Editorial. 



H3 



tion of the impinging force ; in otlier word, when the organism 

 is oriented. The anterior end will be directed toward the source 

 of the stimulating agent or away from it, according as it is the 

 decrease or increase in intensity that is the effective stimulus^ 

 The transference of the organism along the axis of progress has 

 in these cases no effect on the relation of the organism to the 

 stimulating agent, so that so far as this reaction is concerned 

 backward progression might take place as well as forward. 

 Under these stimuli a certain orientation of the organism, not a 

 certain direction of progress, is the essential result of the reac- 

 tion. This class of reactions forms the basis, so far as the in- 

 fusoria are concerned, for taxis proper (Massart, Davenport, 

 etc.), strophic taxis (Rothert), and topotaxis (Pfeffer). 



Thus in the first class of reactions the essential result is 

 a certain direction of progression — toward a region of greater 

 or of less intensity — while in the second class the essential re- 

 sult is the orientation. But in both classes the nature of the 

 stimulus is the same and the reaction is the same. The stimu- 

 lus is some change in the relation of the organism to the sur- 

 rounding conditions — a change in the intensity of action of 

 some agent. The response is a motor reaction that consists of 

 a series of trial movements. The response continues in each 

 case until the effective stimulus comes to an end, then the usual 

 motion is resumed. In the first class this does not produce ori- 

 entation (save sometimes by "exclusion"), because stimulation 

 comes to an end without it. In the second class it does produce 

 orientation, because stimulation does not come to an end with- 

 out it. The difference between the two classes depends on the 

 peculiar difference in the distribution of the stimulating agent, 

 not upon different ways of reacting on the part of the organism. 

 The reaction is by "trial" movements continued till a cessation 

 of the effective stimulation is brought about ; this will lead to 

 orientation or not, just as the conditions require. 



H. S. JENNINGS. 



'For details as to this relation, see this Journal, 14, 1904, pp. 470, 472, and 

 478 ; Carnegie Institution, Publication 16, 1904, p. 60. 



