  From a sample of $1172 \pm 61$ $D^+ \to \pi^- \pi^+ \pi^+$ decay, we find
$\Gamma (D^+ \to \pi^- \pi^+ \pi^+) / \Gamma (D^+ \to K^- \pi^+ \pi^+) =
  0.0311 \pm 0.0018 ^{+0.0016}_{-0.0026}$. Using a coherent amplitude analysis
to fit the Dalitz plot of this decays, we find strong evidence that a scalar
resonance of mass $478^{+24}_{-23} \pm 17$ MeV/$c^2$ and width $324^{+42}_{-40}
\pm 21$ MeV/$c^2$ accounts for approximately half of all decays.
