Writing rapidly in pencil in 1842, Charles Darwin produced a sketch of ideas that would grow to become his Origin of Species. Much that would revolutionize our understanding of biology was already present, not least his conclusion that ‘specific forms are not immutable’. In this paper, I consider how that mutability bears upon the theological conviction that every creature is related to God as a likeness to its exemplar, drawing particularly on the work of Thomas Aquinas. It is clear from a letter, dated 11 January 1844, that Darwin saw his insight to be a disruptive one, writing to his friend Joseph Dalton Hooker that ‘I am almost convinced (quite contrary to opinion I started with) that species are not (it is like confessing a murder) immutable.’