188 
DIFFICULTIES OK THEORY. 
Chap. YI. 
With these facts, here far too briefly and imperfectly 
given, which show that there is much graduated diver¬ 
sity in the eyes of living crustaceans, and bearing in 
mind how small the number of living animals is in 
proportion to those which have become extinct, I caH 
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 this 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 by 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 
imagination to take a thick layer of transparent tissue, 
with a nerve sensitive to light beneath, and then sup¬ 
pose every part of this layer to be continually changing 
