Over 850 illustrations from the printed volumes of The Correspondence of Charles Darwin have been added to the online transcripts of the letters. The contents include maps, diagrams, drawings, sketches and photographs, covering geological, botanical, zoological and philosophical topics. Many of the letters would be difficult to understand without them.

Here are some of the highlights in chronological order:

Adam Sedgwick's report on his geological work in N. Wales since he and CD parted, 4 September 1831

Thomas Sutcliffe's map of the route between Santiago and San Fernando, Chile, [28 August - 5 September 1834]

CD to W. A. Leighton: map of where specimens of Cynoglossum sylvaticum may be seen growing, [1-23 July 1841]

William Hopkins's comments on a compass diagram designed to show the dip, strike, and anticlinal lines of a geological formation, 3 March 1845

Edward Forbes's "Atlantis" theory, [25 February 1846]

E. A. Darwin's calculations on the structure of bees' cells, [before 8 June 1858], and their geometry, [19 June 1858].

W. E. Darwin's observations on Pulmonaria, 14 April [1864]

Ernst Haeckel's diagram illustrating that he considers Selachius the ancestral form of the fish and hence of all higher vertebrates, 23 March 1868

Roland Trimen on the coloration of moths, 26 March 1868

Adolf Reuter's sketches of Robinia rubra and Pirus malus, 23 September 1869

Alexander Agassiz's description of instances of sexual differences in viviparous fish, [before 1 June 1871]

Fritz Müller's observations on butterflies and termites, 16 January 1872

CD asks G. C. Wallich for the negative of a photograph of smiling girl (enclosed) and permission to publish it in Expression, 24 February [1872]

Gerard Krefft's "Review of Owen's Cuvierian principle of palaeontology", sketches of Thylacoleo carnifex, 15 May 1872

Hubert Airy's description of the curious disposition of leaves on some Acacia twigs, 24 September 1872

Hubert Airy's illustrations of the role of twisting in the development of leaf arrangement in different species of Gasteria, 7 December 1873

F. F. Hallett's rough sketch showing his system of selection, 21 May 1875

J. G. Joyce's report of investigations carried out at Silchester with Frank and Horace [Darwin] on earthworm activity at the site of a Roman villa, 15 November 1877

W. T. Thiselton-Dyer's information on Cyclamen and other plants enclosing a fern specimen, both sides of which are reproduced, 29 January 1878

S. B. J. Skertchly's researches on the age and divisions of the Palaeolithic period, 4 March 1878

C. B. Clarke's dimorphism in Rubiaceae with enclosures containing bud samples, 12 May 1878

G. H. Darwin's drawings of Thalia dealbata requested by CD, 12 July 1878

W. T. Thiselton-Dyer's enclosure specimen of Commelina benghalensis, 30 December 1878

C. A. Lindvall's map of the sandhills in central Sweden, 16 February 1879

C. W. Hamilton's observations [on stem structure?], 21 February 1879

Thomas Comber's specimens showing gynodioecism in Plantago lanceolata, 15 June 1879

Anthony Rich's description of the movement of a caterpillar, 1 July 1879

E. Desrousseaux's theory that all phenomena originate in movement, 30 September 1879

Eduard Schulte's drawing and description of a butterfly discovered in Celebes, 23 October 1879

A. S. Wilson on clubroot fungus of cultivated Cruciferae, 17 February 1880

Volney Rattan's drawings of germination in Megarrhiza californica, enclosed in a letter from Asa Gray, 4 April 1880

Adolf Ernst's sketch of the flower of Lisianthus vasculosus Griseb., 7 August 1880

Fritz Müller on the sleep movements and paraheliotropism of Maranta and other plants and the fertilisation of figs by Hymenoptera, 9 January 1881

CD's instructions to the engraver on illustrations for his paper ["The action of carbonate of ammonia on the roots of certain plants", Collected papers 2: 236-56], 22 February 1882