  Shocks of supernova remnants (SNRs) are important (and perhaps the dominant)
agents for production of the Galactic cosmic rays. Recent $\gamma$-ray
observations of several SNRs have made this case more compelling. However,
these broadband high-energy measurements also reveal a variety of spectral
shape demanding more comprehensive modeling of emissions from SNRs. According
to the locally observed fluxes of cosmic ray protons and electrons, the
electron-to-proton number ratio is known to be about 1%. Assuming such a ratio
is universal for all SNRs and identical spectral shape for all kinds of
accelerated particles, we propose a unified model that ascribes the distinct
$\gamma$-ray spectra of different SNRs to variations of the medium density and
the spectral difference between cosmic ray electrons and protons observed at
Earth to transport effects. For low density environments, the $\gamma$-ray
emission is inverse-Compton dominated. For high density environments like
systems of high-energy particles interacting with molecular clouds, the
$\gamma$-ray emission is $\pi^0$-decay dominated. The model predicts a hadronic
origin of $\gamma$-ray emission from very old remnants interacting mostly with
molecular clouds and a leptonic origin for intermediate age remnants whose
shocks propagate in a low density environment created by their progenitors via
e.g., strong stellar winds. These results can be regarded as evidence in
support of the SNR-origin of the Galactic cosmic rays.
