Now, let us ask, why do we, without any hesitation whatever, or any exception whatever, always and immediately arrive at this conclusion respecting intelligence and design? Nothing could be more unphilosophical, nay, more groundless, than such a process of reasoning, if we had only been able to trace design in one or two instances; for instance, if we found only the eye to show proofs of contrivance, it would be wholly gratuitous, when we saw the ear, to assume that it was adapted to the nature of sound, and still more so, if, on examination, we perceived it bore no perceptible relation to the laws of acoustics. The proof of contrivance in one particular is nothing like a proof, nay, does not even furnish the least presumption of contrivance in other 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.