Kohler, R. E. Subspecies classification and biological survey, 1850s-1930s: Robert E. Kohler. Spaces of classification.(2003).
Lua i Stíl Chicago (17ú heag.)Kohler, Robert E. "Subspecies Classification and Biological Survey, 1850s-1930s: Robert E. Kohler." Spaces of Classification.(2003) .
Lua MLA (8ú heag.)Kohler, Robert E. "Subspecies Classification and Biological Survey, 1850s-1930s: Robert E. Kohler." Spaces of Classification.(2003), .
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