Charles Darwin wasn't just a writer and a naturalist; he was also the father of ten children. And when those children got a hold of his manuscript for On the Origin of Species, the did what kids do: they drew all over them.
The Cambridge University Library has many of Darwin's papers, including pages from On the Origin of Species that his children used as scrap paper. Darwin himself often used the back side of the paper on his manuscripts for notes, effectively destroying his own manuscript, and let his children draw on the back side of his papers as well. He kept a pile of papers in the cupboard under his stairs for his children to grab as they desired, and at one point, he apparently kept pages from his most famous manuscript there. The Darwin Manuscripts Project has made Darwin's surviving papers available online, including those containing his children's works of art.
Darwin was himself a skilled artist, using his skills in his observations of various species of plants and animals, and if the top image is any indication, it seems that at least one of his children picked up his interest in illustrating the natural world.
On the other hand, some of their drawings were more fanciful, like this one, possibly drawn by Francis Darwin (who would follow in his father's naturalist footsteps), which the Darwin Manuscripts Project has logged as the "Aubergine and Carrot Cavalry":
At least one of these drawings captured something from Darwin's own life. At the Appendix, Benjamin Breen points to a drawing one of the children did of the Darwins' home, which appears to include the sandwalk Darwin liked to tread as he thought. Now, though, the drawings are immortalized not for what they depict, but for what they were drawn upon.