130 DARWINISM chap. 



this time soft and flexi])le, the constant repetition of this effort 

 causes the eye gradually to move round the head till it comes 

 to the upper side. Xow if we suppose this process, which in 

 the young is completed in a few days or weeks, to have been 

 spread over thousands of generations during the development of 

 these fish, those usually surviving whose eyes retained more 

 and more of the position into which the young fish tried to 

 twist them, the change becomes intelligible ; though it still 

 remains one of the most extraordinary cases of degeneration, by 

 which symmetry — which is so universal a characteristic of 

 the higher animals — is lost, in order that the creature may be 

 adapted to a new mode of life, whereby it is enabled the better 

 to escape danger and continue its existence. 

 , The most difficult case of all, that of the eye — the thought 

 of which even to the last, Mr. Dar\\an says, " gave him a cold 

 shiver " — is nevertheless shown to be not unintelliirible ; 



I . . . . o ' 



granting of course the sensitiveness to light of some forms of 

 nervous tissue. For he shows that there are, in several of the 

 lower animals, rudiments of eyes, consisting merely of pigment 

 cells covered "with a translucent skin, which may possibly 

 serve to distinguish light from darkness, but nothing more. 

 Then we have an optic nerve and pigment cells ; then we 

 find a hollow filled with gelatinous substance of a convex 

 form — the first rudiment of a lens. Many of the succeeding 

 steps are lost, as would necessarily be the case, o'snng to the 

 great advantage of each modification Avhich gave increased 

 distinctness of vision, the creatures possessing it incAdtably 

 surviA'ing, while those below them became extinct. But we 

 can Avell understand how, after the first step was taken, every 

 variation tending to more complete A'ision would be preserved 

 till we reached the perfect eye of birds and mammals. Even 

 this, as Ave know, is not absolutely, but only relatively, perfect. 

 Neither the chromatic nor the spherical aberration is absolutely 

 corrected ; while long- and short- sightedness, and the various 

 diseases and imperfections to Avhich the eye is liable, may be 

 looked upon as relics of the imperfect condition from Avhich 

 the eye has been raised by variation and natural selection. 



These few examples of difficulties as to the origin of remark- 

 able or complex organs must suffice here ; but the reader Avho 

 wishes further information on the matter may study carefully 



