  We propose a robust and scalable scheme to generate an $N$-qubit $W$ state
among separated quantum nodes (cavity-QED systems) by using linear optics and
postselections. The present scheme inherits the robustness of the Barrett-Kok
scheme [Phys. Rev. A {\bf 71}, 060310(R) (2005)]. The scalability is also
ensured in the sense that an arbitrarily large $N$-qubit $W$ state can be
generated with a quasi-polynomial overhead $\sim 2^{O[(\log_2 N)^2]}$. The
process to breed the $W$ states, which we introduce to achieve the scalability,
is quite simple and efficient, and can be applied for other physical systems.
