Within the narrowly defined context of a strictly continuum-mechanical formulation, a body is a differentiable manifold and its material properties are described by one or more functions of its deformations. It is shown that the corresponding differential-geometric object is given precisely by a groupoid, whose variation in time characterizes the various phenomena of material evolution. In this context, one can identify and distinguish between pure remodeling and other evolutive phenomena, which can be further classified according to whether or not material symmetries are preserved.