In order to illustrate what I mean by contrivance, adaptation, and design, let me refer to a familiar example--the human eye. Made up of three coats and three humors, of solids and fluids, of nerves, blood-vessels, and muscles, and rivalling the most perfect optical instrument, it must have required the most consummate contrivance to give the requisite quantity and position to parts so numerous and unlike, for producing the phenomena of vision. Yet how perfectly it is done! How few, out of the hundreds of millions of eyes of men and other animals, fail of vision through any natural defect!
No less marvellous are the adaptations of the eye. In order to be adapted to the wonderful effect which we call light, its coats and humors must be transparent, and possess a certain density and opacity, that the rays may form an image on the retina. Yet to prevent confusion in the image, the transparency must be confined to the central parts of the eye, and a dark plexus of veins and muscles must be so situated as to absorb the scattering rays. In order to adapt the eye to different distances, and to the greater or less intensity of the light, delicate muscles must be so situated as to contract and dilate the pupil, and lengthen and shorten the axis. That the eye might be directed to different objects, strong muscles must be attached to its posterior surface; and that the eyelid might defend it from injuries in front, a very peculiar muscle must give it power to close. No less perfect is the adaptation of the eye to the atmosphere, or, rather, there is a mutual adaptation; and it is as proper to say that the atmosphere is adapted to the eye, as that the eye is adapted to the atmosphere. In like manner, there is a striking relation between the eye and the sun and other heavenly bodies, and between the eye and day and night; so that we cannot doubt but they were made for one another. We might, indeed, extend the relations of the eye to every object in the universe; and the same may be said of every organ of plants and animals. The adaptation between them is as wide as creation. And it is the wonderful harmony between so many millions of objects that makes us feel that infinite wisdom alone could have produced it.