448 BIOLOGY IN RELATION TO OTHER NATURAL SCIENCES. 



want, with this advaiita.ye for our ])reseiit purpose, that the i)lieiioinena 

 are sucli as we all luive it in our power to observe in ourselves. In the 

 visual apparatus the principle of voruxdify of reaction is fully exem- 

 plified. In the physical sense the word " light'' stands fora»ther vibra- 

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

 are the stinuilus, the sensations are the reaction. Between the two 

 conies the link, the "letting" off," which it is our business to understand. 

 Here let us remember that the man who first recognized this distinction 

 between the physical and the physiological was not a biologist, but a 

 l)]iysicist. It was Young who first made clear (though his doctrine fell 

 on unappreciative ears) that, although in vision the external intluences 

 Avhich 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 apparatus. 

 We speak of the organ of vision as hif/JiJy differentiated, an expression 

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

 diff'eient 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 

 hi<jher animals consist in a series, placing at the lower end of the series 

 those of which the functions are continuous, and therefore called vege- 

 tative; at the other, those highly specialized structures, as e. e/., those, 

 in the brain, which in response to ])hysical light produce physiological, 

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

 cells of the surfaci^ of the brain, which in response to a stiiiinhis of much 

 greater complexity i)roduce voluntary motion. And just as in civilized 

 society an individual Is valued according to his power of doing one 

 thing Avcll, 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 specialization. And if it be asked how this conformity is 

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

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

 an exami)le, that we can readily understand how it happened that it 

 was in this field that the relation between response and stimulus was 

 first clearly recognized. I need scarcely say that, however interesting 

 it might be to follow out the lines of inquiry thus indicated, we cannot 

 attempt it this evening. All that I can do is to mention one or two 

 recent observations which, while they serve as illustrations, may per- 

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

 subject. 



Probably every one is acquainted with some of the familiar proofs 

 that an object is seen fi)r a much longer period than it is actually 

 exi)Osed to view; that the visual reaction lasts much longer than its 

 cause. More precise observations teach us that this response is regu- 

 lated according to laws which it has in common with all the higher 

 functions of an organism. If, for example, the cells in the l)rain of the 

 torpedo are "let olf " — that is, awakened by an external stimulus — the 



