These letters, mostly in German, refer to evolution, Darwin, monism, phylogeny, radiolaria, morphology, and anatomy, and other scientific work of Haeckel and his correspondents.

Includes correspondence, autobiographical notes, addresses, articles, and drawings. The collection reflects Riddle's interests in breeding, heredity, and evolution. He was interested in humanism and the presentation of biological and evolutionary theories. There is much information on Riddle's activities in favor of freedom of scientific teaching, especially in high schools.