The idea of reincarnation generally describes how a consciousness gradually changes from incarnation to incarnation. It also describes how the body is determined by the consciousness. Thus the fact that we see a "gradual change" from species to species when we look at them, the living ones, or the archeological findings, can not only be explained by Darwinistic evolution, but also by reincarnation.

Here is another way to say it, which might be clearer:

The gradual change we see between one species and another, both among now living beings and in fossils, can be explained by reincarnation, a gradual change of the consciousness of individual beings, reflected in the shape of their bodies. Darwinistic ideas are not the only ideas to explain that observation.