KNOWLEDGE AND BELIEF. 303 



particular activity of the epidermic cells in which 

 they terminate. In harmony with the great law of 

 " division of labour " the originally indifferent " sense- 

 cells of the skin " undertook different tasks, one group 

 of them taking over the stimulus of the light rays, 

 another the impress of the sound waves, a third the 

 chemical impulse of odorous substances, and so on. 

 In the course of a very long period these external 

 stimuli effected a gradual change in the physiological, 

 and later in the morphological, properties of these 

 parts of the epidermis, and there was a correlative 

 modification of the sensitive nerves which conduct 

 the impressions they receive to the brain. Selection 

 improved, step by step, such particular modifications as 

 proved to be useful, and thus eventually, in the course 

 of many millions of years, created those wonderful 

 instruments, the eye and the ear, which we prize so 

 highly ; their structure is so remarkably purposive 

 that they might well lead to the erroneous assumption 

 of a "creation on a preconceived design." The 

 peculiar character of each sense-organ and its specific 

 nerve has thus been gradually evolved by use and 

 exercise — that is, by adaptation — and has then been 

 transmitted by heredity from generation to generation. 

 Albrecht Eau has thoroughly established this view in 

 his excellent work on Sensation and Thought, a 

 physiological inquiry into the nature of the human 

 understanding (1896). It points out the correct 

 significance of Muller's law of specific sense-energies, 

 adding searching investigations into their relation to 

 the brain, and in the last chapter there is an able 

 "philosophy of sensitivity" based on the ideas of 

 Ludwig Feuerbach. I thoroughly agree with his 

 convincing work. 



