16 BEPOET— 1893. 



The repeater may serve as a good example, inasmuch as it, is, in 

 biological language, a highly differentiated structure, to which a single 

 function is assigned. So also in the living organism, we find the best 

 examples of specific energy where Miiller found them, namely, in the 

 most differentiated, or, as we are apt to call them, the highest structures. 

 The retina, with the part of the brain which belongs to it, together con- 

 stitute such a structure, and will afford us therefore the illustration we 

 want, with this advantage for our present purpose, that the phenomena 

 are such as we all have it in our power to observe in ourselves. In 

 the visual apparatus the principle of riormality of reaction is fully 

 exemplified. In the physical sense the word ' light ' stands for ether 

 vibrations, but in the sensuous or subjective sense for sensations. The 

 swings are the stimulus, the sensations are the reaction. Between the 

 two comes the link, the ' letting off,' which it is our business to under- 

 stand. Here let us remember that the man who first recognised this 

 distinction between the physical and the physiological was not a bio- 

 logist, but a physicist. It was Young who first made clear (though his 

 doctrine fell on unappreciative ears) that, although in vision the external 

 influences which give rise to the sensation of light are infinitely varied, 

 the responses need not be more than three in number, each being, in 

 Miiller's language, a ' specific energy ' of some part of the visual appa- 

 ratus. We speak of the organ of vision as highly differentiated, an 

 expression which carries with it the suggestion of a distinction of rank 

 between different vital processes. The suggestion is a true one ; for it 

 would be possible to arrange all those parts or organs of which the 

 bodies of the higher animals consist in a series, placing at the lower end 

 of the series those of which the functions are continuous, and therefore 

 called vegetative ; at the other, those highly specialised structures, as, e.g., 

 those in the brain, which in response to physical light produce physiolo- 

 gical, that is subjective, light ; or, to take another instance, the so-called 

 motor cells of the surface of the brain, which in response to a stimulus 

 of much greater complexity produce voluntary motion. And just as in 

 civilised society an individual is valued according to his power of doing 

 one thing well, so the high rank which is assigned to the structure, or 

 rather to the 'specific energy' which it represents, belongs to it by 

 virtue of its specialisation. And if it be asked how this conformity is 

 manifested, the answer is, by the quality, intensity, duration, and ex- 

 tension of the response, in all which respects vision serves as so good an 

 example, that we can readily understand how it happened that it was in 

 this field that the relation between response and stimulus was first 

 clearly recognised. I need scarcely say that, however interesting it 

 might be to follow out the lines of inquiry thus indicated, we can- 

 not attempt it this evening. All that I can do is to mention one or two 

 recent observations which, while they serve as illustrations, may perhaps 

 be sufficiently novel to interest even those who are at home in the subject. 



