874 



SCIENCE 



[N. S. Vol. XXXVII. No. 962 



growing controversy, I too have failed to be 

 convinced. Controversies fatten on misunder- 

 standings. There is evidence that this one 

 has been no exception to the rule. Some of it 

 may be found in a recent book by Mast° and 

 in a review and reply^ that have since ap- 

 peared. Accordingly, silence might seem to 

 be the better part. Nevertheless, I am tempted 

 to brave the possibility of further difficulty by 

 accepting what seems to be a reasonable chance 

 of focusing attention more sharply than be- 

 fore on the issue raised in my former paper. 



It will be my endeavor, then, to show why 

 the view that tropic reactions are developed 

 through selection of overproduced or trial 

 movements is unsatisfactory. And I shall 

 attempt to do this by calling attention to cer- 

 tain relations between the structure and be- 

 havior of non-symmetrical and bilaterally 

 symmetrical organisms that have heretofore 

 met with much scantier consideration than, in 

 my opinion, they deserve. Let me refer first 

 of all to the behavior of the non-symmetrical 

 flagellate Euglena. 



n 



Euglena, as is well known, is a non-sym- 

 metrical unicellular organism, with a single 

 non-symmetricaUy placed photoreceptive re- 

 gion near the gullet, and swims in a spiral 

 path by means of a flagellum, or, in the ab- 

 sence of the latter, assumes a crawling habit 

 that is accompanied by a side-to-side oscilla- 

 tion suggesting the spiral swimming of the 

 flagellated form. 



I have been very much impressed by Mast's 

 account of the orientation of the crawling 

 Euglena to light. It appears that the organ- 

 ism, crawling in a path perpendicular to a 

 beam of light, on entering the beam executes 

 a turning movement toward the source of 

 light; that this turning movement is accom- 

 plished by a series of sharp reactions, defined 

 by a bending of the body away from the 

 source of light; that each reaction foUows an 



= ' ' Light and the Behavior of Organisms, ' ' New 

 York, 1911. 



^Parker, Jour. An. Beh., Vol. I., 1911, p. 461; 

 and Mast, ihid., 11., p. 209. 



abrupt change in intensity of light falling on 

 the photoreceptive region; and that these ab- 

 rupt changes are connected with the oscilla- 

 tion of the organism from side to side, leading 

 to an intermittent presentation of the photo- 

 receptor to the light in a position effective for 

 stimulation. 



Few will deny that constant stimulation 

 plays no obvious role here. But many, I am 

 sure, will not fail to be impressed with certain 

 elements of similarity in structure and be- 

 havior between Euglena and various bilat- 

 erally symmetrical organisms with one of a 

 s ym metrical pair of eyes blinded. And just 

 here appears a point that has not been always 

 clearly apprehended by the author of " Light 

 and the Behavior of Organisms." Somehow 

 Mast has obtained from my former paper the 

 impression that I evidently consider "that 

 orientation [in Euglena'] is due to the local 

 effect of unequal stimulation of symmetrically 

 situated points on the body " — and yet do not 

 " explain where the symmetrically located 

 points which are subject to local stimulation 

 are situated in Euglena." I have looked 

 carefully over my paper since this statement 

 came to my notice, but have not been able to 

 find any attempt to explain the orientation of 

 Euglena on any such basis. That was per- 

 haps a result to be expected, as I had not been 

 conscious of an attempt so to explain the 

 orientation of Euglena or any other non-sym- 

 metrical organism. Of the symmetrical points 

 to which " it is evident " I refer, I frankly 

 confess my ignorance. Nor do I feel any 

 more hopeful than Mast himself of a success- 

 ful issue to the most exhaustive search for 

 them. Our reasons for this attitude of mind, 

 however, do not coincide. For mine, the 

 reader is referred to that suggestive resem- 

 blance between the structure and behavior of 

 non-synunetrical organisms and symmetrical 

 organisms non-symmetricaUy stimulated to 

 which I would again call attention. 



Let us imagine Euglena crawling horizon- 

 tally along a definite axis of locomotion, di- 

 rectly toward a source of light. At successive 



