We consider the existence of approximate travelling waves of generalized KPP equations in which the initial distribution can depend on a small parameter $\mu $ which in the limit $\mu \rightarrow $ 0 is the sum of some $\delta $-functions or a step function. Using the method of Elworthy & Truman (1982) we construct a classical path which is the backward flow of a classical newtonian mechanics with given initial position and velocity before the time at which the caustic appears. By the Feynman-Kac formula and the Maruyama-Girsanov-Cameron-Martin transformation we obtain an identity from which, with a late caustic assumption, we see the propagation of the global wave front and the shape of the trough. Our theory shows clearly how the initial distribution contributes to the propagation of the travelling wave. Finally, we prove a Huygens principle for KPP equations on complete riemannian manifolds without cut locus, with some bounds on their volume element, in particular Cartan-Hadamard manifolds.