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.

ARW’s wife will accompany him to Down.

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.

Sexual differences in coloration. Sexual selection versus natural selection as explanations. ARW continues to argue against sexual selection, saying that natural selection, in keeping the female dull for protection, would account for differences in sexual colouring more effectively than inheritance and partial transmission of sexually selected male colours. Colours of female birds of paradise. Protective coloration. Disagrees with CD on coincidence of hidden nests and bright colours of females.