I definitely deny that any pathological process, i.e. any life-process taking place under unfavourable circumstances, is able to call forth qualitatively new formations lying beyond the customary range of forms characteristic of the species. All pathological formations are either degenerations, transformations, or repetitions of typical physiological structures.

'Cellular-Pathologie', Archiv für pathologische Anatomie und Physiologie und für klinische Medizin, 1855, 8, 13–4. Trans. Lelland J. Rather, 'Cellular Pathology', in Disease, Life, and Man: Selected Essays by Rudolf Virchow (1958)