THE REACTIONS OF ORGANISMS TO LIGHT 213 



with Loeb's definition. And finally in the summary, p. 205, we 

 find the following: "Under certain conditions, if one side is 

 illuminated, they [earthworms] always turn toward the shaded 

 side without preliminary movements and therefore orient 

 directly " (no trials). Can it be possible that my critic actually 

 read the section of my book containing the quotation given 

 above and still came to the conclusion that I have " attempted 

 to apply the trial and error theory ... to the 

 exclusion of the tropism idea," assuming that he meant by the 

 "tropism idea" merely orientation without trial movements. 

 The truth of the matter is that no attempt was made to apply any 

 theory whatsoever, but there was a very serious attempt made 

 to describe precisely how orientation occurs in all of the different 

 forms studied. In the fly larvae it is true I found no evidence 

 of direct orientation, no reactions in accord with Parker's defini- 

 tion of tropism. But by merely glancing over the general sum- 

 mary, pp. 228-235, it will be seen that according to my conclu- 

 sions, direct orientation, i. e. orientation without trial is found 

 in coelenterates, earthworms, planaria, mollusks, arthropods, 

 and vertebrates. Note the following statement found on page 

 233: "mollusks, arthropods and vertebrates all orient directly. 

 There is little evidence of preliminary trial movements in this 

 process in these forms." Do not these statements show that if 

 there was an attempt made to apply trial reactions to the exclu- 

 sion of other factors in explaining orientation the attempt 

 must have been a miserable failure? 



Orientation is unquestionably direct in many instances, i. e., 

 without trial and in accord with Parker's definition of tropisms, 

 provided he includes under the phrase "unequal stimulation of 

 symmetrical parts," cases in which the stimulus may be confined 

 to either side, it being zero on the opposite side ; which would in 

 reality mean that orientation may be due to a differential response 

 to localised stimulation. If however he means to imply in his 

 definition that both sides of the organism are continuously 

 unequally stimulated during the process of orientation and 

 continuously equally stimulated after orientation in accord with 

 Loeb's last theory of tropisms, I have no conclusive evidence in 

 support of his " tropism idea." 



6. In discussing the efficiency of trial in the process of orienta- 

 tion Parker bases his argument upon a hypothetical case. Assum- 



