In Chapter 5, Darwin insists that horses must all be descended from a striped ancestor because so many of them present with stripes. He uses ethos and logos as he rejects the alternate view that God created each species of horse independently:
To admit this view is, as it seems to me, to reject a real for an unreal, or at least for an unknown, cause. It makes the works of God a mere mockery and deception; I would almost as soon believe the old and ignorant cosmogonists, that fossil shells had never lived, but had been created in stone so as to mock the shells living on the sea-shore.
Darwin knows that he stands the chance of alienating religious readers because he is downplaying God's role in creating life. To bolster his credibility in this passage, he turns himself into God's defender. He then uses logic to back the reader into believing that to be faithful, they must agree with Darwin. Humans know that stripes can be bred into a line of horses. To reject this "known" means of getting a striped horse and instead to believe, with no evidence, that God made sure each and every species crossed with a zebra-like animal at some point is foolish, Darwin claims. It makes God's interventions on earth "a mere mockery and deception." The implication is that busy work like this is beneath God. If people instead believe, as Darwin does, that God created the means for all these species to evolve on their own, God's creative power comes to seem far stronger and more impressive. Readers should thus align themselves with him if they want to believe in a powerful God.
Darwin further tells readers about a debunked theory that fossils were artwork, not geological records of life. The idea that people had created fossils to mimic real shells they saw at the beach was as good an explanation as any for their existence before people knew better, but anyone ascribing to that belief in Darwin's day would be comically out of touch. Darwin compares the theory of individually created species to this quaint theory about fossils, framing it as a similarly outdated explanation. Science, he is implying, progresses over time. This passage suggests that Darwin's theory is the only one that accommodates both updated scientific views and faithful Christianity.
In Chapter 7, Darwin announces that he is about to engage with a "formidable array" of objections that a scientist name St. George Jackson Mivart has made to Darwin's theory. In this passage, Darwin uses verbal irony to develop his own ethos as a responsible scientist and debater:
When thus marshalled, [Mivart's complaints] make a formidable array; and as it forms no part of Mr. Mivart’s plan to give the various facts and considerations opposed to his conclusions, no slight effort of reason and memory is left to the reader, who may wish to weigh the evidence on both sides.
On its face, this passage is respectful and even admiring toward Mivart. Darwin acknowledges the strength of his opponent's argument. He admits that Mivart's argument is so effective that it leaves the reader incapable of considering any other stance—unless, that is, the reader has excellent recall of what they have read elsewhere and is smart enough to reason out a counterargument on their own. Darwin is not outright dismissing Mivart, but he is being ironic. Obliquely, he is pointing out that Mivart has not walked his reader through the whole field of evidence. Failure to engage with opposing evidence or counterarguments is in fact the hallmark of a weak argument. If Mivart had helped his reader "weigh the evidence on both sides" and had demonstrated that his argument was still stronger than Darwin's, he may have been said to win the debate. Instead, he leaves room for readers to come to their own conclusions and for Darwin to be the one to show the full picture.
What Darwin goes on to do is exactly what he at first "compliments" Mivart for not doing: in the rest of the chapter, he engages fully with each one of Mivart's points and with his other critics' points. Darwin does not leave any room for people to accuse him of failing to address objections to his theory. He comes off as a scientist who cares more about getting his theory right than simply being right. Whereas Mivart appears to have dismissed Darwin's theory without fully considering it, Darwin demonstrates himself to be a deeply principled debater who is determined to rise above and take his opponents seriously.
