Let B be a two-dimensional ball with radius R. Let (u(x, y), ξ ) be a nonconstant steady state of the shadow systemwhere f and g satisfy the following: f ξ (u, ξ ) < 0, g ξ (u, ξ ) < 0 and there is a function k(ξ ) such that g u (u, ξ ) = k(ξ )f ξ (u, ξ ). This system includes a special case of the Gierer-Meinhardt system and the FitzHugh-Nagumo system. We show that if Z[U θ (·)] 3, then (u, ξ ) is unstable for all τ > 0, where U(θ) := u(R cos θ, R sin θ) and Z[w(·)] denotes the cardinal number of the zero level set of w(·) ∈ C 0 (R/2π Z). The contrapositive of this result is the following: if (u, ξ ) is stable for some τ > 0, then Z[U θ (·)] = 2. In the proof of these results, we use a strong continuation property of partial differential operators of second order on the boundary of the domain.