In Chapter 3, Darwin uses personification and a simile to emphasize the incredible power of natural selection:
We have seen that man by selection can certainly produce great results, and can adapt organic beings to his own uses, through the accumulation of slight but useful variations, given to him by the hand of Nature. But Natural Selection, as we shall hereafter see, is a power incessantly ready for action, and is as immeasurably superior to man’s feeble efforts, as the works of Nature are to those of Art.
Darwin begins his entire argument by describing variation in domesticated species. Victorians were very familiar with crossbreeding and the way it was possible to select for specific traits when breeding plants and animals. Darwin then builds on this familiar foundation to argue that the same kind of variation happens in nature. Here, in Chapter 3, he builds to his point that nature selects specific traits to "breed" into species as well. In fact, he claims, natural selection results in species beyond anything humans could dream up.
Darwin begins this passage by personifying nature as a powerful being that "hands" living beings (plants and animals alike) to humans to breed as they see fit. "The hand of Nature" echoes the idea of "the hand of God," shorthand Christians used to describe God's role in nudging the mortal world in one direction or another. It also echoes Adam Smith's theory of the "invisible hand of capitalism," which itself builds on the idea of "the hand of God" to describe capitalism as an intangible force that nudges the free market in certain directions. Especially given Smith's precedent, Darwin is not exactly suggesting that Nature is God. But he is using the indirect comparison to suggest that Nature is a mysterious and powerful force that provides humans with the raw materials they use to create life.
But as Darwin goes on, he claims that what humans create out of these raw materials pales in comparison to what natural selection creates. He writes that the relationship between the two is like the relationship between nature and art: nature can make "works" (e.g. geographical formations) more staggering than human hands ever could. Likewise, natural selection can create species that far surpass the limits of what humans can create or even imagine. This is the core of Darwin's argument. He wants his readers to see that natural selection can, indeed, explain how every life form came to be.
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.
