[Skidoo]

Matt R. (Matter? Feel the science) and Magic_Man (by all means, keep on the magic):

The definition of "species" as an exclusive set whose individuals are fertile amongst themselves is a functional definition for the purposes of this discussion. It is justified insofar as the development of a mutually fertile group A into a mutually fertile group B that is infertile with A is a necessary condition of all proposed processes of natural evolution.