of bilateral animals to light have . . . been 

 developed by any process of selection based on such movements." 



Bancroft bases his conclusions largely upon my description 

 of the process of orientation in which it is stated (1911, p. 104) 

 that orientation may occur without any increase in the diameter 

 of the spiral. I am inclined to believe that this statement holds 

 literally unter certain conditions, although I established by direct 

 observation, only that when Eiiglena, as it proceeds on its spiral 

 course, is illuminated from the side, it swerves farther toward the 

 light than in the opposite direction, and that the swerving from 

 the light does not carry the animal as far in that direction as it 

 was in the same relative position of the spiral on preceding turns. 

 All this could evidently occur with or without an increase in the 

 diameter of the spiral. Bancroft holds that if it occurs without 

 an increase in the diameter of the spiral. Euglena orients "as 

 directly as its locomotor mechanism allows", and consequently 

 orientation contains nothing of the nature of a "trial-and-error" 

 reaction. This is a good point; but the question remains: does 

 orientation actually ever occur thus? 



Without pausing to analyse Torrey's evidence (1913) against 

 indirect orientation, evidence which in my estimation is far less 

 conclusive than Bancroft's, let us grant that Euglena actually 

 does orient directly, that is, without trial, and see what bearing 

 this has on the idea that "trial-and-error" reactions are more primi- 

 tive than responses which have a more direct relation to the location 

 of the stimulus. Any one who has studied the reactions of Eu- 

 glena knows that under certain conditions a very large majority 



