  In the large $N_c$ limit, all hyperon decays involving the same quark diagram
$Q\to Q'$ are described by a single weak form factor $\eta_{QQ'}(w)$. No
assumption on the mass of $Q$ or $Q'$ is necessary, making our results
applicable to both $b\to c$ and $c\to s$ transitions. This same form factor
describes both $\Lambda_Q\to\Lambda_{Q'}$ and
$\Sigma^{(*)}_Q\to\Sigma^{(*)}_{Q'}$ transitions. The (non-)commutativity
between the heavy quark and the large $N_c$ limits is briefly discussed under
the definite example of $\Lambda_b\to\Lambda_c$ decay.
