Biological structures such as the mammalian eye are complex in the sense that they contain many parts arranged in specific ways. It is unlikely that such structures could have been discovered by chance. No one, the astrophysicist N. C. Wickramasinghe once observed with some asperity, expects a tornado touching on a junkyard to produce a Boeing 747. The suggestion provokes Dawkins to indignation. This is a fine image, one introduced originally by the American bio-mathematician Sewell Wright.
Random variation offers the mountaineer an allowance of small changes.
Chance is at work. Natural selection freezes the successful changes in place. And this process owes nothing to chance. In time, the successful changes form a connected path, a staircase to complexity. The example that Dawkins pursues in greatest detail is the eye.