ou, like some good people always put in a
text. My health keeps much the same, or rather improves, and I am able
to work some hours daily.--With many thanks for your interesting letter,
believe me, my dear Wallace, yours sincerely,

CH. DARWIN.

P.S.--I suppose you have read the last number of H. Spencer; I have been
struck with astonishment at the prodigality of original thought in it.
But how unfortunate it is that it seems scarcely ever possible to
discriminate between the direct effect of external influences and the
"survival of the fittest."

* * * * *

_9 St. Mark's Crescent, Regent's Park, N.W. Nov. 19, 1866._

Dear Darwin,--Many thanks for the fourth edition of the "Origin," which
I am glad to see grows so vigorously at each moult, although it
undergoes no metamorphosis. How curious it is that Dr. Wells should so
clearly have seen the principle of Natural Selection fifty years ago,
and that it should have struck no one that it was a great principle of
universal application in nature!

We are going to have a discussion on "Mimicry, as producing Abnormal
Sexual Characters," at the Entomological to-night. I have a butterfly
(Diadema) of which the female is metallic blue, the male dusky brown,
contrary to the rule in all other species of the genus, and in almost
all insects; but the explanation is easy--it mimics a metallic
_Euploea_, and so gets a protection perhaps more efficient than its
allies derive from their sombre colours, and which females require much
more than males. I read a paper on this at the British Association. Have
you the report published at Nottingham in a volume by Dr. Robertson? If
so, you can tell me if my paper is printed in full.

I suppose you have read Agassiz's marvellous theory of the Great
Amazonian glacier, 2,000 miles long! I presume that will be a _little_
too much, even for you. I have been writing a little popular paper on
"Glacial Theories" for the _Quarterly Journal of Science_ of January
next, in which I stick up for glaciers in North America and icebergs in
the Amazon!

I was very glad to hear from Lubbock that your health is permanently
improved. I hope therefore you will be able to give us a volume per
annum of your _magnum opus_, with all the facts as you now have them,
leaving additions to come in new editions.

I am working a little at another family of my butterflies, and find the
usual interesting and puzzling cases of variation,