It cannot hold with plants, or the lower fixed aquatic animals.

I saw clearly what an immense aid this would be, but gave it up.

Disinclination to cross seems to have been independently acquired, probably by Natural Selection; and I do not see why it would not have sufficed to have prevented incipient species from blending to have simply increased sexual disinclination to cross.
(Paragraph 11.) I demur to a certain extent to amount of sterility and structural dissimilarity necessarily going together, except indirectly and by no means strictly.

Look at vars.

of pigeons, fowls, and cabbages.
I overlooked the advantage of the half-sterility of reciprocal crosses; yet, perhaps from novelty, I do not feel inclined to admit probability of Natural Selection having done its work so queerly.
I will not discuss the second case of utter sterility, but your assumptions in Paragraph 13 seem to me much too complicated.