st thanks, and assure you that there
is no one living to whose kindness in such a matter I could feel myself
indebted with so much pleasure and satisfaction.--Believe me, dear
Darwin, yours very faithfully,

ALFRED R. WALLACE.

* * * * *

_Down, Beckenham, Kent. July 9._

My dear Wallace,--Dr. G. Krefft has sent me the enclosed from Sydney. A
nurseryman saw a caterpillar feeding on a plant and covered the whole
up, but, when he searched for the cocoon [pupa], was long before he
could find it, so good was its imitation, in colour and form, of the
leaf to which it was attached.

I hope that the world goes well with you. Do not trouble yourself by
acknowledging this.--Ever yours,

CH. DARWIN.

Accompanying this letter, which has been published in "Darwin and Modern
Science" (1909), was a photograph of the chrysalis (_Papilio sarpedon
choredon_) attached to a leaf of its food-plant. Many butterfly pupae
are known to have the power of individual adjustment to the colours of
the particular food-plant or other normal environment; and it is
probable that the Australian _Papilio_ referred to by Darwin possesses
this power.

* * * * *

_Nutwood Cottage, Frith Hill, Godalming, July 9, 1881._

My dear Darwin,--I am just doing, what I have rarely if ever done
before--reading a book through a second time immediately after the first
perusal. I do not think I have ever been so attracted by a book, with
perhaps the exception of your "Origin of Species" and Spencer's "First
Principles" and "Social Statics." I wish therefore to call your
attention to it, in case you care about books on social and political
subjects, but here there is also an elaborate discussion of Malthus's
"Principles of Population," to which both you and I have acknowledged
ourselves indebted. The present writer, Mr. George, while admitting the
main principle as self-evident and as actually operating in the case of
animals and plants, denies that it ever has operated or can operate in
the case of man, still less that it has any bearing whatever on the vast
social and political questions which have been supported by a reference
to it. He illustrates and supports his views with a wealth of
illustrative facts and a cogency of argument which I have rarely seen
equalled, while his style is equal to that of Buckle, and thus his book
is delightful reading. The title of the book is "Progress and Pover