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Chapter 5
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 acknowledged that his readers might have objections at this point in the book and stated that he wanted to address them. He identified four main objections: 1. If species all descend through gradual variation, where is the record of all the transitional forms? 2. How could an animal currently living (like the bat) have been formed from another animal with such different habits? 3. Can habits and instincts really be inherited through natural selection? 4. How come when species are crossed the offspring are sterile, but when varieties are crossed, the offspring are still fertile?
Darwin spends a large portion of the book responding to the objections of his critics, sometimes even addressing them by name. While Darwin’s narrator persona remains amiable, the sheer amount of space that Darwin spends responding to critics suggests that his ideas faced a lot of criticism but that Darwin nevertheless believed that his arguments were enough to overcome this criticism.
Themes
On the Absence or Rarity of Transitional Varieties. Darwin addressed the first objection by noting that the geological record is imperfect—perhaps even more so than some of his colleagues believed. More challenging to Darwin was the question of why intermediate forms seem to be so rare among currently existing species.
Darwin tries to base his arguments on solid evidence, which is why the imperfect nature of the geological record makes things difficult for him—the conditions that cause a specimen to be preserved as a fossil are specific and sometimes haphazard. But while Darwin often admits to the limits of the available data, he still argues that nevertheless his theories make more sense based on the available data than his opponents’ theories do.
Themes
Literary Devices
Darwin observed in his travels that species were generally limited to inhabiting a continuous area. He found that in general, whenever two similar varieties of a species range over large, continuous areas, there is a third variety that exists in a small intermediate zone between them. The small population size means that the varieties in this zone are much rarer and more prone to extinction. This explains why species are relatively well-defined instead of being a chaotic mix of intermediate forms.
Once again, geography and environment turn out to play an essential role in the development of species. Because of the aforementioned gaps in the geological record, Darwin is forced to make observations based on available data, including observations about modern species distributions, and he uses this information to make logical inferences that are sometimes surprising but always supported by evidence.
Themes
On the Origin and Transition of Organic Beings with peculiar Habits and Structure. Darwin noted that some of his opponents questioned how it was possible that a terrestrial carnivore could have descended from an aquatic one. Darwin admitted it was a difficult problem and that he had only collected a few specimens that seemed to show transitional species of this sort. Bats presented a similar problem.
Explaining how aquatic species became terrestrial species has long been one of the greatest challenges for theories of evolution, and can be difficult to understand intuitively. Darwin does not dismiss the issue—in fact, he admits that it is an objection worth serious consideration.
Themes
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Nevertheless, however, Darwin believed that modern creatures like flying squirrels provided an example of what a transitional species between bats and other mammals might look like. He also noted that many crustaceans and mollusks can breathe underwater but are adapted to live on land.
As with all the objections Darwin considers, however, there is a simple answer. In fact, there are many currently existing species that live a hybrid life between land and sea or land and air. This shows Darwin’s skill at crafting arguments—he anticipates or addresses objections specifically so that he can reveal how his own theories are better.
Themes
Organs of extreme Perfection and Complication. Darwin confessed that it seemed at first absurd to him that something as complex as the eye could have arisen through natural selection. Nevertheless, he believed that by following reason, it was possible to imagine the gradual development of the eye over time, with early imperfect versions being subtly improved upon over generations.
Darwin’s critics focused on the development of the eye, because it is a complex, highly organized organ with a lot of sub-parts. To them, it seemed impossible that such a complex organ could have arisen part-by-part—it must have been created all at once (and many of them believed it was God who did this creation). While Darwin is careful not to antagonize religious readers, he strongly believes that no part of an organism is created all at once—natural selection is always an accumulation of small, gradual changes.
Themes
Because the imperfections of the geological record make it impossible to look back at a species’ descent in a straight line, Darwin believed that explaining the eye involved looking at different species and genera within a larger group. He found that some species have a very simple organ that could nevertheless be called an eye, with just an optic nerve, some pigment-cells, and some translucent skin around it. Other species have slightly more complex eyes, incorporating pupils but still lacking complex features like lenses.
When Darwin can’t draw on the geological record for evidence, he often looks to the wide variety of life that exists in the present. While the species of the present aren’t the same as the species that existed in the past, they can still offer hints about how things used to be, and some current species have descended in more or less the same form for thousands and thousands of years.
Themes
Ultimately, Darwin observed a wide range of eyes and believed these were evidence that the human eye could have been perfected through gradual adaptations over millions of years.
The long time period is important for Darwin’s theory—such small changes need long periods of time in order to be effective. Reading about the geological work of Charles Lyell is part of what encouraged Darwin to think about such long periods of time.
