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Chapter 3
Themes
Key
LitCharts assigns a color and icon to each theme in The Origin of Species, which you can use to track the themes throughout the work.
Natural Selection and the Power of Nature
Reason, Argument, and the Scientific Method
Time and Progress
Collaboration and Science
Summary
Analysis
Darwin asked rhetorically how the struggle for existence could lead to variation. He suggested that because humans had so much evidence of how it was possible to intentionally breed animals for human use, it should be no surprise that similarly helpful variations might arise in a wild population over many generations. Similarly, unhelpful variations would quickly die out. This is what Darwin called natural selection or survival of the fittest.
Now that Darwin has laid the important groundwork for his theories, he transitions into what natural selection actually is. Crucially, natural selection is different from what happens during breeding under domestication, but Darwin nevertheless continues to use domestication to help illuminate what happens in the wild.
Themes
Some writers misunderstood the concept of natural selection. They suggested that natural selection creates variations, but in fact, it just preserves variations, which is an important distinction. Darwin admitted that he found it difficult to avoid personifying the word “Nature” and treating it like an entity, but in fact, he believed that nature was simply an aggregation of laws and events.
Here, and elsewhere, Darwin is careful to avoid creationist language that might suggest that an active divine presence is guiding evolution. Darwin was writing for both a secular and a religious audience, and he carefully chose his words to make his ideas palatable to a wide spectrum within both groups.
Themes
Literary Devices
Darwin argued that if humans could produce such amazing results by breeding domesticated plants and animals, natural selection would produce even more stunning variations, since unlike humans, nature does not have to work with external, visible characteristics. Similarly, humans are limited in the way they create variations in plants and livestock, often staying within the same regions and climates and breeding them for specific purposes, but the struggle to exist in nature is more open-ended.
The key feature of natural selection is that helpful modifications to organisms must not only arise, but also have a way of being preserved and passed on to offspring. Even matters that seem insignificant to humans may prove significant in the natural selection process, such as how leaf-eating insects are green, giving them camouflage.
Darwin further explores the theme of how human perception is limited, and so as a result, human breeders are limited in what they can accomplish with domesticated plants and animals.
Themes
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Though natural selection works to the advantage of individual species, it can have advantages that go beyond an individual organism: for example, natural selection can affect a whole community of social animals. What natural selection can’t do, however, is preserve a modification in one species that gives an advantage to a second species—at least not unless there is also some benefit for the first species.
Darwin continues the theme that adaptations don’t occur in a vacuum. While he shows how adaptations can have effects beyond a single organism, he makes the important distinction that because different species are competing to exist, one species will never develop an adaptation that is solely of benefit to another species.
Themes
Natural selection is not a perfectly orderly process, and sometimes organisms that would have had advantages are destroyed through some accidental means. In such cases, natural selection might seem powerless, but these exceptions don’t negate the fact that natural selection is very powerful under other circumstances.
Darwin considers the role that probability plays in natural selection and gives an example of how an organism that is theoretically better adapted could nevertheless fail to pass on its useful adaptations to offspring due to a freak accident.
Themes
Sexual Selection. Darwin wrote how, as part of natural selection, the sexes of the species may be modified in relation to each other. Darwin called this sexual selection, by which he meant the struggle of “one sex, generally the males, for the possession of the other sex.” Some species of males, particularly carnivores, will literally fight each other for females. Others, like birds, have more peaceful competitions, but they nevertheless compete by performing strange ceremonies to show off.
Here, Darwin expands the idea of what competition among individuals means. He has already clarified that competition can be metaphorical and not necessarily physical. In this passage, Darwin emphasizes, through the concept of sexual selection, that much of natural selection revolves around reproduction.
Themes
Darwin concluded that if males and females of a species have the same habits but differ in form or external appearance, the main cause of the difference is usually sexual selection. Sexual selection can involve giving organisms ways to attack, ways to defend, or even simply ways to charm.
Though the struggle for food is important, ultimately organisms are struggling to pass on their traits to offspring. Sexual selection shows the important role of reproduction in the process of survival of the fittest.
Themes
Illustrations of the Action of Natural Selection, or the Survival of the Fittest. Darwin suggested illustrating his points with some imaginary examples. He imagined wolves, where some of the wolves have to be slim enough to chase after deer but still muscular enough to overpower their prey, while other wolves have to be stockier but don’t need to be as fast to hunt sheep. Over time, wolves with one of these advantages would tend to out-survive others and through chance would pass some of their traits on to offspring.
