In an article by John Angus Campbell entitled The Invisible Rhetorician: Darwin's "Third Party" Strategy, the author makes the point that rather than respond directly, Darwin habitually had his allies, (particularly his "inner guard" of Joseph Dalton Hooker, Charles Lyell, Asa Gray, and Thomas Henry Huxley) publicly defend the ideas he put forward in Origin - even when he philosophically disagreed with these men privately. As Samuel Butler writes it: "a powerful literary backing at once came forward to support him [Darwin]." Campbell describes Darwin as "a rhetorician in a distinct but complementary sense: his skill as the manager of his own campaign."
Campbell documents Darwin's correspondence with his "inner guard" to solicit their support of his doctrine. He wanted to win them over personally because, for example, "Lyell's public stature was so great that his hesitation would slow general acceptance, while his "conversion" would virtually spell victory." In addition to explicitly directing these four colleagues in how to respond to his troublesome critics, Darwin also had copies of their articles (if complimentary to Origin) republished, at his own expense, and sent to journals, critics, scientists, and clerics to bolster belief in his ideas. In this way, Darwin was able to indulge his "desire for action without compromising his appearance of being above the fray."
Darwin's tactics seem to have been to win detractors over by persuasion rather than combat: "While Darwin himself certainly used battle imagery, for every reference to battle in his letters, there are two to "conversion" ... When we consider that Darwin's "enemies" were also his friends [i.e. orthodox colleagues], even those of his own household [his religious wife], his dominant imagery is consistent with his rhetorical aim." Campbell makes the analogy: "Not Huxley's mythical battle between the scientific David and the religious Goliath, but the subtle diplomacy of Jacob versus Essau prepares us to grasp the kind of tactics Darwin used to convert his culture to his theory."
The article also provides some insight into Darwin's scientific philosophy:
Darwin's empiricism was narratological; that is, he believed that the testable unit in science was less the isolated fact than the unified narrative. In Darwin's view, natural selection should be accepted, not because it could be proven experimentally but because it could bring greater coherence to larger bodies of facts than any other theory then available.
Campbell quotes Darwin writing to Huxley (who "'accepted' Darwin's theory on the caveat that natural selection had not yet been confirmed experimentally") on the topic:
You speak of finding a flaw in my hypothesis, and this shows you do not understand its nature. It is a mere rag of an hypothesis with as many flaws and holes as sound parts. My question is whether the rag is worth anything? I think by careful treatment I can carry in it my fruit to market for a short distance over a gentle road; but I fear you will give the poor rag such a devil of a shake that it will fall all to atoms: and a poor rag is better than nothing.
But even according to his personal & intellectual critic, Samuel Butler, the effect of Darwin's "poor rag" is influential and lasting. He writes that, unlike Darwin's evolutionary theory predecessors, who "did not bring people round to their opinion ... Mr. Darwin and Mr. Wallace did, and the public cannot be expected to look beyond this broad and indisputable fact."Campbell's article shows Darwin's savvy ability not only to successfully write rhetorically in Origin, but also to draw attention and support for his ideas by persuading others to defend them on his behalf - though perhaps this tactic backfired with Samuel Butler, who might have been content to be Darwin's ardent fan, had he early on been offered the personal touch.