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Chapter 13
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
Classification. Since the beginning of history, humans have classified organisms based on their similarities and differences. Darwin observed that the process wasn’t simple—you couldn’t simply group all land animals in one category and water animals in another, since even members of the same subgroup often have very different habits.
Darwin has a complicated relationship with classification. On the one hand, he frequently points out the limits of current classification systems, particularly when it comes to distinguishing between a species and a variety. Nevertheless, classification systems also provide a way to order observations, and ultimately Darwin believes in refining classification systems rather than throwing them out.
Themes
In ancient times, many naturalists believed that certain parts and characteristics of an organism determined where and how it lived, and so organisms could be classified based on specific parts of their structure. Darwin, however, believed this idea was totally false. The external similarity of a whale to a fish, for example, is not very useful for classification purposes.
Before getting to his modern ideas about classification, Darwin explores how classification worked in the past. By looking at how classification developed, he explores the logic behind these classifications. Understanding this logic (including what it got right and what it got wrong) will be important later when Darwin proposes his own ideas.
Themes
Characteristics of organisms are not totally useless for classification, however, and both important and unimportant characteristics played a role in classification during Darwin’s time. The seeming importance of a characteristic doesn’t relate to its importance for classification, and characteristics in the embryo can be just as important as characteristics in a full-grown adult.
While Darwin criticizes classification systems in the past, he also tries to draw a bridge between those old systems and the present. In this way, he downplays the potentially disruptive nature of his own ideas, suggesting that his beliefs are simply a logical refinement of what eminent thinkers in the past believed.
Themes
Darwin described classification in his time as based on “chains of affinities.” While, for example, it might be relatively easy to define characteristics common to birds, crustaceans provide a more complicated case. Geography also played a role in classification, though in Darwin’s view, it was sometimes applied arbitrarily.
Darwin looks at the many difficulties inherent in constructing an accurate classification system. While he criticizes some aspects of contemporary classification, he acknowledges that creating a comprehensive system to classify the full diversity of life is a very difficult task.
Themes
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Darwin believed that any logical system of classification should be based on genealogy, but he allowed that in such a system, the amount of difference between different branches of the classification tree might seem to vary wildly. He likened it to the classification of languages: how some ancient languages might have been preserved in more or less the same form, while others changed extensively due to periods of spreading or isolation by the language’s speakers. Regardless of the degree of differences, it still made sense to classify and group languages based on how they descended.
Darwin proposes a classification system that is similar to the tree of life diagram that he described elsewhere (particularly in Chapter IV). His focus on genealogy gives evolution and natural selection a central role in the classification process. Darwin also expands on an idea he introduced earlier: how some species descend over time with minimal changes from their ancestors. For example, earlier, Darwin mentions his belief that modern rock-pigeons are similar to an ancient progenitor, while other pigeon varieties show a little more divergence. While change is inevitable under natural selection, the rate and extent of the change is not inevitable.
Themes
Darwin argued that all naturalists used descent and genealogy to some extent when classifying species, because some species have males and females with very different external forms or larvae that didn’t resemble adults, but naturalists nevertheless classified them together. Since species in nature don’t come with written pedigrees, Darwin argued that the best way to determine genealogy was to focus on traits that were least likely to be modified by an organism’s condition of life (perhaps the angle of a jaw or the way an insect’s wing folds).
Darwin knows that his statements in the previous section about basing classification on genealogy could appear controversial, so in this section, he shows how it resembles something familiar. Males and females of some species look very different—a classification system based solely on appearance or attributes would have to classify the males and females as something separate. The advantage of Darwin’s genealogical approach is that it easily factors in differences between males and females (since all individuals in a species, regardless of sex, would have a common genealogy).
Themes
Analogical Resemblances. Darwin drew a distinction between what he called “real affinities” and “analogical or adaptive affinities,” the latter of which may appear to suggest a relationship between organisms but in fact are just the result of natural selection working in similar ways. Though analogical adaptations may play an important role in an organism’s survival, they are mostly useless for classification.
