Nucleation plays
a critical role in the birth of crystals
and is
associated with a vast array of phenomena, such as protein crystallization
and ice formation in clouds. Despite numerous experimental and theoretical
studies, many aspects of the nucleation process, such as the polymorph
selection mechanism in the early stages, are far from being understood.
Here, we show that the hitherto unexplained excess of particles in
a face-centered-cubic (fcc)-like environment, as compared to those
in a hexagonal-close-packed (hcp)-like environment, in a crystal nucleus
of hard spheres can be explained by the higher order structure in
the fluid phase. We show using both simulations and experiments that
in the metastable fluid phase, pentagonal bipyramids, clusters with
fivefold symmetry known to be inhibitors of crystal nucleation, transform
into a different cluster, Siamese dodecahedra. These clusters are
closely similar to an fcc subunit, which explains the higher propensity
to grow fcc than hcp in hard spheres. We show that our crystallization
and polymorph selection mechanism is generic for crystal nucleation
from a dense, strongly correlated fluid phase.