288 The Evolution of Optics. 



about, but when they settle down in the lungs of the Chinese sea- 

 slugs the brow grows over the eyes. 



There are three families of vertebrates that live as parasites in ants' 

 nests. They are blind, or nearly so, two having lost their limbs. In- 

 deed, it is the rule that the young of most blind parasites have eyes. 

 The number of such blind degenerate species tells a sad story of the 

 fall of animals as profound as any " fall of man." 



All these facts are corollaries of the great law that use develops 

 function and disuse is followed by atrophy. Another controlling law 

 operating under and with this law consists in the influence of dimin- 

 ished and denied light. The most interesting examples of this law 

 are the eyes of deep-sea fish. A writer in the Cornhill Magazine has 

 thus stated the facts : 



" Fish that live at very great depths have either no eyes at all or 

 enormously big ones. Indeed, there are two ways you may get on in 

 these gloomy abysses by delicate touch organs, or by sight that col- 

 lects the few rays of light due to phosphorescence or other accidental 

 sources. Now, as we go down in the water we find at each depth that 

 the effects produced upon the eyes of fish are steadily progressive in 

 one direction or the other. Species that live at a depth of eighty 

 fathoms have the eye already a good deal bigger than their nearest rep- 

 resentatives that live at or near the surface. Down to the depth of 

 200 fathoms, where daylight disappears, the eyes get constantly bigger 

 and bigger. Beyond that depth small-eyed forms set in, with long 

 feelers developed to supplement the eyes. Sight, in fact, is here begin- 

 ning to atrophy. In the greatest abysses the fish are mostly blind, 

 feeling their way about entirely by their sensitive bodies alone over 

 the naked surface of rock at the bottom. Some of them have still ex- 

 ternal relics of functionless eyes ; in others, the oldest and most con- 

 firmed abysmal species, the eye has altogether disappeared externally, 

 though its last representative may still be recognized, imbedded deep 

 in the tissues of the head." 



You all know about the blind fish of the Mammoth and other caves. 

 Before birth the optic nerve is connected with the eye of the mole, but 

 during adult life it is usually atrophied, and the mole is of course 

 blind. The Spolax d 1 Olivier, or mole-rat, also lives underground, and 

 is blind, though having some rudiments of eyes left under the skin. 

 An aquatic reptile Proteus, living in obscure caverns, has only traces 

 of the organ of vision. The number of blind or partially blind species 

 is said to number hundreds. So fearfully does the law of hunger sac- 

 rifice everything else to its implacable rule. A remarkable illustration 

 is also that of the African tunnel ants, or termites. No one can ever 

 forget Drummond's beautiful essay on them. There are three intoler- 



