Conduct of Debate
Darwin usually avoided public controversy, and he is sometimes thought to have withheld his views on religion or human nature because he feared adverse public reaction. This exchange of letters with the zoologist Ernst Haeckel, an ardent proponent of Darwinism and a stern critic of religious authority in Germany, shows that Darwin had strong reservations about the value of polemical debate in science, and a deep optimism that the truth would eventually prevail without such aggressive tactics.
Letter 5500 — Darwin, C. R. to Haeckel, E. P. A., 12 Apr 
Darwin is sympathetic to Haeckel’s position, and is struck by singular clarity of his Generelle Morphologie. However, he is concerned that the remarks on various authors seem too severe. He believes that severity leads the reader to take the side of the attacked person.
Letter 5533 — Haeckel, E. P. A. to Darwin, C. R., 12 May 1867
Haeckel thanks Darwin for the new edition of Origin [4th ed. (1866)]. He comments on Darwin’s criticism of the harsh tone of Generelle Morphologie. He may have harmed himself, but does not believe he has harmed his cause. He believes a radical reform of the science necessary. Since most scientists take a prejudiced view of the matter, a vigorous attack is essential.
Letter 5544 — Darwin, C. R. to Haeckel, E. P. A., 21 May 
Darwin discusses his previous criticisms of Haeckel’s Generelle Morphologie. He fears Haeckel will make enemies. He discusses reception of descent theory in England.