Natural Theology (1802) by William Paley represents part of a movement of scientific thought that was extremely prevelent in the first half of the 19th Century. Biology was not a terribly well definied discipline just yet, and a great many naturalists were also clergymen. The logic goes something like this: God created the world; the world is beautiful; therefore by studying the natural world we honor God.
Natural Theology attempts to prove the existence of a deity based on the complexity and perfection of the natural world. The introduction is as follows:
In crossing a heath, suppose I pitched my foot against a stone, and was asked how the stone came to be there, I might possibly answer, that for any thing 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 given before, that for any thing I knew the watch might have always been there. Yet why should not this answer serve for the watch as well as for the stone; why is it not as admissible in the second case as in the first? For this reason, and for no other, namely, that when we come to inspect the watch, we perceive--what we could not discover in the stone--that its several parts are framed and put together for a purpose...
The sentiment is not new; indeed, it was probably just such wonder of nature that led the earliest human societies to worship deities. Neither has this line of reasoning been abandoned; several late twentieth century scientific creationists, most notably Michael Behe, reissued a very nearly identical argument. In "Darwin's Black Box" (1996), Behe writes:
To a person who does not feel obliged to restrict his search to unintelligent causes, the straightforward conclusion is that many biochemical systems were designed. They were designed not by the laws of nature, not by chance and necessity; rather, they were planned. The designer knew what the systems would look like when they were completed, then took steps to bring the systems about. Life on earth at its most fundamental level, in its most critical components, is the product of intelligent activity.
The conclusion of intelligent design flows naturally from the data itself--not from sacred books or sectarian beliefs. Inferring that biochemical systems were designed by an intelligent agent is a humdrum process that requires no new principles of logic or science. It comes simply from the hard work that biochemistry has done over the past forty years, combined with consideration of the way in which we reach conclusions of design every day.
Natural Theology was an extremely influential book in its day; Charles Darwin was strongly influenced by it, and originally intended to structure his own work along similar lines. Although scientifically obsolete, Natural Theology is historically interesting, besides being exceptional writing. Paley's argument continues:
The watch is found, in the course of its movement, to produce another watch similar to itself; and not only so, but we perceive in it a system or organization separately calculated for that purpose. What effect would this discovery have, or ought it to have, upon our former inference? What, as hath already been said, but to increase beyond measure our admiration of the skill which had been employed in the formation of such a machine? Or shall it, instead of this, all at once turn us round to an opposite conclusion, namely, that no art or skill whatever has been concerned in the business, although all other evidences of art and skill remain as they were, and this last and supreme piece of art be now added to the rest? Can this be maintained without absurdity? Yet this is atheism.