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.
At the end of Chapter 4, Darwin summarizes his argument thus far with an extended simile and a metaphor comparing the evolution of life on earth to the growth of an old tree:
As buds give rise by growth to fresh buds, and these, if vigorous, branch out and overtop on all sides many a feebler branch, so by generation I believe it has been with the great Tree of Life, which fills with its dead and broken branches the crust of the earth, and covers the surface with its ever-branching and beautiful ramifications.
Darwin describes earlier in the passage how early life was like the early stages of tree growth: a single species shot up like a sapling and then branched into a few "budding" species, like little offshoots. Like the new growth on a maturing tree, species have fought one another to establish themselves as "branches" on the metaphorical "Tree of Life." Only some species make it, and often new species beat out older ones. The species that go extinct fall to the ground like "dead and broken branches." Darwin has some fun here with the way his simile slides neatly into metaphor: the "dead and broken branches," he writes, incorporate into the "crust of the earth." Technically, dead tree branches on the ground do decompose into the ground and become part of the very top layer of the earth's crust. What Darwin is getting at, though, is the fossil record. Species that have fallen off the "Tree of Life" have turned into the fossils that humans have unearthed and begun studying as records of natural history. The species still alive today are the living branches and buds on the Tree of Life, but eventually many of them, too, will fall to the ground and turn into fossils.
Darwin's Tree of Life simile is an interesting counterpoint to some of his other similes and metaphors comparing natural selection to war and other forms of fierce and brutal competition. While he does emphasize the way buds and branches are always fighting to "overtop" one another, ultimately the tree is a single organism. Darwin refers elsewhere to the "division of physiological labor," the notion that different kinds of tissue perform different functions within a single organism. The branches of the Tree of Life compete with one another to accomplish their own ends, but each of them is also working to keep the tree itself standing. In fact, the more competition there is, the more the tree flourishes.
In Chapter 6, Darwin addresses several challenges to his theory of natural selection. He uses a simile to explain why his theory is not invalidated by the fact that unrelated animals sometimes share features, like wings:
As two men have sometimes independently hit on the same invention, so in the several foregoing cases it appears that natural selection, working for the good of each being, and taking advantage of all favourable variations, has produced similar organs, as far as function is concerned, in distinct organic beings, which owe none of their structure in common to inheritance from a common progenitor.
If traits are acquired through slow, incremental selection from one generation to the next, Darwin admits that it might seem unlikely that two unrelated species, such as butterflies and bats, would each develop wings. Surely there would be too much divergence in each species' development for such a specific trait to be a natural coincidence—these animals must be created by an intelligent being. But Darwin doesn't think the coincidence is so far-fetched. He compares the situation to that of two men who, without talking to one another, happen to invent the same thing. In the case of the inventors, they are likely inspired by a similar problem. For instance, ancient pyramids can be found all around the world. The most likely explanation is not that all these ancient civilizations were in contact with one another, but rather that pyramids are a strong shape. Many civilizations have independently come up with the pyramid design, and many pyramids have lasted for a long time. Likewise, Darwin argues, many unrelated animals may have developed wings because they all found themselves in circumstances under which wings would be advantageous.
By comparing parallel developments through natural selection to parallel inventions, Darwin once again uses modern society as a reference point for understanding nature. England was industrializing throughout the 19th century. Capitalism was heating up, and the pressure to compete and innovate was intensifying. For Darwin, this feeling of economic competition was a useful way to explain the dynamics he believed existed in nature.
In Chapter 12, Darwin describes many possible ways plant seeds might be dispersed around the world. He uses a simile and imagery as he describes a swarm of locusts that may have introduced invasive plant species to Madeira through seeds in their dung:
The Rev. R. T. Lowe informed Sir C. Lyell that in November, 1844, swarms of locusts visited the island of Madeira. They were in countless numbers, as thick as the flakes of snow in the heaviest snowstorm, and extended upwards as far as could be seen with a telescope. During two or three days they slowly careered round and round in an immense ellipse, at least five or six miles in diameter, and at night alighted on the taller trees, which were completely coated with them.
Darwin does not simply state that a swarm of locusts visited Madeira in 1844. Instead, he impresses upon the reader how awe-inspiring the sight of the locusts was, at least to the man who wrote to Charles Lyell about the swarm. Darwin compares the "thick" swarm of insects to the "flakes of snow in the heaviest snowstorm." This simile not only gives the reader a sense of what the swarm looked like, but also associates the swarming of locusts with big climate events, such as "the heaviest snowstorm." Darwin is trying to suggest a natural interdependence between insects and plants. If a swarm of locusts is as natural and consequential an event as a snowstorm, it follows that locust swarms might be part of the broader story of natural selection and plant species' fight for survival.
The simile fits into the broader set of images Darwin uses to describe the locusts. He claims that the cloud of insects extended upward past what could be seen with the naked eye. He describes the constantly buzzing, "immense ellipse" they formed as they flew overhead for 2 or 3 days, and he describes how they completely encased the tall trees at night. This imagery is not necessarily an exaggeration. Locust swarms really can be classified as a natural disaster in some cases. But the way Darwin pauses over these images is notable given that his book is more scientific and academic than literary. By emphasizing the impressive sight of the locust swarm, Darwin captures his readers' attention and shows them just how overwhelmingly powerful and violent nature can be. Given that his thesis holds that natural forces are entirely responsible for the creation of all simple and complex life forms, these images help Darwin convince his readers that nature is as powerful as it needs to be for his theory to hold true.
