Popper believed that a scientific theory could never be proven, but only disproven. The scientist sets forth his theory and then makes attempts to find falsifying instances. This is, according to Popper, the appropriate activity of the scientist, much more so that finding confirming instances. Because of this, you find Darwinists readily saying (as they have on this blog) that you can't prove anything (scientifically).
The problem is, they will just as readily say, in other contexts, that evolution has been "proven." Put these two propositions together, and you get the following syllogism:
No scientific theory can be provenJust thought I'd mention it.
Evolution is a scientific theory
Therefore, Evolution cannot be proven