240 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 
"ck of transparent skin, furnished with a nerve and lined with pig- 
ment, but destitute of any other apparatus. In fishes and reptiles, 
as Owen has remarked, “the range of gradations of dioptric structures 
is very great.” It is a significant fact that even in man, according to 
the high authority of Virchow, the beautiful crystalline lens is formed 
in the embryo by an accumulation of epidermic cells, lying in a sack- 
like fold of the skin; and the vitreous body is formed from embryonic 
sub-cutaneous tissue. To arrive, however, at a just conclusion 
regarding the formation of the eye, with all its marvellous yet not 
absolutely perfect characters, it is indispensable that the reason should 
conquer the imagination; but I have felt the difficulty far too keen 
to be surprised at others hesitating to extend the principle of 
natural selection to so startling a length. 
It is scarcely possible to avoid comparing the eye with 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 spaces 
filled with fluid, and with a nerve sensitive to light beneath, and then 
suppose every part of this layer to be continually changing slowly in 
density, so as to separate into layers of different densities and thick- 
nesses, placed at different distances from each other, and with the sur- 
faces of each layer slowly changing inform. Further we must suppose 
that there is a power, represented by natural selection or the survival 
of the fittest, always intently watching each slight alteration in the 
transparent layers; and carefully preserving each which, under varied 
circumstances, in any way or in any degree, tends to produce a dis- 
tincter image. We must suppose each new state of the instrument to 
be multiplied by the million; each to be preserved until a better one 
is produced, and then the old ones to be all destroyed. In living 
bodies, variation will cause the slight alterations, generation will 
multiply them almost infinitely, and natural selection will pick out 
with unerring skill each improvement. Let this process go on for 
millions of years; and during each year on millions of individuals of 
many kinds; and may we not believe that a living optical instrument 
might thus be formed as superior to one of glass, as the works of the 
Creator are to those of man? 
