This paper studies the undiscounted utilitarian optimal paths of the canonical Dasgupta--Heal--Solow model when the stock of natural capital is a direct argument of well-being, besides consumption. We use a Keynes--Ramsey rule which yields a generalization of Hartwick's rule. We characterize solutions in the Cobb-Douglas utility case, and analyse the influence of the intertemporal elasticity of substitution on the time profile of the optimal paths. We show that the ratio of the values of the resource and capital stocks remains constant along the optimal path, and is independent of the initial conditions.
