Williams, D W (2007) Otto Kleinschmidt (1870-1954), biogeography and the ‘origin’ of species: From Formenkreis to progression rule. Biogeografía 1:3-9 PDFArkivigite je 2009-09-28 per la retarkivo Wayback Machine
unm.edu
elibrary.unm.edu
Stresemann, Erwin (1936) The Formenkreis-Theory. Auk 53(2):150-158 PDFArkivigite je 2011-06-07 per la retarkivo Wayback Machine
web.archive.org
Stresemann, Erwin (1936) The Formenkreis-Theory. Auk 53(2):150-158 PDFArkivigite je 2011-06-07 per la retarkivo Wayback Machine
Williams, D W (2007) Otto Kleinschmidt (1870-1954), biogeography and the ‘origin’ of species: From Formenkreis to progression rule. Biogeografía 1:3-9 PDFArkivigite je 2009-09-28 per la retarkivo Wayback Machine