PHYSIOLOGICAL STATES AS DETERMINING FACTORS. 119 



the light grows stronger has given rise to much discussion (see 

 Holmes, 1901 and 1903). We have here, of course, a parallel case to 

 the reversal of the reaction in Stentor or the flatworm under mechani- 

 cal stimuli of varying intensity. In the weak light we must suppose 

 the organism to be thrown into a certain physiological condition, the 

 concomitant of which is a certain type of reaction. In a more intense 

 light a different physiological condition is induced, corresponding to a 

 different reaction. The fact that different intensities of stimuli do 

 cause different physiological conditions and different reactions is, of 

 course, familiar to us, both from experimentation on animals and from 

 our own experience ; in the latter case we usually call the distinctive 

 reactions to very intense stimuli pain reactions. In the reversal of the 

 reaction to light as the light becomes stronger we have, it seems to me, 

 merely an instance of this general phenomenon, not differing in funda- 

 mental character from other instances. 



In most of these cases we have, of course, a further problem in regard 

 to those features of the reaction which concern direction. Why does 

 the weak stimulus on the left side of Planaria cause a turning toward 

 that particular side? Or, why does a weak light from a certain direc- 

 tion cause Volvox to swim in that particular direction ? These problems 

 of direction are, of course, not touched in the foregoing discussion, 

 which, however, loses none of its force because these problems remain. 

 They are simply farther problems. The tropism theory gave a simple, 

 direct answer to these questions ; but, as we have alreadv shown in 

 the foregoing paper, this answer was, in many cases at least, not a 

 correct one. Possiblv some combination of certain features of the 

 tropism theory with a consideration of the facts of changes in physio- 

 logical condition may give us a satisfactory answer to the problems 

 of direction. 



INTERFERENCE OF STIMULI. 



Again, we have in the interaction or interference of stimuli certain 

 phenomena which seem to fall under our present point of view. First, 

 we have the so-called cases of u heterogeneous induction," where the 

 action of one stimulus reverses or modifies the reaction to another. 

 For example, many cases are known in which animals positively photo- 

 tactic become negative, or vice versa, when the temperature is changed. 

 (For a collection of such cases see Loeb, 1893, and Davenport, 1897, 

 p. 199.) In these cases the physiological condition of the organism 

 seems altered by one stimulus (as heat or cold), in such a way that it 

 no longer reacts to another stimulus (light) as it did before. In the 

 ciliate infusoria, specimens which are in contact with solids do not 

 react at all to many agents which under other circumstances call 

 forth a marked reaction. Putter (1900) has made a special study of 



