As an instance of this last point, let me remind you that Sight was originally a differentiation of Touch. The light, the shadows, falling on the sensitive general surface of a primitive organism provoke a tactile irritation. In the course of evolution this sense specialises itself at some point of the surface into what we call Sight. Now, to-day, when the little picture formed by the fore-part of the Human Eye falls upon the Retina at the back, it falls upon a screen formed by the myriad congregated finger-tips, so to speak, of the optic nerve--the rods and cones, so-called--which cover like a mosaic the whole ground of the Retina, and _feel_ with their sensitive points the images of the objects in the outer world. And so Sight is still Touch--it is the power of feeling or touching at a distance--as one sometimes in fact becomes aware in looking at things.
But then again on and beyond all these things--beyond the focussing and photographing of rays, beyond the latent adaptations to the needs of innumerable creatures, and the epitomising of ages of evolution--the Human Eye has faculties even more far-reaching perhaps and wonderful. It is the marvellous organ of human Expression. By the dilatations and contractions of the iris, by the altering convexities of the lens and the eyeball, and in a hundred other ways, it manages somehow to convey intelligence of Command, Control, Power, of Pity, Love, Sympathy, and all those myriad emotions which flit through the human mind--an endless series--a perfect encyclopædia. It is difficult even to imagine the eye without this power of language. And what other functions it may have it is not necessary to inquire. Highly specialised though it is, it is already obvious enough that to call it a Machine for focussing rays of light is monstrously and ludicrously inadequate--even as it would be to call the Heart (the very centre of emotion and life, and the symbol of human love and courage) a common Pump.
Nature is an infinitude, and can at no point be circumscribed by the human intellect. Nor obviously is there any sense in taking one little portion of Nature and isolating it from the rest, and then describing it exhaustively _as if_ it really were so isolated. A thousand mechanicians will agree, as I have said, in their description of a machine, because in fact they will agree to view the machine just in the one aspect of its particular action; but ask a thousand people to describe one and the same face--or, better still, get a thousand portrait-painters, skilled in their art, to paint portraits of the same face--and you know perfectly well that all the likenesses will be different. And why will they be different? Simply because every face, however rude, has infinite sides, infinite aspects, and each painter selects what he paints from his own point of view. And the same is true of every object and process in Nature.