Because real populations can be difficult to track and study over time, sometimes Darwin uses hypothetical examples in order to demonstrate certain principles. Wolves make an obvious choice for an example, since they resemble domestic dogs, whose breeding has been well-studied. Here, Darwin shows how a population can diverge.
Themes
In the case of wolves or other animals, the different regions where the different varieties live are often what keep them from intermixing with others of their species (which might result in the varieties not being passed on).
Geography and environment continue to play a significant role in natural selection.
Themes
Darwin put forward another example of plants that put out nectar, which attracts insects. While these visits from insects don’t benefit the individual plant, they help spread pollen, crossing different individual flowers of the same species. Because the plants with the most nectar would be visited the most frequently and therefore spread their pollen most widely, these plants tended to survive in populations.
Darwin’s hypothetical example with plants is more or less based on real life, but he has simplified the process in order to make the principles simpler to understand. He uses this case to show how relationships between different species can play a role in natural selection.
Themes
From the insects’ perspective, some individuals may also have variations that help them get more nectar. For example, some bees might have a proboscis shaped to give it a slight advantage in getting nectar, even if the change is too small for humans to perceive.
Crucially, the plants do not adapt specifically for the insects—the plants adapt for their own benefit, and the fact that some insects also benefit is simply a side effect, at least from the plants’ perspective.
Themes
Darwin ended the section by admitting that some might object to his ideas, but that his work was like that of the renowned geologist Charles Lyell, whose ideas inspired Darwin and fundamentally changed the way that people in the field of geology thought.
In addition to being Darwin’s friend, Lyell was well-respected in his field, and Darwin appeals to Lyell’s authority in order to hopefully generate more respect for his own work.
Themes
On the Intercrossing of Individuals. According to Darwin, with some exceptions (like parthenogenesis), animals and plants have different sexes, and an individual of each sex is required to produce offspring. Even in the case of hermaphrodites, Darwin noted that reproduction in animals usually involved two individuals that pair (although there remain some exceptions that do not pair). Among plants, the majority are hermaphrodites.
Parthenogenesis is when an organism reproduces without being fertilized (and is a type of asexual reproduction). Some hermaphrodite plants can reproduce by combining their own male and female parts, but Darwin notes that it is still more common for a plant to be fertilized by a different plant. He explores the reasons for this in the following sections.
Themes
Darwin found that with plants and animals, interbreeding helped produce more vigorous and fertile offspring, except for very close interbreeding, which had the opposite effect. With plants, certain varieties have pollen that are more potent than other varieties, but the pollen of a plant’s native species takes precedence over pollen from other species.
It was common knowledge in Darwin’s time that inbreeding of plants and animals can lead to unusual and potentially harmful characteristics in the offspring. By contrast, wider interbreeding can add diversity and lead to healthier offspring.
Themes
Some animal species, like earthworms, are hermaphrodites that pair to reproduce. However, Darwin wasn’t aware of any terrestrial animals that could fertilize themselves. Many aquatic animals self-fertilize, although this may be because the currents of the water create the opportunity for crossings.
Darwin was fascinated by worms and would write about them in greater depth in one of his later books. In this passage, he notes how reproduction in animals is usually (although not always) quite different than reproduction in plants.
Themes
Circumstances favourable for the production of new forms through Natural Selection. Darwin noted that variability in a population was always favorable, since it offers increased probability for helpful variations to arise and eventually be preserved. It might seem that free crossing between animals would get rid of the effects of natural selection, since it might cause traits to move toward the average instead of toward new varieties. In fact, however, there are many factors like geographic isolation or even different niches within the same environment that keep varieties of animals distinct from each other.
Darwin explains why free interbreeding results in greater variety rather than simply smoothing all of a population toward the average. As is often the case, the crucial factors are geography and environment. A diversity of environments means that beneficial adaptations for a species won’t be uniform and that a variation that is beneficial in one environment could be detrimental in another.
Themes
Isolation is an important condition for allowing new varieties to arise through natural selection. The passage of time itself is not enough to create natural selection; it is only important because natural selection can be a slow process, and more time gives more opportunity for variations to arise and be preserved.
Darwin’s ideas about long time scales are influenced by his readings of Charles Lyell’s books about geology and the long time periods involved in that field. Like the movements of the earth, changes in species also happen gradually.
