Immediately behind each of these head-tentacles you see a little wart,
which has a black bead set as it were in its substance. You have
often, doubtless, observed the similar black points that are placed
at the tips of the upper pair of the horns (tentacles) of the common
garden snail; and I daresay, when a child, you have amused yourself by
touching them, and noticing how instantaneously the sensitive creature
would roll them in, so to speak, concealing them far in the interior of
the inturned horns. And every child is taught that these black spots
are the snail’s _eyes_; and so, indeed, they are; and these spots on
our Top’s warts are its eyes too, notwithstanding that some learned
naturalists, apparently from the mere love of paradox, have affected
to doubt the fact that such is their function. If you could dissect
out one of these points, and submit it to careful examination with a
good microscope, you would find all the parts essential to an organ of
vision; there is a sclerotic coat, a distinct little pupil and iris,
a cornea in front, and a dark pigment layer within, with vitreous and
aqueous humours, and even a crystalline lens for the condensation of
the rays of light. Minute these parts are, to be sure, but not less
exquisitely finished for that. Indeed, the more skill they require in
the demonstrator, the more they reflect the inimitable skill of the
Creator. Swammerdam, the Dutch physiologist, who so beautifully showed
the structure of the snail’s eye, seems to have feared the doubts of
his conclusions that would ensue from the difficulty of repeating his
investigations. “But who will credit this?” he says; “for does it not
seem impossible that on a point not larger than the nib of the pen
with which I write, such exquisite art and so many miracles should be
displayed?”