particulars; because, _a priori_, it is just as easy to suppose one part
of nature to be designed for a purpose, and another part, nay, all other
parts, to be formed at random and without any contrivance, as to suppose
that the formation of the whole is governed by design. Why, then, do we,
invariably and undoubtedly, adopt the course of reasoning which has been
mentioned, and never for a moment suspect anything to be formed without
some reason--some rational purpose? The only ground of this belief is,
that we have been able distinctly to trace design in so vast a majority
of cases as leaves us no power of doubting that, if our faculties had
been sufficiently powerful, or our investigation sufficiently diligent,
we should also have been able to trace it in those comparatively few
instances respecting which we still are in the dark.

It may be worth while to give a few instances of the ignorance in which
we once were of design in some important arrangements of nature, and
of the knowledge which we now possess to show the purpose of their
formation. Before Sir Isaac Newton's optical discoveries, we could not
tell why the structure of the eye was so complex, and why several lenses
and humors were required to form a picture of objects upon the retina.
Indeed, until Dolland's subsequent discovery of the achromatic effect of
combining various glasses, and Mr. Blair's still more recent experiments
on the powers of different refracting media, we were not able distinctly
to perceive the operation and use of the complicacy in the structure of
the eye. We now well understand its nature, and are able to comprehend
how that which had at one time, nay, for ages, seemed to be an
unnecessary complexity; forms the most perfect of all optical
instruments, and according to the most certain laws of refraction and of
dispersion.

So, too, we had observed for some centuries the forms of the orbits in
which the heavenly bodies move, and we had found these to be ellipses
with a very small eccentricity. But why this was the form of those
orbits no one could even conjecture. If any person, the most deeply
skilled in mathematical science, and the most internally convinced of
the universal prevalence of design and contrivance in the structure
of the universe, had been asked what reason there was for the planets
moving in ellipses so, nearly approaching to circles, he could not
have given any good reason, at least beyond a guess. The force of
grav