I cannot believe so universal an attribute as utter sterility between remote species was acquired in so complex a manner.

I do not agree with your rejoinder on grafting: I fully admit that it is not so closely restricted as crossing, but this does not seem to me to weaken the case as one of analogy.

The incapacity of grafting is likewise an invariable attribute of plants sufficiently remote from each other, and sometimes of plants pretty closely allied.
The difficulty of increasing the sterility through Natural Selection of two already sterile species seems to me best brought home by considering an actual case.

The cowslip and primrose are moderately sterile, yet occasionally produce hybrids.

Now these hybrids, two or three or a dozen in a whole parish, occupy ground which might have been occupied by either pure species, and no doubt the latter suffer to this small extent.