As Darwin says, it hasn't been disputed. Creationists agree there is variation among creatures, but say that the variation is within limits. It seems to me that the further back in time you find an organism, the less time there would have been for it to evolve. For some reason these discoveries never seem to be counted against evolution. If I understand correctly, living beings have a genetic clock that seems programmed to end our lives.
Cells simply stop operating properly after a certain amount of time.
In Origin of Species Darwin outlines his theory, then spends much of his book defending the theory against objections. Today's evolutionists seem not so brave. They seem bent on preventing any criticism of Darwin's theories in public schools. In Origin of Species Darwin called it a theory so many times I lost track. If we're going to stop calling it a theory, there must have been some great discoveries that prove the theory.