The genesis of Darwin’s thoughts towards 'evolution' are found in the various notebooks that he began while on the Beagle. They include the 'Transmutation Notebooks', and Notebooks A-E, covering everything from geology, biological issues, and speculations on the relation of human beings to his general evolutionary thinking. On display is a facsimile edition of his 'Red Notebook', started in late spring 1836, which has his early thoughts on transmutation as well as patterns of abrupt replacement of species: 'Speculate on neutral ground of 2. Ostriches...'
The Red Notebook of Charles Darwin. Edited by Sandra Herbert. Ithaca: Cornell University Press, 1980. Science QH 31 D2 D6587
'When we see Avestruz [rheas] two species. Certainly different. Not insensible change. – Yet one is urged to look for common parent? Why should two of the most closely allied species occur in the same country?' So wrote Darwin in his 'Red Notebook'. His observation on these birds in South America was crucial in his realization of the pattern of geographic replacement. This is John Gould’s rendering of 'Darwin's Rhea', a species that was actually given its full scientific name (Rhea pennata) by the French naturalist Alcide d’Orbigny in 1834.
John Gould's Rhea, from The Zoology of the Voyage of the H. M. S. Beagle.
The distillation of the Notebooks and his experiences on the Beagle became evident in Darwin's 1842 Sketch, a 48 paged essay in two parts that help shape the development of his evolutionary theory. Importantly, one chapter is headed 'natural selection', with phrases and jottings such as 'DeCandolle’s war of nature' and 'Nature's variation far less, but such selection far more rigid and scrutinizing.' In 1844, he compiled his Essay, a much longer treatise of 164 printed pages. This is the 1887 reprint, edited by his son Francis.
Life and Letters of Charles Darwin. Vol. II. London: John Murray, 1887. Science Storage Bliss E4 D