The Mind of the Simplest Animals 45 



in the following chapter discuss more fully the nature of the 

 evidence that helps us to conjecture the existence of different 

 sensation qualities in an animal's mind ; but it is clear that 

 where an animal so simple in its structure as the Amoeba 

 makes no difference in its reactions to various stimuli, there 

 can be no reason for supposing that if it is conscious, it is 

 aware of them as different. The reaction to edible sub- 

 stances is, however, unlike that to other stimulations. The 

 peculiarity of edible substances which occasions this differ- 

 ence must be a chemical one. In our own case, the classes 

 of sensation which result from the chemical pecularities of 

 food substances are smell and taste ; evidently to a water- 

 dwelling animal smell and taste would be practically indis- 

 tinguishable. We may say, then, that supposing conscious- 

 ness to exist in so primitive an animal as the Amoeba, we 

 have evidence for the appearance in it of a specific sensation 

 quality representing the chemical or food sense, and standing 

 for the whole class of sensations resulting from our own 

 organs of smell and taste. The significance of the positive 

 reaction is harder to determine. It seems to be given in re- 

 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 tempera- 

 ture and visual sensations. We do, however, find that 

 varying the strength of the stimulus will produce different 

 affective qualities; it is a familiar fact that moderate 

 intensities of stimulation in the human organism are 

 accompanied by pleasantness, and stronger intensities 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 con- 



