More than 70 years have elapsed, since, in 1866, de Bary enunciated the hypothesis that Lichens are dual organisms, the socalled gonidia being Algae. As about 1899, the year when Nylander died, the dual nature of the Lichens had become generally accepted, lichenologists have had 40 years to realize the consequences of this theory. Nevertheless even now opinions differ widely. While practically all botanists admit that a Lichen is composed of a Fungus and an Alga, most lichen-taxonomists apply, perhaps for a good deal unintentionally, the species-name to the consortium, while others emphasize the necessity of restricting the specific and generic names to the Fungus. Reinke, Wainio, Zahlbruckner and Keissler may be regarded as adherents of the first procedure; Sernander, Fink, Clements and Nannfeldt of the second. While the attitude of Zahlbruckner c.s. is perhaps largely due to practical considerations, Asahina recently advocates this conception as a logical consequence of the dual nature of Lichens: “Aus der dualistischen Natur der Flechten muss man aber eine Flechten-Art A (Pilz) + B (Alge) als verschieden von der A+B’ betrachten”. But, in my opinion, the very fact of the dual nature of Lichens leads to the conclusion that a Lichen is no more a species than a plum pocket is one. In general, as stated above, it seems to me that the acknowledgement of the consortium as specific is semiconscious, and rests on practical reasons. Nannfeldt remarks that Werner, for instance, has used specific names as well for the Lichen as for the Fungous component. In many cases, it must be admitted, the procedure though arbitrary, is perfectly harmless. Lichen taxonomy is obliged to use other methods than taxonomic mycology, the vegetative thallus being often as important a feature as the fructification: and with the thallus, one naturally describes the gonidia, though for practical reasons the description remains as a rule incomplete. If the Fungus is strictly monophagous, the presence of a special gonidial partner may be regarded as a character of the Fungus. In many cases, however, especially in groups where the consortium bears a primitive character, difficulties arise. The latter regard not only the delimitation of the species, but their place in the classification as well. There are numerous instances of Fungi living either without or with gonidia, or with different kinds of gonidia. As long ago as 1866, Fries remarked that it would be unnatural to bring Peltigera aphthosa and P. malacea or Pannaria brunnea and P. hypnorum in different genera. The same holds good, for instance, for Peltigera canina and P. variolosa. Reinke argues that the species containing Cyanophyceae and those containing Chlorophyceae might have developped independently, and placed in all these cases the second species in a different genus. But if this procedure is applied, it strikes one that almost or perhaps quite identical species are divided indescriminately over both groups. It appears, for instance, that even the subdivision of Peltigera in Peltidea and Eupeltigera can not be accepted. In such derived groups as Peltigera it happens but rarely that in one species widely different gonidia are met with, and the presence of Algae belonging to the same genus can only be demonstrated by cultivating them. In this connection the forms of Parmelia caperata (Jaag) and of Xanthoria parietina (Waren) must be mentioned. Asahina has suggested that chemical differences in morphologically identical Lichens might be due to physiological differences in the gonidia, but, as Thomas has demonstrated now that parietin, one of the substances which have always been considered as specific for definite consortia, is produced in pure cultures by the Fungi Caloplaca murorum and C. elegans, this hypothesis seems rather doubtful.