The distinction Darwin makes here is similar to the distinction he made earlier about how fish and whales look alike but should be classified very differently. To use Darwin’s language, “real affinities” have a genealogical basis, whereas “analogical or adaptive affinities” don’t suggest a genealogical relationship and simply show how natural selection can arrive at similar results through different paths.
Themes
On the Nature of the Affinities connecting Organic Beings. Descendants of a dominant species are likely to spread widely because they are well-adapted to do so. This is why, at the higher levels of classification like family, order, and class, the organisms native to Australia are not substantially different from the ones from Europe.
Darwin begins describing the implications of a genealogical classification system. For example, even species from very different parts of the world are likely to be connected at the high levels of classification. This is a result of migration and of divergence.
Themes
The enormous differences between species that share a similar lineage can often be explained by extinction, where over time intermediate forms died out, leaving only the most extreme versions. This is all consistent with natural selection, which makes it more or less inevitable that the descendants of any parent species will eventually diverge.
Darwin knows it’s strange that two modern species that are supposedly similar from a genealogical perspective might look very different on the outside. His explanation—that, in fact, intermediate species probably existed at some point but have died out—fits well with the ideas about extinction in natural selection that he advanced earlier in the book.
Themes
Morphology. Organisms from the same class often have a “unity of type” where they share parts and organs that serve a similar purpose. Naturalists found it odd and hard to explain that, for example, the bones of a bat resemble other mammals, but the wing bones are used for such a different purpose. Similarly, crustaceans with the most complex multi-part mouths tended to have fewer legs and vice versa.
As is often the case, Darwin describes a complex problem and lays out the issues before proposing a solution. In this example, the problem is that genealogically similar animals can have very different features. For example, most mammals don’t have wings like a bat, even though bats and many other mammals share similar bone structures.
Themes
As Darwin observed, natural selection once again provided an explanation for these strange morphology cases. He noted that the more often a part in an organism is repeated, the more opportunity there is for variability, and so this is why features like vertebrae are particularly responsive to adaptation. Over time, these changes can become great, with a skull, for example, being formed from fused vertebrae.
Darwin notes that the vertebrae of some creatures have bones that seem to correspond with the multiple skull bones of others. He reasons that, due to sheer probability, small distinct parts like vertebrae would be more likely to be affected by natural selection than other body parts, since there is more chance for variability and for gradual improvement.
Themes
Development and Embryology. Darwin considered development in embryology to be one of the most important subjects in all of natural history. He also believed that metamorphoses in insects provided a particularly good way to observe how development worked.
Embryology, with its close connection to reproduction, is central to Darwin’s arguments about natural selection. It provides a unique way to study characteristics of an organism that may not be reflected in full-grown adult specimens.
Themes
Some animals resemble each other as embryos, but their parts differentiate and serve vastly different functions in the adult animal. This is in part because any embryos have to act in similar ways to ensure their own survival. As embryos develop, the organisms tend to become more complex in their organization.
Darwin notes that even organisms that appear very different as adults might have similarities while still in embryonic form. This suggests similarities and supports his idea that organisms that have different characteristics as adults might nevertheless have strong genealogical similarities.
Themes
At whatever age a variation appears in an adult parent organism, it will tend to reappear in offspring at the same age. For example, if mature cattle have a variation in their horns, the offspring are most likely to develop the same variation around the same age (although this is not a universal rule and has exceptions). This is part of the reason why embryos can look more similar than adults—because the organism has not developed enough for the adult variations to manifest.
Darwin has explored how natural selection has worked over long time periods, but here he explores how the results of natural selection are reflected over the course of an individual organism’s lifespan. He notes that organisms grow and change over the course of their lives, and he makes the natural inference that, in most situations, offspring will tend to develop in ways similar to their parents.
