Pseudochirus is a well-defined genus of Phalangers, which, up to this time, embraced three species: cookii, bernsteinii and albertisii. Pseudochirus viverrinus Ogilby (1837), from Van Diemen’s Land, is regarded as specifically distinct from Ps. cookii Desmarest (1817), from the Australian Continent. They are also separated by Gould (Mammals of Australia, 1863), who stated that the island examples are of a larger size and always dressed in a softer and longer fur and who said, that for the present he regarded them as distinct. Gould remarked an analogical phenomenon in Trichosurus lemurinus or vulpinus from the Australian Continent and Trichosurus fuliginosus from Van Diemen’s Land. He says „the skins of the Island and Continental Animals are both made into sleeping rugs, but the former are esteemed so much more highly, that a rug formed of them is considered worth three times the price of one of the latter.” A conscientious comparison of the skulls however shows that there is no trace of difference between these parts of the continental and of the island forms, neither in form of skull nor in dentition. In my opinion such facts are proofs to illustrate the great variation of a certain species within certain limits; these variations depend upon external causes, as there are: locality, climate, food a. s. o. The study of such influencing circumstances learns us the laws of nature.