  Metric $f(R)$ gravity theories behave like GR with cosmological constant when
$f"(R)$ = 0, and like scalar-tensor theories elsewhere. I investigate the
transition from $f"(R) \neq 0$ to $f"(R) = 0$, and show that this theory may
offer a way to explain Dark Energy and/or Dark Matter without having to make
use of the chameleon mechanism to agree with solar system tests.
