"5. This has already been confirmed by reference to fact; in the History of Physiology, I have shown that those who studied the structure of animals were irresistibly led to the conviction that the parts of this structure have each its end or purpose;--that each member and organ not merely produces a certain effect or answers a certain use, but is so framed as to impress us with the persuasion that it was constructed _for_ that use;--that it was _intended_ to produce the effect. It was there seen that this persuasion was repeatedly expressed in the most emphatic manner by Galen;--that it directed the researches and led to the discoveries of Harvey;--that it has always been dwelt upon as a favourite contemplation, and followed as a certain guide, by the best anatomists;--and that it is inculcated by the physiologists of the profoundest views and most extensive knowledge of our own time. All these persons have deemed it a most certain and important principle of physiology, that in every organized structure, plant or animal, each intelligible part has its allotted office:--each organ is designed for its appropriate function:--that nature, in these cases, produces nothing in vain: that, in short, each portion of the whole arrangement has its _final cause_; an end to which it is adapted, and in this end, the reason that it is where and what it is. "6. This Notion of Design in organized bodies must, I say, be supplied by the student of organization out of his own mind: a truth which will become clearer if we attend to the most conspicuous and acknowledged instances of _design_. The structure of the eye, in which the parts are curiously adjusted so as to produce a distinct image on the retina, as in an optical instrument;--the trochlear muscle of the eye, in which the tendon passes round a support and turns back, like a rope round a pulley;--the prospective contrivances for the preservation of animals, provided long before they are wanted, as the milk of the mother, the teeth of the child, the eyes and the lungs of the foetus:--these arrangements, and innumerable others, call up in us a persuasion that Design has entered into the plan of animal form and progress. And if we bring in our minds this conception of Design, nothing can more fully square with and fit it, than such instances as these. But if we did not already possess the Idea of Design;--if we had not had our notion of mechanical contrivance awakened by inspection of optical instruments, or pulleys, or in some other way;--if we had never been conscious ourselves of providing for the future;--if this were the case, we could not recognize contrivance and prospectiveness in such instances as we have referred to. The facts are, indeed, admirably in accordance with these conceptions, when the two are brought together: but the facts and the conceptions come together from different quarters--from without and from within.