  We analyse observed fractions of core-collapse SN types from the Lick
Observatory SN Search, and we discuss corresponding implications for massive
star evolution. For a standard IMF, observed fractions of SN types cannot be
reconciled with expectations of single-star evolution. The mass range of WR
stars that shed their H envelopes via their own mass loss accounts for less
than half the observed fraction of SNeIbc. Progenitors of SNeIbc must extend to
a much lower range of initial masses than classical WR stars, and we argue that
most SNIbc and SNIIb progenitors must arise from binary Roche-lobe overflow.
SNeIc still trace higher mass and metallicity, because line-driven winds in the
WR stage remove the He layer and propel the transition from SNIb to Ic. Less
massive progenitors of SNeIb and IIb may not be classical WR stars; they may be
underluminous with weak winds, possibly hidden by overluminous mass-gainer
companions that appear as B[e] supergiants or related objects having aspherical
circumstellar material. The remaining SN types (II-P, II-L, and IIn) are
redistributed across the full range of initial mass. We consider direct
collapse to black holes without visible SNe, but find this problematic. Major
areas of remaining uncertainty are (1) the influence of binary separation,
rotation, and metallicity, (2) mass differences in progenitors of SNeIIn
compared to SNeII-L and II-P, and (3) SNeIc arising from single stars with
eruptive mass loss, its dependence on metallicity, and how it relates to
diversity within the SNIc subclass. (abridged)
