THE THEORY OF TROPISMS. 93 



similar physical differences. The organism is considered to continue 

 to move its motor organs in exactly the same way after the external 

 change (usually called a stimulus) has taken place ; the reason for turn- 

 ing lies only in the different mechanical effect produced when the motor 

 organs act on a medium of greater or less internal friction than before. 



It is difficult to conceive how anyone having any acquaintance with 

 the movements of organisms could propose such a theory as that of 

 Ostwald, and indeed this author states (p. 24) that his account is purely 

 theoretical, and that he has not attempted to test his theory by experi- 

 ment. We need not, therefore, dwell upon the theory, further than to 

 point out the fact that the reactions of many of these lower organisms 

 have been studied thoroughly, and the reflex movements which they 

 perform when subjected to directive stimuli have been fully described, 

 and that these movements are entirely incompatible with such a theory 

 as that which Ostwald sets forth.* If details are desired, it may be 

 pointed out that all the observations brought in the following that are 

 inconsistent with the theory of tropisms as dependent upon direct 

 stimulation of the motor organs are a fortiori inconsistent with such 

 a theory as that of Ostwald. 



We may, then, turn to the theory of tropisms as set forth in the above 

 quotations from Verworn, Loeb, and Holt & Lee. Diagrams illus- 

 trating the method of action of a stimulus, on this theory, are given in 

 the first of these contributions (Figs, i and 2). 



How far does this theory go in explaining the behavior of the lower 

 organisms? "Tropisms" has become the keyword everywhere in 

 animal behavior ; it is supposed to furnish a ready explanation of most 

 of the puzzles which we here encounter. How far is this justified? 



This question can be answered only by accurate observation of just 

 what organisms do under the influence of stimuli. The theory of 



*Some of the assumptions which Ostwald makes as a basis for his physical 

 analysis of the swimming of the lower organisms are so extraordinary as to 

 deserve mention as curiosities. He states, for example, that as a rule the lower 

 swimming organisms which exhibit the tropisms show active movement vertically 

 onlv upward; he thinks it probable that cases where they have been described 

 as swimming actively downward are errors; that such downward movement is 

 really only passive falling. Yet everyone who has worked with Paramecium or 

 other Ciliata must know how far from the facts is this idea. In a vertical tube 

 Paramecia hasten as freely, and almost as frequently, downward as upward. 

 These infusoria by no means collect at the top in a vertical tube so regularly as 

 the literature on geotropism might lead one to suppose; Paramecia of this region 

 at least are almost as likely to collect at the bottom as at the top. And there is 

 little more difficulty in Paramecium in distinguishing an active movement down- 

 ward from a passive one than there is in man. From my own observations I 

 know that parallel statements could be made for many other free-swimming 

 organisms, including Metazoa (Rotifera and Crustacea), as well as Protozoa. 



