tic 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. 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. But can you conceive that any individual plants of the primrose
and cowslip which happened to be mutually rather more sterile (i.e.
which, when crossed, yielded a few less seed) than usual, would profit
to such a degree as to increase in number to the ultimate exclusion of
the present primrose and cowslip? I cannot.

My son, I am sorry to say, cannot see the full force of your rejoinder
in regard to second head of continually augmented sterility. You speak
in this rejoinder, and in Paragraph 5, of all the individuals becoming
in some slight degree sterile in certain districts: if you were to admit
that by continued exposure to these same conditions the sterility would
inevitably increase, th