Darwin was not the first to challenge theological/natural design, nor is it clear that by challenging design, he provided a position completely incompatible with all forms of natural theology popular at the time. The following three sets of letters offer different perspectives on the controversial topic of design. The first is between Darwin and Harvard botanist Asa Gray, taking as their point of departure reviews of Origin. The second is a single letter from naturalist A. R. Wallace to Darwin on design and natural selection. The third is a single letter from Darwin to philosopher and economist William Graham on natural laws.
Darwin and Gray
Letter 2814 — Darwin, C. R. to Gray, Asa, 22 May 
Darwin writes to Gray about the opinions and reviews of Origin. He also shares his view on design in nature. Although he does not believe in the necessity of design, he finds it hard to conclude that everything is the result of “brute force”.
Letter 2855 — Darwin, C. R. to Gray, Asa, 3 July 
Darwin writes to Gray and tells him Origin has “stirred up the mud with a vengeance”; Gray and three or four others have saved him from annihilation and are responsible for the attention now given to the subject. He poses Gray a question on design in nature, as he is in a “muddle” on this issue.
Letter 3256 — Darwin, C. R. to Gray, Asa, 17 Sept 
Darwin writes to Gray about botany and his botanical experiments. He also discusses his views on design. He shares a witty thought experiment about an angel.
Letter 3342 — Darwin, C. R. to Gray, Asa, 11 Dec 
Darwin writes to Gray about politics and his forthcoming botanical papers. He says he is in a “thick mud” regarding design in nature, and more inclined to “show a white flag than to fire my usual long-range shot”. He asks Gray some questions about design.
Letter 6167 — Darwin, C. R. to Gray, Asa, 8 May 
Darwin writes to Gray about his review of Variation [Nation 6 (1868): 234–6] which he thinks is very good. Darwin also thinks he gives an “excellent idea of Pangenesis”. He talks about Gray giving him a good slap at his concluding paragraph, where he ought to have brought in and contrasted natural and artificial selection. He says that it seemed so obvious that natural selection depended on contingencies even more complex than those which must have “determined the shape of each fragment at the base of my precipice”.
Darwin and Wallace
Letter 5140 — Wallace, A. R. to Darwin, C. R., 2 July 1866
Wallace writes a lengthy analysis of sources of misunderstanding of natural selection. He worries about the accusation in Darwin & his teachings “Natural Selection” requires the constant watching of an intelligent ‘chooser’ like man’s selection to which you so often compare it.” He discusses the advocacy of Spencer’s term “survival of the fittest” instead of “Natural Selection”. Wallace urges Darwin to stress frequency of variations.
Darwin and Graham
Letter 13230 — Darwin, C. R. to Graham, William, 3 July 1881
Darwin praises Graham’s Creed of science, but disagrees that the existence of natural laws implies purpose. His “inmost conviction” is that “the Universe is not the result of chance” but has horrid doubt whether convictions of man’s mind, which has been developed from lower animals, are at all trustworthy. He also believes natural selection is doing more for progress of civilisation than Graham admits.