Themes
Extinction caused by Natural Selection. As better adapted organisms grow in number, less well-adapted organisms become rarer, and this leads to the possibility of extinction. New varieties and species tend to put the most pressure on their closest relatives, often driving them to extinction.
Though natural selection is about organisms becoming better adapted to their environments, one of the unavoidable consequences of this process is that less-well-adapted organisms will tend to die off, often eventually becoming extinct.
Themes
Divergence of Character. Darwin believed that varieties were “incipient species” and that they were often in the process of becoming different species. In small areas with a lot of competition for resources, there is often a lot of diversity, coming from different genera and orders. This advantage of diversity is why foreign plants sometimes thrive compared to indigenous plants when they are introduced in a new country.
This section is why Darwin made the point earlier that the distinction between varieties and species is somewhat arbitrary. Under his theory, varieties can eventually become species, and this explains why it is sometimes difficult to draw the line between them.
Themes
The Probable Effects of the Action of Natural Selection through Divergence of Character and Extinction, on the Descendants of a Common Ancestor. The descendants of a species will be most adapted to survive if they become more diverse. In this way, the descendants of a species can be plotted as a sort of “tree” with lines branching out, some of which end (in extinction) and others of which continue branching off.
The tree image that Darwin describes helps to illustrate both how life becomes more diverse over time but also how not all of that diversity survives into the present (because of many extinctions). It provides a visual representation of an idea Darwin explores in greater detail later: how diverse modern species and varieties are often descended from only a couple of progenitors.
Themes
Darwin admitted that in the real world, natural selection probably doesn’t proceed as methodically as in his tree diagram. Nevertheless, the diagram shows important ideas, like how extinction plays a role in natural selection. It also shows how, over generations, one species tends to become many different descendant species, and sometimes branches become so separated that they become different genera or even families.
Like many scientific diagrams, Darwin’s tree of life is a simplification of messier ideas. Nevertheless, it provides a clear demonstration of how seemingly unrelated species may nevertheless share important connections, particularly if you look back far enough in evolutionary history.
Themes
Though it might seem that this means the future will have more diversity and the past had less diversity, because of the role extinction plays, it’s possible that diversity in both periods would be similar to the present.
Darwin explains how extinction and diversity maintain a careful balance and how the future outcomes of natural selection are not inevitable based on the past.
Themes
On the Degree to which Organisation tends to advance. Because natural selection preserves positive traits, organisms tend to become better adapted to their conditions. This means natural selection tends to lead toward organization, such as the development of specialized, interlocking systems of organs within a creature.
One of the challenges from critics that Darwin addresses later in this book is how complex organs like the eye could have arisen through natural selection. Here, Darwin lays the groundwork for arguments that he’ll explore in greater detail later.
Themes
As Darwin wrote, however, this seems to present a problem: if natural selection leads to organization, why do simple organisms still exist? He argued that this wasn’t actually a problem: if there is no advantage for an earthworm to have a more complex system of organs, then it will not be naturally selected. All life would have originally started out as simple unicellular organisms before new variations arose.
Though natural selection leads toward helpful adaptations in species, Darwin stresses that this does not necessarily mean greater complexity. He notes that for many organisms, more complexity would have no benefit and in fact might even be detrimental.
Themes
Convergence of Character. One of Darwin’s critics believed that he overestimated the importance of divergence between species. This critic believed that Darwin didn’t put enough emphasis on convergence, and that creatures of different species might actually come to resemble each other over time and go from two genera to one genus. Darwin believed this view was unlikely—that if species did converge like this, they would be vulnerable to extinction.
While it is easy to dismiss Darwin’s critics today, it is important to remember that his theories were competing against many others, some of which were advanced by eminent naturalists. The idea that species could converge might seem just as feasible as the idea of divergence to some observers, but Darwin correctly points out that such a convergence would provide little advantage under natural selection.
Themes
Summary of Chapter. Darwin repeated some of the main ideas from the chapter: how helpful variations arise in nature and are preserved; how sexual selection plays a role; how extinction acts as part of natural selection; how widely diffused species show more variation; and how the process of new species descending from ancestors can be diagrammed as a tree.
With this chapter, Darwin has finished his broad overview of natural selection. Though there is more detail to explain in future chapters, he pauses here to review what he covered previously, since this information is the foundation of all that comes after it in the book.
Themes
Literary Devices
