Using Galois theory of functional equations, we give a new proof of the main result of the paper "Transcendental transcendency of certain functions of Poincaré" by J.F. Ritt, on the differential transcendence of the solutions of the functional equation R(y(t)) = y(qt), where R(t) ∈ C(t) verifies R(0) = 0, R ′ (0) = q ∈ C, with |q| > 1. We also give a partial result in the case of an algebraic function R.