  Type Ia supernovae (SNe Ia) occur in both old, passive galaxies and active,
star-forming galaxies. This fact, coupled with the strong dependence of SN Ia
rate on star formation rate, suggests that SNe Ia form from stars with a wide
range of ages. Here we show that the rate of SN Ia explosions is about 1% of
the stellar death rate, independent of star formation history. The dependence
of SN Ia rate on star formation rate implies a delay time distribution
proportional to t^{-0.5+-0.2}. The single degenerate channel for SNe Ia can be
made to match the observed SN Ia rate -- SFR relation, but only if white dwarfs
are converted to SNe Ia with uniform efficiency of ~1%, independent of mass.
Since low-mass progenitors are expected to have lower conversion efficiencies
than high mass progenitors, we conclude that some other progenitor scenario
must be invoked to explain some, or perhaps all, SNe Ia.
