rmediate forms die out, without also the
other extreme form also dying out from not having the advantages of the
first selected form? for, as I understand, both female forms occur on
the same island. I quite agree with your distinction between dimorphic
forms and varieties; but I doubt whether your criterion of dimorphic
forms not producing intermediate offspring will suffice; for I know of
a good many varieties, which must be so called, that will not blend or
intermix, but produce offspring quite like either parent.

I have been particularly struck with your remarks on geological
distribution in Celebes. It is impossible that anything could be better
put, and [it] would give a cold shudder to the immutable naturalists.

And now I am going to ask a question which you will not like. How does
your Journal get on? It will be a shame if you do not popularise your
researches.

My health is so far improved that I am able to work one or two hours a
day.--Believe me, dear Wallace, yours very sincerely,

CH. DARWIN.

* * * * *

_9 St. Mark's Crescent, Regent's Park, N.W. February 4, 1866._

My dear Darwin,--I am very glad to hear you are a little better, and
hope we shall soon have the pleasure of seeing your volume on "Variation
under Domestication." I do not see the difficulty you seem to feel about
two or more female forms of one species. The _most common_ or _typical_
female form must have certain characters or qualities which are
sufficiently advantageous to it to enable it to maintain its existence;
in general, such as vary much from it die out. But occasionally a
variation may occur which has special advantageous characters of its own
(such as mimicking a protected species), and then this variation will
maintain itself by selection. In no less than three of my _polymorphic_
species of Papilio, one of the female forms mimics the _Polydorus_
group, which, like the _AEneas_ group in America, seems to have some
special protection. In two or three other cases one of the female forms
is confined to a restricted locality, to the conditions of which it is
probably specially adapted. In other cases one of the female forms
resembles the male, and perhaps receives a protection from the
abundance of the males, in the crowd of which it is passed over. I think
these considerations render the production of two or three forms of
female very conceivable. The physiological difficulty is to me greater,