e in number over other families of A, which are not more sterile
when crossed with B. But I do not know that I have made this any clearer
than in the chapter in my book. It is a most difficult bit of reasoning,
which I have gone over and over again on paper with diagrams.

I shall be intensely curious to see your article in the _Journal of
Travel_.

Many thanks for such answers as you could give. From what you say I
should have inferred that birds of paradise were probably polygamous.
But after all, perhaps it is not so important as I thought. I have been
going through the whole animal kingdom in reference to sexual selection,
and I have just got to the beginning of Lepidoptera, i.e. to end of
insects, and shall then pass on to Vertebrata. But my ladies next week
are going (ill-luck to it) to take me nolens-volens to London for a
whole month.

I suspect Owen wrote the article in the _Athenaeum_, but I have been told
that it is Berthold Seeman. The writer despises and hates me.

Hearty thanks for your letter--you have indeed pleased me, for I had
given up the great god Pan as a stillborn deity. I wish you could be
induced to make it clear with your admirable powers of elucidation in
one of the scientific journals.

I think we almost entirely agree about sexual selection, as I now follow
you to large extent about protection to females, having always believed
that colour was often transmitted to both sexes; but I do not go quite
so far about protection.--Always yours most sincerely,

CH. DARWIN.

* * * * *

_Hurstpierpoint. March 1, 1868._

My dear Darwin,--I beg to enclose what appears to me a demonstration,
_on your own principles_, that Natural Selection _could_ produce
_sterility of hybrids_.

If it does not convince you I shall be glad if you will point out where
the fallacy lies. I have taken the two cases of a slight sterility
overcoming a perfect fertility, and of a perfect sterility overcoming a
partial fertility--the beginning and end of the process. You admit that
variations in fertility and sterility occur, and I think you will also
admit that if I demonstrate that a considerable amount of sterility
would be advantageous to a variety, that is sufficient proof that the
slightest variation in that direction would be useful also, and would go
on accumulating.

Sir C. Lyell spoke to me as if he greatly admired pangenesis. I am very
glad H. Spencer at once acknowledges th