Leonardo da Vinci:

 A Grotesque Head (c.1503-07)
(Black Chalk - Guise Bequest 1765)

Although this drawing is thought to date from somewhat later, it seems to be the culmination of this series of studies. Some scholars have suggested this sheet might be "Scaramuccia, king of the gypsies" ; others think it was a cartoon used for the head of an onlooker in a now lost painting of the Mocking of Christ.

Regardless of the interpretation, the drawing demonstrates Leonardo's dedication to the scientific study of nature in all its forms, even its most unattractive.