December 2, 1904.] 



SCIENCE. 



751 



of the body either toward or away from the 

 source of stimulation. The animal is per- 

 fectly oriented with rv.ference to the stimulus 

 when symmetrical points on the body are 

 equally stimulated. 



By careful observation of the behavior of 

 several tyijes of lower organism Jennings has 

 determined to his own satisfaction, and judg- 

 ing from the nature of his descriptions with 

 certainty, that the reactions of the unicellular 

 organisms, and also those of some of the 

 metazoa, are not in harmony with the current 

 theories of orientation. The reaction of 

 ciliate infusoria to heat and cold is thus de- 

 scribed, in summary, by the author, " the re- 

 sponse, on coming into a region where the tem- 

 perature is above or below the optimum, is by 

 backing and turning toward a structurally 

 defined side, followed by a movement forward. 

 This reaction is repeated as long as an effect- 

 ive supraoptimal or suboptimal temperature 

 continues. The result is to prevent the organ- 

 isms from entering regions of marked supra- 

 optimal or suboptimal temperature, and to 

 cause them to form collections in regions of 

 optimal temperature. The common orienta- 

 tion of a large number of individuals some- 

 times produced in this way is an indirect re- 

 sult of the method of reaction. Since move- 

 ment in any other direction than a certain 

 one is stopped, the organisms after many trials 

 come into this direction. Orientation is, 

 therefore, by ' exclusion,' or by the method of 

 trial and error" (p. 28). 



Similarly the author has shown that the 

 reactions of ciliates and flagellates to light are 

 not in accordance with the assumptions of the 

 theories of orientation. 



Among the multicellular organisms the 

 Eotifera, according to the observation of Jen- 

 nings, turn as a rule toward a structurally de- 

 fined side — the dorsal — ^without relation to the 

 side stimulated most strongly, and continue 

 turning, or alternately turning and darting 

 forward, until the anterior end of the body is 

 directed away from the source of stimulation. 

 " Thus the direction of turning is throughout 

 dependent upon an internal factor, not pri- 

 marily on the way in which the stimulus im- 

 pinges on the organism. These reactions of 



the Eotifera are thus inconsistent with a 

 theory of tropisms which regards orientation 

 as a jDrimary feature of the reactions, and 

 which holds that the action of the stimulating 

 agent is a direct one on the motor organs of 

 that part of the body on which it impinges " 

 (p. 88). 



Of the correctness of Jennings's observa- 

 tions there can be little doubt, for all his work 

 is characterized by admirable care in observa- 

 tion and accuracy in description, but in criti- 

 cism of the manner in which he discusses the 

 bearing of his own results upon the orientation 

 theories it might be said that he does not 

 make sufficient allowance for the asym- 

 metry of the organisms with which he 

 worked. The fact that the various unicellular 

 organisms and rotifers whose behavior is de- 

 scribed in these papers do not orient according 

 to the theories of Verworn or Loeb by no 

 means proves that their theories are in prin- 

 ciple wrong. Jennings's work does, however, 

 very emphatically call attention to certain 

 weaknesses and false assumptions of the 

 theories in question, for their advocates have 

 too often adduced the presumably well-known 

 reactions of these very organisms in support 

 of one or another form of the theory of the 

 tropisms. 



It is only fair to the author to state that his 

 arguments are directed more especially against 

 the ■' direct action ' assumption of the tropism 

 theory than against the theory of orien- 

 tation. He believes that his observations 

 prove beyond a doubt, for the animals studied, 

 that stimuli bring about reactions by their 

 general action upon the organism rather than 

 by direct local action, and he, therefore, con- 

 tends with good reason that the theory of the 

 tropisms as stated by Verworn, Loeb and others 

 is radically wrong. 



It is worthy of note, in connection with the 

 facts of Jennings's studies, that the turning 

 toward a structurally defined side which is so 

 common among the unicellular organisms 

 serves usually, although not always, to direct 

 the organism away from the source of stimula- 

 tion. For example, Paramecium turns towards 

 its aboral side, hence away from the currents 

 of stimulating substance which are drawn 



