450 BIOLOGY IN RELATION TO OTHER NATURAL SCIENCES. 



The fact thus deuiuiistiated, that the vi.sual reactiou eonsequeut on 

 aninstantaueousilliimination exhibits tlie alternations I have described, 

 has enabled M. Charpeutier to make out another fact in relation to the 

 visual reaction which is, I think, of equal importance. In all the 

 instances, excepting the retina, in which the i^hysiological response to 

 stimulus has a definite time limitation, ,ind in so far resembles an explo- 

 sion — in other words, in all the higher forms of specific energy, it can 

 be shown experimentally that the process is propagated from the part 

 first directly acted on to other contiguous pai'ts of similar endowment. 

 Thus, in the simplest of all known phenomena of this kind, the elec- 

 trical change, by which the leaf of the Dionjiea plant responds to the 

 slightest touch of its sensitive hairs, is propagated from one side of the 

 leaf to the other, so that in the opposite lobe the response occurs after 

 a delay which is proportional to the distance between the spot excited 

 and the spot observaxl. That in the retina there is also such propaga- 

 tion has not only been surmised from analogy, but inferred from certain 

 observed facts. M. Chari)entier has now been able by a- method which, 

 although simple, I must not attempt to describe, not only to jirove its 

 existence, but to measure its rate of progress over the visual field. 



There is another aspect of the visual response to the- stinuilus of light 

 which, if I am not trespassing too long on your patience, may, I think, 

 be interesting to consider. As the relations between the sensations of 

 color and the physical properties of the light which excites them, are 

 among tlie most certain and invariable in the whole range of vital reac- 

 tions, it is obvious that they aftord as fruitful a field for physiological 

 investigation as those in which wliite light is concerned. We have on 

 one side physical facts, that is, wave lengths or vibration-rates; on the 

 other, facts in consciousness — namely, sensations of c(^)lor — so simple 

 that notwithstanding their subjective character there is no difficulty in 

 measuring either their intensity or their duration. Between these there 

 are lines of influence, neither physical nor jisychological, which pass 

 from the former to the latter through the visual apparatus (retina, nerve, 

 brain). It is these lines of influence which interest tlie physiologist. 

 The structure of the visual api)aratus aftbrds us no clews to trace them 

 by. The most important fact we know about them is that they must 

 be at least three in numbei". 



It has been lately assumed by some that vision, like every other 

 specific energy, having been developed progressively, objects were seen 

 by the most elementary forms of eye only in chiaroscuro, that after- 

 wards some colors were distinguished, eventually all. As regards 

 hearing it is so. The organ which, on structural grounds, we consider 

 to represent that of hearing in animals low in the scale of organiza- 

 tion — as, e. g., in the Ctenophora — has nothing to do with sound,* but 



* Verworn, "Gleichgewiclit u. Otolithenorgan," Pjiilger's Archiv., vol. 1. 423; also 

 Ewald's Kesearclies oiv the Labyriuth as a Seuse-orgiiu ("Ueber ilas Endorgau ilea 

 Ncrvus octavus," Wiesbaden, 1892). 



