Par. 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 the case 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 the
probability of Natural Selection having done its work so clearly.
I will not discuss the second case of utter sterility; but your
assumptions in par. 13 seem to me much too complicated. 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.
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