But if variations useful to any organic being ever do occur, assuredly individuals thus characterised will have the best chance of being preserved in the struggle for life; and from the strong principle of inheritance, these will tend to produce offspring similarly characterised. This principle of preservation, or the survival of the fittest, I have called Natural Selection.”

Organs of extreme perfection

One of the books which most impressed the young Darwin was the treatease of theologian William Paley “Natural Theology - or Evidences of the Existence and Attributes of the Deity Collected from the Appearances of Nature” (1802).
The script begins in this way: “In crossing a heath, suppose I pitched my foot against a stone, and were asked how the stone came to be there; I might possibly answer that, for anything I knew to the contrary, it had lain there for ever: nor would it perhaps be very easy to show the absurdity of this answer. But suppose I had found a watch upon the ground, and it should be inquired how the watch happened to be in that place; I should hardly think of the answer which I had before given, that for anything I knew, the watch might have always been there… no, the answer is that the watch must have had a maker: that there must have existed, at some time, and at some place or other, an artificer or artificers, who formed it for the purpose which we find it actually to answer; who comprehended its construction, and designed its use… Every indication of contrivance, every manifestation of design, which existed in the watch, exists in the works of nature; with the difference, on the side of nature, of being greater or more, and in a degree which exceeds all computation.”
Paley goes on with a discussion of anatomical organs, and in the case of the eye he finds it perfectly natural comparing it with the telescope. They are both optical instruments, and Paley concludes that “there is precisely the same proof that the eye was made for vision as there is that the telescope was made for assisting it”.