Notes the differences in haemoglobin between species; this proves, to WP, that natural selection does not act only on morphological development.

Remarks on the differences and similarities in the effects of prussic acid on different groups of animals.

Questions CD on his early education and influences.

Sends his "Charles Darwin, eine biographische Skizze" [Das Ausland 2 Apr 1870].

Thanks CD for Origin, 5th ed.

Comments on reviews of Descent by the Duke of Argyll and A. R. Wallace.

Lists the Darwinian professors at Jena.

WP’s work shows external ear to have no physiological functions.

W. Müller’s book not yet arrived. Will send Müller’s next works.

Delighted that CD does not hesitate to follow his theory to its last consequences. "This is far more difficult in England than in Germany."

Wilhelm Müller in his new book [Beiträge zur pathologischen Anatomie und Physiologie des menschlichen Rückenmarks (1871)], adduces many facts in favour of CD’s theory.

Various observations on the human ear. [See Descent, 2d ed., p. 14 n.]

Asks for CD’s "Sketch of an infant" [Collected papers 2: 191–200]. He has made observations on new-born children and mammals to determine what behaviour is inherited and what acquired.