Illustration emerges from complex and diverse motives. The portrayal of an objective reality may seem to lie at its heart, but there may be other, subtle factors at work. Preconception, for example, guides many an illustrator’s hand. A wish to project known realities onto nascent concepts distorts reality in its own ways, and the process of transmuting the subtle realism of Nature into an engraver’s line imposes constraints and conventions of its own.

There is a general principle in artwork, often unrecognized: the culture of each era dictates its own arbitrary realities. Our experience of this is largely intuitive, but it explains why we can relate a specific image (a saint from a thirteenth-century psalter or the countenance of the Statue of Liberty) more easily to the time it was produced than to the identity of the artist or the name of the subject. In just this way, a scientific illustration is a mirror of contemporaneous preoccupations and a clue to current prejudice. It is more than a didactic symbol. Some illustrations create, and then perpetuate, icons that transcend reality and provide a synthesized convention that passes from one generation of books to the next. These icons are created for textbooks, and they populate their pages as decorative features that do little to reveal reality.

Early in the century, François Legaut’s Voyages et Aventures (1708) featured a rhinoceros with a second horn projecting forward from its brow. This structure is never found in life. Why should it be featured in an eighteenth-century illustrated textbook? The first published study of a rhinoceros (made by Albrecht Dürer in 1515), although powerful and otherwise realistic, boasts a small secondary horn on the shoulders, which projects forward. The image was repeatedly plagiarized and – with each generation of copying – this imaginary forward-projecting second horn increased in size. By the time it was included in Legaut’s book, the imaginary horn was equal in size to the real one.