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and if any variation or modification in tlie organ be ever useful 
to an animal under changing conditions of life, then the diffi- 
culty of believing that a perfect and complex eye could be 
formed by natural selection, though insuperable by our ima- 
gination, can hardly be considered real. How a nerve comes 
to be sensitive to light, hardly concerns us more than how life 
itself first originated ; but I may remark that several facts 
make me suspect that nerves sensitive to touch may be made 
sensitive to light, and likewise to those coarser vibrations of 
the air which produce sound. In looking for the gradations 
by which an organ in any species has been perfected, we ought 
to look exclusively to its lineal ancestors ) but this is scarcely 
ever possible, and we are forced in each case to look to species 
of the same group, that is to the collateral descendants fiom 
the same original parent form, in order to, see what gradations 
are possible, and for the chance of some gradations having 
been transmitted from the earlier stages of descent, in an un- 
altered or little altered condition. Amongst existent verte- 
brata, we find but a small amount of gradation in the structure 
of the eye ; though in the fish amphioxus the eye is an ex- 
tremely simple condition without a lens ; and from fossil 
species we can learn nothing on this head. In this gieat 
class we should probably have to descend far beneath the 
lowest known fossiliferous stratum to discover the earlier 
stages by which the eye has been perfected. 
ff In the great kingdom of the articulata we can start from 
an optic nerve, simply coated with pigment, which sometimes 
forms a sort of pupil, but is destitute of a lens or any other 
optical mechanism. From this rudimentary eye, which can 
distinguish light from darkness, but nothing else, there is an 
advance towards perfection along two lines of structure, which 
Muller thought were fundamentally different ; namely, — firstly, 
stemmata, or the so-called f simple eyes/ which have a lens 
and cornea ; and secondly, f compound eyes/ which seem to 
act mainly by excluding all the rays from each point of the 
object viewed ; except the pencil that comes in a line per- 
pendicular to the convex retina. In compound eyes besides 
endless differences in the form, proportion, number, and posi- 
tion of the transparent cones coated by pigment, and which 
act by exclusion, we have additions of a more or less perfect 
concentrating apparatus. Thus in the eye of the meloe the 
facets of the cornea are f slightly convex, both externally and 
internally, that is, lens-shaped/ In many crustaceans there 
are two cornea, — the external smooth, and the internal divided 
into facets — within the substance of which, as Milne Edwards 
says, ‘ renflemens lenticulaires paraissent s^etre developpes ; 
