130 
DARWINISM 
CHAP. 
this time soft and flexible, the constant repetition of this effort 
causes the eye gradually to move round the head till it comes 
to the upper side. Now 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. Darwin says, “gave him a cold 
shiver ”—is nevertheless shown to be not unintelligible; 
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, owing to the 
great advantage of each modification which gave increased 
distinctness of vision, the creatures possessing it inevitably 
surviving, while those below them became extinct. But we 
can well understand how, after the first step Avas taken, every 
variation tending to more complete vision would be preserved 
till Ave reached the perfect eye of birds and mammals. Even 
this, as we 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 which the eye is liable, may be 
looked upon as relics of the imperfect condition from which 
the eye has been raised by variation and natural selection. 
These feAv examples of difficulties as to the origin of remark¬ 
able or complex organs must suffice here ; but the reader who 
Avishes further information on the matter may study carefully 
