Columbia subject headings:
Biologists.
Deutscher Monistenbund.
Germany.
Monism--History.
Zoologists.
Harvard subject headings:
Biologists--Germany--Correspondence.
Chemists--Germany--Correspondence.
Deutscher Monistenbund--History.
Deutscher Monistenbund.
Haeckel, Ernst Heinrich Philipp August, 1834-1919--Correspondence.
Haeckel, Ernst, 1834-1919--Correspondence.
Monism--History.
Naturalists--Germany--Correspondence.
Ostwald, Wilhelm, 1853-1932--Correspondence.
Zoologists--Germany--Correspondence.
Princeton subject headings:
Biologists--Germany--Correspondence.
Chemists--Germany--Correspondence.
Deutscher Monistenbund--History.
Haeckel, Ernst, 1834-1919--Correspondence.
Ostwald, Wilhelm, 1853-1932--Correspondence.
Zoologists--Germany--Correspondence.