NOTES
Reading edition
In the following references, L.L.L. indicates the "Life and Letters of Sir Charles Lyell" by Mrs K. Lyell (1881), D.L.L. the "Life and Letters of Charles Darwin" by F. Darwin (1887), M.L.D. "More Letters of Charles Darwin" edited by F. Darwin and A. C. Seward (1903), and H.C.E. Huxley's "Collected Essays."