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.
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