I demand of you, and of the whole world, that you show me a generic character—one that is according to generally accepted principles of classification, by which to distinguish between Man and Ape. I myself most assuredly know of none.... But, if I had called man an ape, or vice versa, I should have fallen under the ban of all the ecclesiastics. It may be that as a naturalist I ought to have done so.

Letter to J. G. Gmelin (1747) as quoted by Jeffrey H. Schwartz, Sudden Origins: Fossils, Genes, and the Emergence of Species (1999)