Themes
Modes of Transition. Darwin believed that if there existed an organ so complex that it couldn’t have been formed through a process of slight, successive variations, it would break his whole theory; he also believed there was no solid evidence that any such organ existed. While human organs may seem complex, in many cases they have analogous forms in less complex animals that hint at what transitional organs may have looked like.
Darwin had ambitious goals for his book and aimed to develop a theory that would cover how virtually all species came to be in their present forms. He offers opponents a chance to prove him wrong, but ultimately, he shows that natural selection is such a powerful, flexible process that it would be difficult, if not impossible, to prove that any organ was truly too complex for natural selection.
Themes
Special Difficulties of the Theory of Natural Selection. Darwin addressed some of the most serious criticisms of his theory of natural selection. Neuter insects were a strange case that Darwin promised to explore in the next chapter. The electric organs of some fish also seemed odd, but Darwin argued that the real difficulty here was that so little was known about these organs to begin with, making it difficult to infer how they might have developed gradually. Luminous organs in insects presented a similar issue.
Even for a theory as comprehensive as natural selection, however, there are some odd examples from nature that, on the surface, seem difficult to account for. As always, Darwin adopts a reasonable tone and never dismisses objections before considering whatever possible value the objections might have.
Themes
Darwin noted that with electric organs and luminosity, a similar function was performed by organs that might have developed in very different ways. The wings of a bird, a bat, and a butterfly are similar in some ways but also very differently constructed, suggesting a different development process. Darwin summed up this situation by quoting another naturalist who suggested that Nature presented a lot of variety but not much actual novelty. He reiterated his belief in natural selection and how it acted only in small steps, never taking big, sudden leaps.
Ultimately, Darwin argues that while electricity or luminosity seem like extraordinary developments on the surface, a closer inspection reveals that these traits are just like any other trait that arises through natural selection. When Darwin says that nature has variety but not novelty, he means that things in nature that look very different often serve purposes that, in the end, are quite similar.
Themes
Literary Devices
Organs of little apparent Importance, as affected by Natural Selection. Darwin found difficulty explaining how natural selection was involved with parts and characteristics of creatures that seemed to have little importance. He cautioned, however, that it was not always easy to tell the difference between an unimportant part and one that only appears that way. It’s also possible that today’s unimportant organs once played a very important role in a progenitor species.
Darwin has touched on the issue of seemingly useless body parts earlier in the book and explained it through the theory of economy of growth. His answer here shows his cautious approach to making sweeping statements. His reluctance to declare part of an organism useless demonstrates that he is methodical and always careful to avoid making statements that he doesn’t necessarily have the evidence to prove.
Themes
Utilitarian Doctrine, how far true: Beauty, how acquired. Some naturalists protested against Darwin by arguing that some beautiful things in nature seem to have been created for the pleasure of man or a Creator. Darwin argued, however, that beauty is subjective and is not necessarily innate or unchangeable. While he admitted that beauty certainly played a role in sexual selection, he believed that other types of beauty may have simply arisen as a result of other forces like symmetry of growth.
The arguments of Darwin’s opponents about beauty here are maybe not the most serious from a scientific standpoint, but Darwin often uses grand language about the beauty of nature, and the idea that evolution only enhances the wonder of nature is central to his argument. In particular, Darwin often talks about beauty and natural selection to refute the idea that his theories are anti-religious.
Themes
Natural selection doesn’t modify one species exclusively for the good of another species. It can lead to features that hurt other species (like the venomous fangs of a snake), but it can’t lead to structures that solely hurt the organism itself with no benefit. While it might seem like the sting of the bee violates this principle (since the bee dies after stinging), Darwin argued that such an adaptation is good for the species as a whole, and that it is still possible to imagine such a mechanism arising from earlier versions that did not cause the insect’s death.
Darwin is not arguing that species don’t develop traits that benefit over other species. For example, some species of plants are better adapted to accept certain bees. What Darwin is arguing is that these adaptations cannot only benefit the other species. In the case of the plant adapted for bees, it benefits from having its pollen spread more widely by the bees.
Themes
Summary: the Law of Unity of Type and of the Conditions of Existence embraced by the Theory of Natural Selection. Darwin summarized that while the objections to his theories are worth serious consideration, he believed they all had explanations. He repeated his conviction that the best explanation for the similarities among present organisms (“Unity of Type”) is to look back for a common ancestor. He believed that an even more important principle than Unity of Type was Conditions of Existence, by which he referred to the external conditions of an organism’s life that led to the inheritance of variations and adaptations.
Despite dedicating a full chapter to the objections of his critics, he isn’t finished with the topic and will consider it for a couple more chapters. Ultimately, Darwin is not only addressing the objections of critics, but also using the criticisms as a way to organize the structure of his book, presenting the criticisms in an order that will allow him to naturally build his argument up.
Themes
