admit all your assumptions, and even if they were all
admitted, the process is so complex and the sterility (as you remark in
your note) so universal, even with species inhabiting quite distinct
countries (as I remarked in my chapter), together with the frequency of
a difference in reciprocal unions, that I cannot persuade myself that it
has been gained by Natural Selection, any more than the difficulty of
grafting distinct genera and the impossibility of grafting distinct
families. You will allow, I suppose, that the capacity of grafting has
not been directly acquired through Natural Selection.

I think that you will be pleased with the second volume or part of
Lyell's Principles, just out.

In regard to sexual selection. A girl sees a handsome man, and without
observing whether his nose or whiskers are the tenth of an inch longer
or shorter than in some other man, admires his appearance and says she
will marry him. So, I suppose, with the pea-hen; and the tail has been
increased in length merely by, on the whole, presenting a more gorgeous
appearance. Jenner Weir, however, has given me some facts showing that
birds apparently admire details of plumage.--Yours most sincerely,

C. DARWIN.

* * * * *

_Hurstpierpoint. March 24, [1868?]._

Dear Darwin,--Many thanks for the photo, which I shall get when I go to
town.

I return your son's notes with my notes on them.

Without going into any details, is not this a strong general argument?--

1. A species varies occasionally in two directions, but owing to their
free intercrossing they (the variations) never increase.

2. A change of conditions occurs which threatens the existence of the
species, but the _two varieties_ are adapted to the changing conditions,
and, if accumulated, will form two new _species adapted to the new
conditions_.

3. Free crossing, however, renders this impossible, and so the species
is in danger of extinction.

4. If _sterility_ could be induced, then the pure races would increase
more rapidly and replace the old species.

5. It is admitted that _partial sterility_ between _varieties_ does
occasionally occur. It is admitted the _degree_ of this sterility
_varies_. Is it not probable that Natural Selection can accumulate these
variations and thus save the species?

If Natural Selection can _not_ do this, how do species ever arise,
except when a variety is isolated?

Closely allied species in distinct c