"On the Origin of Species" by Charles Darwin - The Wonderful Eye

This passage, if read in isolation, seems to concede Darwin's inability to explain the most wonderfully complex features that we encounter in biology, such as our own eyes. But in the pages that follow his rhetorical confession, he presents lucid evidence that “organs of extreme perfection and complication” arose through “numerous gradations” with “each grade being useful to its possessor”.

To evolve an eye, start with a “nerve merely coated with pigment” that will respond to light. Darwin also introduces the possibility that “an organ originally constructed for one purpose … may be converted into one for a wholly different purpose”. To evolve a lung, start with an air sac used for flotation. In essence, Darwin sets himself the seemingly impossible challenge of leaping across a large pond, but then shows it's really not too difficult when there are stepping stones along the way. 

-- Richard E. Lenski, Michigan State University