98 BEHAVIOR OF THE LOWER ORGANISMS 



cent) and the current is passed through the vessel. The strong chemical 

 causes the animals to swim backward ; th^ current orients them in the 

 usual way ; the result is that they swim backward to the anode. This 

 phenomenon is to be observed in solutions of various chemicals, as 

 acids, potassium iodide, sodium carbonate, etc. It will probably be 

 found to occur in any solution that causes the animals to swim back- 

 ward for a considerable time. It should be investigated further. As 

 soon as the Paramecia have become accustomed to the chemical, so that 

 they no longer swim backward within it, they react to the current in the 

 usual way, swimming to the cathode. 



Thus we find that under the action of more than one stimulus Para- 

 mecium may behave in any of the ways which we mentioned in our 

 first paragraph as conceivable. It may react to the first stimulus with- 

 out regard to the second, or to the second without regard to the first, 

 depending on which is the more effective. Such results are often pro- 

 duced when both the stimuH are sufficiently strong to cause reaction if 

 acting alone. Which stimulus shall produce its characteristic effect some- 

 times depends on which comes into action first. Thus, Paramecia in 

 contact may not react to the electric current or to heat; while free 

 Paramecia subjected to the same strength of current or degree of heat 

 do not show the positive contact reaction. This condition of affairs 

 seems to occur throughout the animal series; in higher animals we 

 express the same phenomenon subjectively by saying that attention to 

 one thing prevents attending to others. 



' In some cases the behavior shown is a resultant of the action of the 

 , two stimuli. Examples of this are seen in the movement along a surface 

 under the simultaneous action of the contact reaction and a mechanical 

 shock, or in swimming around a chemical in solution, under the influence 

 of the electric current ; or in swimming backward to the anode when in 

 solutions of strong chemicals. 



Finally, the effect of one stimulus is sometimes merely to change 

 the method of reaction to another. Thus heat and strong chemicals 

 cause the animal to respond to contact by the avoiding reaction in place 

 of the positive contact reaction ; carbon dioxide has the contrary effect. 

 The modifications of the reaction to gravity above mentioned are ex- 

 amples of the same thing. Cases of this character have much theoret- 

 ical interest. We shall return to them in considering the variability 

 and modifiability of the reactions of Paramecium. 



2. VARIABILITY AND MODIFIABILITY OF REACTIONS 



We have seen in the last section that the behavior of Paramecium 

 under a given stimulus may be determined by the simultaneous presence 



