470 Journal of Comparative Neurology and Psychology. 



meets in swimming, or the pressure of the water upon it. Such 

 an environmental change produces, then, Hke many other 

 changes, the avoiding reaction, with its "trial" of different di- 

 rections. The same result is produced by setting the water in 

 motion in other ways, as by causing the vessel containing the 

 animals to vibrate back and forth. 



If now we produce a more extensive current, and allow it 

 to continue, as in the experiment shown in Fig. lO, we find 

 the same result produced. The animals at first pause, then 

 swing the anterior end about in a circle, thus "trying" many 

 different directions. They then swim forward in one of these 

 directions. The reaction is then repeated, and this occurs as a 

 rule several times, until they have come into a position with 

 anterior end directed up the stream. The reaction then ceases ; 

 the animals swim forward in the usual spiral manner. 

 They have become oriented by the method of "trial and error," 

 the "trials" continuing till the position of orientation was 

 reached. 



We have seen that the original cause of the reaction was a 

 change in the environment — the movement of the water — 

 causing a change in the resistance or pressure the Paramecium 

 meets. But why does the reaction continue till orientation is 

 reached, then cease ? Consideration of the relation of the cur- 

 rent to the spiral course followed by the animal shows that this 

 is exactly what we should expect from all that we know of the 

 behavior of the animal and the cause of the present reaction. 

 Consider a specimen that is swimming transversely or obliquely 

 to the current, as in Fig. 1 1. In its spiral course it swings the 

 anterior end first against the current, to the point a, then with 

 the current to the point /k In the swing toward a the move- 

 ment is resisted by the current ; in the swing toward d it is aid- 

 ed by the current.' Its relation to the current thus changes 

 during each turn of the spiral ; in one phase the movement is 

 "easier" from being aided, in the next more difficult, from be- 



1 The upward and downward movement of the swing may be neglected for 

 our present purpose. 