Themes
Darwin believed it likely that for many species, embryonic and larval stages of animals helped show what a possible progenitor species may have looked like. For example, crustaceans that look vastly different as adults look surprisingly similar as larvae (called the “nauplius”), suggesting what a possible progenitor species for crustaceans may have been like.
Ultimately, Darwin’s genealogical theory of classification reflects his belief that existing forms of life are more closely related to each other than many of his peers realized. He proves the worth of a genealogy-based classification system by showing how it can account for both the similarities and the big differences among crustaceans.
Themes
Literary Devices
Rudimentary, Atrophied, and Aborted Organs. Darwin observed that many creatures had organs that just seemed to be useless—in fact, almost all higher animals had something of the sort. Snakes, for example, have a useless lung lobe, and birds have a “bastard-wing” digit that doesn’t affect flight in any way.
The issue of rudimentary features is one Darwin has explored before, but he brings it up again here because it is also relevant to the issue of classification.
Themes
Some organs used to serve two purposes but become useless for one of them. For example, some fish have a swim bladder that doesn’t help with buoyancy as it might be expected to, but which does still play a role with respiration.
Though Darwin believes that natural selection follows certain laws, he also believes those laws can lead to surprising conclusions, including body parts that adapt to new purposes different from the purposes they originally served.
Themes
Darwin clarified that just because an organ was not well developed did not make it useless or “rudimentary.” It might in fact be “nascent,” that is, on the way to developing into something more useful. Such a distinction wasn’t always easy to make. For example, Darwin considered the wings of an ostrich to be rudimentary, since they provide little practical advantage to the bird. Nevertheless, he considered the wings of a penguin to be nascent, because although they no longer enable flight, they serve an important role as fins.
Darwin’s use of the term “nascent” (which refers to something about to be born, either literally or metaphorically) reflects his belief that modern nature is not an endpoint and that natural selection remains an ongoing process. This was controversial for religious reasons (since many took it as doctrine that nature was already in a state of perfection), but it is an unavoidable takeaway from Darwin’s theories, since if natural selection happened in the past, there is no reason why it would stop in the present.
Themes
Some rudimentary organs, like teeth in whales, appear in the embryo before completely disappearing. In adults, rudimentary organs often retain a sort of embryonic quality. Though Darwin admitted that it might seem shocking that a process like natural selection could result in such rudimentary or atrophied organs, he did not find any contradiction. He found it logical that disuse and successive variations over time would lead to a natural selection process where some formerly useful organs became rudimentary.
The strange development of embryos (such as teeth that appear in embryonic whales, then disappear) presents a challenge to Darwin, but embryos also provide an opportunity—a chance for him to show that genealogical classification has real applications. Though rudimentary organs may seem contrary to natural selection’s continual process of adaptation, Darwin has already explained this issue (through the principle of economy of growth and through the idea that some seemingly useless organs are simply “nascent”).
Themes
There was no reason, however, why organs rendered useless through natural selection could not eventually be adapted for some new purpose. After an organ has totally ceased to have a function, disuse alone is not enough to explain how it would be eliminated under natural selection. On the other hand, the principle of economy of growth, which Darwin explored in an earlier chapter (Chapter V), does provide a possible explanation for how an organ could totally disappear.
Again, Darwin emphasizes how natural selection is an ongoing process and how a feature that seems “useless” now may once have served a purpose, or may one day serve a new one.
Themes
Summary. Darwin explained again how his theories of natural selection aligned with the different groups and classifications of organisms. He believed that classification only made sense when based on genealogy and that evidence from many sources, including embryos and larvae, showed that many species that appear different as adults can nevertheless be grouped together based on the fact that they descended from the same common ancestor.
Though Darwin criticizes existing classification systems, he ultimately believes that they get some things correct and that his theory of natural selection helps explain classification better than alternative explanations. His call to focus on genealogical classification is not a call to throw out the old system but simply a call to refine and expand on it.
Themes
