In Chapter 1, Darwin debunks the theory that each variety of domesticated livestock (different breeds of sheep, for example) descends from a different "aboriginal stock," or wild ancestor. He uses verbal irony as he spells out the full implications of this theory:
At this rate there must have existed at least a score of species of wild cattle, as many sheep, and several goats, in Europe alone, and several even within Great Britain.
What Darwin writes is that there "must have existed" a vast number of species of wild cattle, sheep, and goats in Europe and Great Britain before livestock were domesticated. It is fairly apparent that what he really means is the opposite: there is no way there were that many wild species. By writing the opposite of what he means, Darwin gives the reader room to arrive at the same conclusion he does. Instead of simply trusting that Darwin is correct to dismiss the theory of several "aboriginal stock," he helps the reader imagine a world in which that theory is accurate.
Darwin uses verbal irony elsewhere too, but it is usually just as mild as it is here. Whereas many writers use verbal irony sharply to satirize people and institutions, Darwin mainly uses it to help people see that his own theory is more logical than others that have been presented. This more polite use of verbal irony is common in academic writing. It helps Darwin and other writers point out flaws in other researchers' conclusions without lampooning them—after all, the people whose work Darwin is critiquing are some of the same people he is trying to convince with his own argument.
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.
