The Mind of the Simplest Animals 43 



sponse not to a special kind of stimulus, but to a mechanical 

 or food stimulus of slight intensity. In our own experience, 

 we do not have stimuli of different intensity producing sen- 

 sations of different quality, except in the cases of temperature 

 and visual sensations. We do, however, find that varying the 

 strength of the stimulus will produce different affective quali- 

 ties ; it is a familiar fact that moderate intensities of stimula- 

 tion in the human organism are accompanied by pleasant- 

 ness, and stronger intensifies by unpleasantness. The motor 

 effects of pleasantness and unpleasantness in ourselves are 

 opposite to each other in character. Pleasantness produces 

 a tonic and expansive effect on the body, unpleasantness a 

 depressive and contractive effect. In the Amoeba, the posi- 

 tive and negative reactions seem to be opposed. The essen- 

 tial feature of the negative reaction is the checking of move- 

 ment at the point stimulated ; that of the positive reaction is 

 the reaching out of the point stimulated in the direction of the 

 stimulus. This much evidence there is for saying that besideTV 

 a possible food sensation, the Amoeba may have some dim K 

 awareness of affective qualities corresponding to pleasantness J I/ 

 and unpleasantness in ourselves. It should, however, be~~~ 

 borne in mind that wide differences must go along with the 

 correspondence. In us, pleasantness brings a thrill, a "bodily 

 resonance," due to its tonic effect upon the circulation, breath- 

 ing, and muscles ; unpleasantness has also its accompaniment 

 of vague organic sensation, without which we can hardly con- 

 ceive what it would be like. In an Amoeba, it is clear that 

 this aspect, as found in human consciousness, must be wholly 

 lacking. Again, in the human mind pleasantness "and un- 

 pleasantness are connected with various sensation qualities 

 or complexes; we are pleased or displeased usually "at" 

 something definite. The vagueness of the affective qualities 

 in an Amoeba's consciousness can only be remotely suggested 



