  The invasion of one fluid into another of higher viscosity in a quasi-two
dimensional geometry typically produces complex fingering patterns. Because
interfacial tension suppresses short-wavelength fluctuations, its elimination
by using pairs of miscible fluids would suggest an instability producing highly
ramified singular structures. Previous studies focused on wavelength selection
at the instability onset and overlooked the striking features appearing more
globally. Here we investigate the non-linear growth that occurs after the
instability has been fully established. We find a rich variety of patterns that
are characterized by the viscosity ratio between the inner and the outer fluid,
$\eta_{in}$/$\eta_{out}$, as distinct from the most-unstable wavelength, which
determines the onset of the instability. As $\eta_{in}$/$\eta_{out}$ increases,
a regime dominated by long highly-branched fractal fingers gives way to one
dominated by blunt stable structures characteristic of proportionate growth.
Simultaneously, a central region of complete outer-fluid displacement grows
until it encompasses the entire pattern at
$\eta_{in}$/$\eta_{out}$$\approx$0.3.
