In Chapter 4, Darwin comments on the way he personifies "Nature" in his writing, using logos to defend his choice:
It has been said that I speak of natural selection as an active power or Deity; but who objects to an author speaking of the attraction of gravity as ruling the movements of the planets? Every one knows what is meant and is implied by such metaphorical expressions; and they are almost necessary for brevity. So again it is difficult to avoid personifying the word Nature; but I mean by Nature, only the aggregate action and product of many natural laws, and by laws the sequence of events as ascertained by us.
Darwin is addressing complaints that he is treating natural selection as though it is some new force that supplants God as the creator of the universe. He admits that he writes about it as if it has the kind of volition and power God has. However, he draws a distinction between literal and figurative language. He does not literally mean that Nature is God-like. Rather, he writes, he personifies it so that he can speak about the "aggregate and product of many natural laws" under the umbrella of his unified theory about the way life has evolved on earth. To defend his choice to use this figurative language, he reminds readers that other scientific writers personify forces of nature all the time. For instance, it is common to read that gravity "rules" planetary movements. By invoking this norm within scientific writing, Darwin aims to convince his readers not to read too much into his own personification of nature.
While Darwin's logic holds true here, it is also important to note that his personification often serves to dramatize nature and the process of natural selection. He frequently describes nature using comparisons to human dynamics and institutions. For example, he refers to the "physiological division of labor," a metaphor many scientists used at this time. By invoking one of the core principles of capitalism, this concept aims to explain how different tissues in an organism take care of different functions the organism needs to stay alive. Humans invented capitalism long after complex life forms emerged on earth, so it is somewhat odd to describe biological functions in terms of capitalism. Darwin's frequent use of personification and other figurative language invoking human society helps him persuade readers who are compelled by stories of human conflict and triumph. Furthermore, it makes for a good narrative.
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.
