The differences between the most recent complete treatment of this genus by Clausen (1938) and the present revision are, I think, of a rather fundamental nature. For, though Dr Clausen and myself agree that in the troublesome taxa, ‘the small number of characters .... has forced workers .... to base conclusions concerning species often on trivial details such as leaf cutting and size, characters which would not ordinarily be considered of fundamental importance in other groups’ (l.c. p. 5), from there on we have followed a different train of thought. Clausen stated (l.c. p. 6) that ‘If these characters were not adopted as criteria for species, it would be necessary to reduce the species to a very small number and thereby remove the opportunity to keep apart populations which appear to be really distinct enough, but for which the characters available for species differentiation do not seem fundamental.’ This comes very close to Prantl’s critics (1884, 300) on Luerssen’s treatise where he stated: “.... so scheint die Frage, ob sie (i.e. the forms which Luerssen brought together under O. vulgatum) als Vatietäten oder als ebensoviele Arten zu bezeichnen sein, von untergeordneter Bedeutung zu sein Es gibt eine grosse Anzahl von Sammlern, Floristen etc., deren wissenschaftliches Bedürfnis befriedigt ist, wenn sie auf Etiquetten oder in Katalogen einen aus zwei Worten bestehenden Namen schreiben können; auf ‘Varietäten’ wird eine Rücksicht in der Regel nicht genommen.” In order to facilitate geographical studies based on these determinations Prantl then chose to accept a small species concept.