52 MENTAL EVOLUTION IN ANIMALS. 



responses are appropriate. In order to see this, let us take 

 the principal faculties of mind in their ascending order, and 

 consider what they are, in their last analysis, upon their 

 physiological side. First we have the organs of special 

 Sensation, the physiological functions of which clearly con- 

 stitute the basis of the whole structure psychological. Yet 

 no less clearly, these functions in their last analysis are 

 merely so many specially developed aptitudes of response to 

 special modes of stimulation. Thus, for instance, the struc- 

 ture of the eye is specially adapted to respond only to the 

 particular mode of stimulation that is supplied by light, the 

 ear to that which is supplied by sound, and so on. In other 

 words, the organs of special sense are so many structures 

 which have been variously and extremely differentiated in 

 several directions, for the express purpose of attaining a 

 severally extreme sensitiveness to special modes of stimula- 

 tion without reference to any other mode. And this is 

 merely to say that the function of an organ of special sense 

 is that of sorting out, selecting, or discriminating the par- 

 ticular kind of stimulation to which its responsive action is 

 appropriate. 



Again, many of the nervous mechanisms which minister to 

 various Reflex Actions are only thrown into activity by special 

 modes of stimulation. This is notably the case with those highly 

 complicated neuro-muscular mechanisms which are thrown 

 into activity only by the mode of stimulation which we call 

 tickling. Such instances are of special interest in the present 

 connexion from the fact that the distinguishing peculiarity of 

 this mode of stimulation consists in its being a stimulation of 

 low intensity. The comparatively violent stimulation that is 

 caused by the passage of food down the gullet, or by contact 

 of the soles of the feet with the ground, is unproductive of 

 any response on the part of the mechanisms which are 

 thrown into violent activity by the gentlest possible stimula- 

 tion of the same surfaces. Similarly with regard to Instincts. 

 These, physiologically considered, are the activities of highly 

 differentiated nervous mechanisms which have been slowly 

 elaborated, through successive generations, for the express 

 purpose of responding to some particular stimulus of a highly 

 wrought character, and which, on its psychological side, is a 

 recognition of the circumstances to which the instinctive 

 adjustment is appropriate. And so with the Emotions. For. 



