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and sometimes these lenses can be detached in a layer distinct 
from the cornea. The transparent cones coated with pigment, 
which were supposed by Muller to act solely by excluding 
divergent pencils of light, usually adhere to the cornea, but 
not rarely they are separate from it, and have their free ends 
convex ; and in this case they must act as converging lenses. 
Altogether so diversified is the structure of the compound 
eyes, that Muller makes three main classes, with no less than 
seven subdivisions of structure ; he makes a fourth main class, 
namely, f aggregates' of stemmata; and he adds that ‘ this is 
the transition- form between the mosaic-like compound eyes 
unprovided with a concentrating apparatus, and the organs of 
vision with such an apparatus.' 
With these facts, here too briefly and imperfectly given, 
which show how much graduated diversity there is in the eyes 
of our existing crustaceans, and bearing in mind how small 
the number of living animals is in proportion to those which 
have become extinct, I can see no very great difficulty (not 
more than in the case of many other structures) in believing 
that natural selection has converted the simple apparatus of 
an optic nerve merely coated with pigment and invested by 
transparent membrane, into an optical instrument as perfect 
as is possessed by any member of the great articulate class. 
“ He who will go thus far, if he find on finishing this 
treatise that large bodies of facts, otherwise inexplicable, can 
be explained by the theory of descent, ought not to hesitate 
to go further, and to admit that a structure even as perfect as 
the eye of an eagle might be formed by natural selection, 
although in his case he does not know any of the transitional 
grades. His reason ought to conquer his imagination ; though 
I have felt the difficulty far too keenly to be surprised at any 
degree of hesitation in extending the principle of natural selec- 
tion to such startling lengths. 
“ It is scarcely possible to avoid comparing the eye to a 
telescope. We know that this instrument has been perfected 
bv the long-continued efforts of the highest human intellects; 
and we naturally infer that the eye has been formed by a 
somewhat analogous process. But may not this inference be 
presumptuous ? Have we any right to assume that the Creator 
works by intellectual powers like those of man ? If we must 
compare the eye to an optical instrument, we ought in imagi- 
nation to take a thick layer of transparent tissue, with spaces 
filled with fluid, and with a nerve sensitive to light beneath, and 
then suppose every part of this layer to be continually chang- 
ing slowly in density, so as to separate into layers of different 
densities and thicknesses, placed at different distances from 
