Let me address a common example that proponents of Intelligent Design use.  [See general eye diagram.] “Look at the wonderful design of the human eye,” they say.  “Surely this design could not have happened by chance.  It must be that “God did it.”  Actually, it did happen by chance — countless little chance events of changes in the gene pool over generations, all controlled by the harsh realities of natural selection and survival of the fittest.  While the initial changes in the gene pool (mutations) were chance events, survival of the fittest is obviously not random.  This is the heart of the basic Theory of Evolution; individuals can pass their genes and characteristics on to their offspring.  If a gene makes an individual more likely to have offspring that survive, its offspring (carrying that gene) will also be more likely to have offspring that survive.  In effect, species are designed to fit their environment.  The designer is the blind process of evolution, however, not some god or gods.  Evolution creates an illusion of human or supernatural design.  This illusion is so powerful that it took until 1859 for us to discover it, when Charles Darwin put forth one of the greatest ideas in science — evolution by natural selection.  This idea was the progenitor and center of the Theory of Evolution.

Darwin was limited by the scientific knowledge of the time, and thus didn't know about genes — the way that characteristics are inherited.  This limitation was soon filled in by Gregor Mendel, who showed that the inheritance of traits follows particular mathematical laws.