Holmes, Random Movements. iii 



tactic response here considered Etiglena does not react by a num- 

 ber of indiscriminate movements until the right one is accident- 

 ally hit upon, but by a direct reflex whose effect is to bring the 

 organism more nearly parallel to the direction of the rays. The 

 phototaxis of Euglena is not so manifestly the outcome of the 

 trial and error method as that of the earthworm. In the latter 

 case light does not cause directly a movement which makes for 

 orientation. The direct response may or may not have that ef- 

 fect. The successful movement is accidentally hit upon, but 

 one can scarcely say this in the case of Euglena in which the 

 orientation takes place more nearly in accordance with the usu- 

 al scheme. 



It is perhaps difficult to decide where best to draw the line 

 as regards the employment of the expression trial and error. 

 If it is extended to include the phototaxis of Euglena and other 

 protozoa where there is a gradual adjustment of the path by ap- 

 propriate direct responses until it coincides with the direction 

 of the rays, we can hardly stop short of including, at least to a 

 considerable degree, the cases of phototaxis that take place ac- 

 cording to the commonly accepted theory. We may regard all 

 departures from the straight and narrow path as errors accord- 

 ing to whatever theory of phototaxis we may choose to adopt, 

 and we can look upon all movements in that path as successful 

 experiments. I would suggest that if the term trial and error 

 is widened, as seems desirable, so as to include such reactions as 

 are described in the first part of this paper where there is no dis- 

 cernible element of learning involved, its application be limited 

 to those cases in which the adapted movements may be regard- 

 ed as chance successes. This would exclude the tropisms of 

 the orthodox kind; it would exclude the gradual orientation of 

 such forms as Euglena where oblique stimulation causes a direct 

 response which brings the body more nearly parallel to the 

 rays. It would include many of the reactions of the protozoa 

 where, as in the phototaxis of the blue Stentor, the right direc- 

 tion of movement is hit upon by chance, and a large part of the 

 actions of higher forms. y\ll organisms make errors. In some 

 cases these errors are rectified by an appropriate direct reflex, 



