REGULATION OF REACTIONS 287 



Woods Hole. Nor was I able to confirm them in obser- 

 vations on several related species at the Tortugas. 



We have thus presented various instances in which an 

 organism is positive under given external conditions at 

 one time and negative under precisely the same condi- 

 tions at another time. In some cases this change in 

 reaction requires a long time, in others only a few moments, 

 as e.g., in the reaction of Vol vox represented in Fig. 33. It 

 is evident that such changes must be regulated by inter- 

 nal factors, that they must be due to alterations within 

 the organism itself. As a matter of fact, all reactions are 

 directly controlled by internal factors which are in turn 

 influenced by external factors. The interesting point here 

 is however the fact that we may have movements and 

 change in movements without any immediate changes in 

 the environment. Many instances of this have been cited 

 by Jennings (1906), especially in Chapter XVI. 



The facts (i) that the reactions may be affected in the 

 same way in a given organism by so many contrasting 

 conditions, including concentration and dilution of medium, 

 high and low temperature, acids, alkalis, narcotics and 

 salts; (2) that the same change in external conditions may 

 cause opposite reactions in different organisms, e.g., a rise 

 in temperature causes some to become negative and others 

 positive; and (3) that the sense of reaction may change 

 without any immediate external change, — indicate that 

 these responses are due not to a direct and specific effect 

 of the environment on some definite chemical compound 

 within the organism, but rather to the effect on the organ- 

 ism as a whole. 



