If these arguments are sound, it follows that sterility may be accumulated and increased, and finally made complete by Natural Selection, whether the sterile varieties originate together in a definite portion of the area occupied by the two species, or occur scattered over the whole area.

(211/4.

The first part of this discussion should be considered alone, as it is both more simple and more important.

I now believe that the utility, and therefore the cause of sterility between species, is during the process of differentiation.
When species are fully formed, the occasional occurrence of hybrids is of comparatively small importance, and can never be a danger to the existence of the species.

A.R.W.