BIOLOGY IN RELATION TO OTHER NATURAL SCIENCES. 451 



confers on its possessor the powci- of Judging- of the direction of its 

 own movements in tlie water in which it swims, and of guiding these 

 movememts accordingly. In the lowest vertebrates, as, e. g., in the 

 dogfish, although the auditory apparatus is much more complicated 

 in structure, and plainly corresponds with our own, we still find the 

 particular part which is concerned in hearing scarcely traceable. All 

 that is provided for is that sixth sense, which the higher animals also 

 possess, and which enables them to judge of the direction of their own 

 movements. But a stage higher in the vertebrate series we find the 

 special mechanismsby which we ourselves a|5preciate sounds beginning 

 to appear — not snpplauting or taking the place of the imperfect organ, 

 but added to it. As regards hearing, therefore, a uey\' function is 

 acquired without any transformation or fusion of the old into it. We 

 ourselves possess the sixth sense, by which we keep our balance and 

 which serves as the guide to our bodily movements. It resides in the 

 partof the internal ear which is called the labyrinth. At the same time 

 we enjoy along with it the possession of the cochlea, that more comipli- 

 cated apparatus by which we are able to hear sounds and to discriminate 

 their vibration -rates. 



As regards vision, evidence of this kind is wanting. There is, so far 

 as I know, no proof that visual organs which are so imi^erfect as to be 

 -incapable of distinguishing the forms of objects, may not be affected 

 differently by their colors. Even if it could be shown that the least 

 perfectkforms of eye i^ossess only the power of discriminating between 

 light and darkness, the question whether in our own such a faculty 

 exists separately from that of distinguishing colors is one which can 

 only be settled by experiment. As inall sensationsof color the sensation 

 of brightness is mixed, it is obvious that one of the first j)oints to be 

 determined is whether the latter represents a ''specific energy" or 

 merely a certain combination of specific energies which are excited by 

 colors. The ([uestion is not whether there is such a thing as white 

 light, but whether we possess a separate faculty l)y which we judge of 

 light and shade — a question which, although we have derived our 

 knowledge of it chiefly from physical experiment, is one of eye and 

 brain, not of wave-lengths or vibration-rates, and is therefore essen- 

 tially i»hysiological. 



There is a German proverb which says, "Bei Kacht sind alle Katzeu 

 gran." The fact which this proverb expresses presents itself experimen- 

 tally when a S])ectrum projected on a white surface is watched, while 

 the intensity of the light is gradually diminished. As the colors fade 

 away they become indistinguishable as such, the last seen being the 

 primary red and green. Finally, they also disappear, but a gray band 

 of light still remains, of which the most luminous part is that which 

 before was green.* Without entering into details let us consider Avhat 



* HeriDg, " Uutersucb. eines total Farbenbliudeii/' Ffliifjer's Arch., 1891, vol. XLix, 

 p. 563. 



