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THE AMERICAN NATURALIST [Vol. XLIV 



recorded for our fixed plants, can not have their behavior 

 included in this category; for by them there is appar- 

 ently no selection tlmjufjh varied movements. Our typ- 

 ical plant tropisms show that one movement or curve, and 

 that is unvaryingly toward the assumption of the new 

 position of equilibrium. The response is what Bohn has 

 called resistless. The term is expressive, and may be 

 useful ; but we must not forget that all reactions of lower 



There are recorded, however, the results of two series 

 of experiments with plants which seem to me to range 

 themselves with the phenomena described as " selection 

 through varied movements." I refer to the establish- 

 ment of half -hourly geotropic rhythm in the stems of 

 Taraxacum and valerian, and of quarter-hourly helio- 

 tropic rhythm in the cotyledons of Phalaris and Arena, 

 by the work of Francis Darwin and Miss Pertz. If there 

 is doubt as to whether the rhythms just noted illustrate 

 the first part of Jennings's hypothesis for the descrip- 

 tion of the origin of behavior, namely, 1 'the selection 

 through varied movements of conditions not interfering 

 with physiological processes," there can, I believe, be no 

 doubt that the establishment of these rhythms illustrates 

 the second part of the hypothesis, namely, that the "reso- 

 lution of one physiological state into another becomes 

 more rapid after it has taken place a number of times." 

 The gradual shortening of the period of oscillation, in the 

 plants used by Darwin and Pertz, till the half-hourly or 

 quarter-hourly rhythm was established, seems to me to 

 express the more rapid resolution of one physiological 

 state into another. 



Though the idea of trial and error seems capable of 

 application to but few of the recorded reactions of fixed 



