126 MENTAL EVOLUTION IN ANIMALS. 



particular kind ; but, as I did not attend to it, the sensation 

 did not involve any element of cognition, and therefore did 

 not minister to any act of perception. All at once, however, 

 I became conscious that I was looking at a book, and in 

 cognizing that the particular object of sensation was a book, 

 I performed an act of perception. In other words, I men- 

 tally interpreted the sensation in terms of past experience ; I 

 made a mental synthesis of the qualities of the object, and 

 assigned it to the class of objects which had previously 

 produced a like sensation. 



Perception, then, is a mental classifying of sensations in 

 terms of past experience, whether ancestral or individual ; it 

 is sensation plus the mental ingredient of interpretation. 

 Now, as a condition to the possibility of this ingredient, it is 

 clearly essential that there should be present the power of 

 memory ; for only by a memory of past experience can the 

 process be conducted of identifying present sensations or 

 experiences as resembling past ones. Therefore in the 

 diagram I have placed the dawn of Memory on the level, 

 just below that at which the faculty of Perception takes its 

 rise. Both Sensation and Perception are represented as 

 attaining a considerable vertical elevation from base to 

 apex, i.e., from their first origin to their completed evolution. 

 That this ought to be so represented is evident if we reflect 

 on the difference in the sensuous faculties of a medusa and 

 an eagle, or between the perceptive faculties of a limpet and 

 a man. It may, indeed, be thought that in my representative 

 diagram I have not allowed enough for such differences, and 

 therefore have made the vertical elevation of these branches 

 too low. But here we must remember that in the case of Sen- 

 sation, as already shown, the advance of the faculty from its 

 earliest to its latest stages consists essentially, on its morpho- 

 logical aspect, of a greater and greater degree of specializa- 

 tion of end-organs of nerves ; and I think that the degree of 

 such advance is sufficiently expressed by the vertical elevation 

 which I have given to the branch in question, seeing how 

 much more intricate must be the morphological development 

 of the nerve-tissues which are concerned in ministering to 

 the next and to all succeeding faculties. And, as regards 

 Perception, we must remember that in its more highly 

 elaborated phases this faculty shades off into the higher 

 representative branches marked " Imagination," &c. ; so that 



