Coloration of butterflies; brilliantly coloured females.

Commends CD on his paper on specific differences in Primula [J. Proc. Linn. Soc. Lond. (Bot.) 10 (1869): 437–54; reprinted and revised in Forms of flowers] as a test-case proving origin of real species.

On sterility of natural species and natural selection. Closely allied forms from adjacent islands offer best chance of finding good species fertile inter se.

Problem of minute variations and sexual selection.

Returns George Darwin’s criticisms of his notes on sterility and sends further notes in reply. Since there are degrees of sterility between varieties, "is it not probable that natural selection can accumulate these variations?" Varieties that are adapted to new conditions could then survive and form new species without being isolated.

If CD is not convinced by his notes on sterility, ARW has little doubt that he is wrong. In fact he was only half-convinced by his own arguments.

Modifies his first proposition [a species varies occasionally in two directions, but owing to free inter-crossing the variations never increase] and further discusses the subject.

Encloses Berthold Seemann’s notes on flora of the Hawaiian Islands. Presence of European alpine species in Hawaiian volcanoes is a "hard nut" for geographical distribution [but see ARW’s Island life (1880), p. 323].

Various topics related to sexual selection: sexual differences, sexual preferences, coloration.

Answers CD’s objection [see 6121 and 6146] about sexual differences and protective colouring. Summarises his theory of colour in nature.

The problem of sterility, and its relation to natural selection.

George Bentham’s support of Darwinism.

On triumph of "Darwinianism".

Discussion of their differences on subject of protection.

Accepts invitation.

On sounds produced by Euchirus longimanus beetle. Sends a pair by post.

Submits a 15–point argument against CD’s views on the coloration of female birds and insects.

Dedication of Malay Archipelago to CD.

Comments on scientific papers.

Argues that [general variability] of species, not single variations or sports, is basis for modification and adaptation to new conditions.

Weir’s paper on relation of protection to colour of caterpillars [Trans. R. Entomol. Soc. Lond. (1869): 21–6; (1870): 337–9] confirms ARW’s hypothesis.

Comments on Fritz Müller’s book [Facts and arguments for Darwin].

Responds to CD’s corrections of his work [Malay Archipelago].

Plumage of birds of paradise.

Expands upon their differences in regard to man and the question of the existence of forces not yet recognised by science.

Asks whether sexual selection could produce the changing plumules or "battledore" scales on the wings of certain butterflies.

Looks forward to Descent, though he expects to find more to differ with than in any other of CD’s books.

Problems of usefulness of incipient organs and of the independent origin of similar complex organs are real difficulties.

Plans a little book on "Distribution of animals".

Inquires about arrangements for the German translation and publication of their original Linnean Society papers [Collected papers 2: 3–19].

ARW thinks he has hit upon a solution to problem of geological time.

Plans for his new book, Contributions to the theory of natural selection (1870), which will contain his papers on the subject.