Lastly, the vast number of existing animal species (about one million) and the small number of major classes (about fifty) and of major phyla or divisions (about ten), could be compared with the vast number of works of literature and the small number of basic themes or plots. All works of literature are variations on a limited number of leitmotivs, derived from man's archetypal experiences and conflicts, but adapted each time to a new environment-the costumes, conventions and language of the period. Not even Shakespeare could invent an original plot. Goethe quoted with approval the Italian dramatist Carlo Gozzi, according to whom there are only thirty-six tragic situations. Goethe himself thought that there were probably even less; but their exact number is a well-kept secret among writers of fiction. A work of literature is constructed out of thematic holons-which, like homologue organs, need not even have a common ancestor.