For Francis (1848–1925) and Horace (1851-1928), sons of biologist Charles Darwin, the equivalent material was pages from their father’s manuscript for On the Origin of Species (composed 1858). After all, the book had gone to the printer, and there was no use in wasting that paper.
The picture to the left shows Francis’s watercolor titled (perhaps by cataloguers at Cambridge University since I don’t see a label on it) “Aubergine and Carrot Cavalry.” The rider on the eggplant wears a turban and baggy pants, and bottles hang from his saddle. The carrot-rider in red is clearly on a mission to expand the British Empire. Both men carry pointy swords.
Another page displays bright birds, butterflies, fruits, and a castle on the reverse of a discussion of ants. And there’s a proto-Cubist house seen from both inside and outside, with a cat, gun, and clock.
Though on first glance we might think the boys defaced an important manuscript in the history of world thought, their artwork probably caused those pages to survive. Most of the rest are long gone.
(Thanks to the New York Times ArtsBeat blog for the tip.)