  We show that any Banach space contains a continuum of non isomorphic
subspaces or a minimal subspace. We define an ergodic Banach space $X$ as a
space such that $E_0$ Borel reduces to isomorphism on the set of subspaces of
$X$, and show that every Banach space is either ergodic or contains a subspace
with an unconditional basis $ which is complementably universal for the family
of its block-subspaces. We also use our methods to get uniformity results; for
example, in combination with a result of B. Maurey, V. Milman and N.
Tomczak-Jaegermann, we show that an unconditional basis of a Banach space, of
which every block-subspace is complemented, must be asymptotically $c_0$ or
$l_p$.
