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Chapter 8
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
Many naturalists believed that the crossed offspring of different species were sterile in order to stop the chaos that might ensue. Darwin, however, argued that this sterility was just an accident. One of the key differences between species and varieties seems to be how fertile the offspring of crosses between species are vs. how fertile the offspring of crosses between varieties are.
As in the previous chapter, sterility represents an interesting challenge to natural selection, because it is unclear what benefit it provides to a species. Darwin’s belief that sterility doesn’t provide any benefit may seem unusual at first, but he expands on the idea throughout the chapter and shows how it isn’t inconsistent with his other ideas.
Themes
Degrees of sterility. Building off the work of other naturalists, particularly those working with plants, Darwin noted that sterility is not a black-and-white condition, and that different crossings of parents can lead to different degrees of sterility in offspring. He observed that close interbreeding was often the culprit. Animals had been studied less than plants, but Darwin noted that breeders found that hybrid offspring became more sterile after being bred with siblings over successive generations—although none of these rules were necessarily universal.
Darwin sets the tone for a discussion of sterility by defining what sterility is. Here, he makes the important observation that sterility is not an all-or-nothing condition and that in fact, there can be different degrees of sterility or fertility in offspring. As in many other cases, Darwin draws his observations from domestic breeders, who don’t perfectly mirror nature but who have a lot of firsthand experience and data.
Themes
Laws governing the Sterility of first Crosses and of Hybrids. Darwin noted that hybrids between two species that are difficult to cross (since the crossing rarely produces offspring) are usually very sterile. Still, there were exceptions, and some specific crossings of plants yielded extremely fertile offspring.Even among hybrid offspring created under similar conditions, fertility can vary widely.
One theme that Darwin returns to throughout this chapter is that while sterility often has general rules, these rules also frequently have exceptions. He gives consideration to exceptions, because he wants to ensure that his theories are broadly applicable, even in unusual circumstances.
Themes
It is difficult to figure out which factors govern fertility of offspring. Logically, it would seem that similar species would produce more fertile offspring, but in fact, in some genera of plants, very similar species produce infertile offspring, while in other genera of plants, all species within it can be crossbred freely—even when the genera are in the same family. Rules vary by species: some species cross well with others, and some species are better at creating hybrid offspring that look like them (as opposed to looking like the other species in the hybrid crossing).
Once again, Darwin was limited by the fact that he was writing before many of the great scientific breakthroughs about genetics. Without this knowledge, the fertility or lack of fertility of offspring was difficult to explain, particularly with universal rules. Darwin is careful not to force anything into fitting into a pattern simply for the sake of logic. He steps back and tries to observe the diversity of things as they are, applying the scientific method.
Themes
Literary Devices
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Ultimately, Darwin believed that these complex systems of laws did not support the idea that hybrids were infertile simply to limit confusion in nature. He believed that such a complex system suggested that sterility was either incidental or that it depended on differences in reproductive systems that were difficult for naturalists like him to observe.
Because of the complexity of sterility, it is hard both for Darwin and for his opponents to make generalizations. Ultimately, Darwin is cautious, trying to avoid any claims that can’t be backed up by evidence. For him, there is not enough evidence to back up the claim that sterile hybrids provide some benefit by limiting confusion.
Themes
Origin and Causes of the Sterility of first Crosses and of Hybrids. Darwin used to believe that sterility in hybrids arose gradually through natural selection and that preventing species from blending was an advantage. After studying the issue more, particularly in plants, he reached the conclusion that it must be due to some other cause and that similar rules probably held true in animals as well.
When it comes to reproduction, plants can be easier to study than animals, because it is easier and quicker to cross them. Because the same principles are broadly applicable, however, naturalists like Darwin often take insights from plant reproduction and then apply them back to animals.
Themes
Darwin believed that the real cause behind sterility in hybrids involved changes to the reproductive system. Hybrids generally arise from unnatural conditions, and these unnatural crossings are reflected by disturbances in the reproductive system. Darwin likened this reproductive disturbance to the behavior of elephants, which don’t breed in confinement, even in their native countries.
Through the book, reproduction and the reproductive system play a central part in Darwin’s theories. Particularly in Darwin’s time, reproductive systems could be difficult to observe, but it was often possible to draw conclusions about a reproductive system based on other factors, like the condition of the offspring.
Themes
Reciprocal Dimorphism and Trimorphism. Some plants look the same on the outside but have different reproductive organs, such as the length of the pistil and stamen. There can be two or even three such varieties (“dimorphic” and “trimorphic”), and some crossings between these different forms produce fertile offspring and others don’t. These dimorphic and trimorphic plants help demonstrate that infertility of offspring is solely related to reproductive features and that external features of a plant or animal don’t necessarily help predict whether its offspring with a similar-looking variety will be sterile.
Again, Darwin uses reproduction in plants in order to draw conclusions that apply more broadly to other organisms that reproduce sexually too. Plants were particularly interesting to Darwin because reproductive organs like the pistil and stamen were clearly visible, making them easier to observe (and particularly making it easier to see variations).
Themes
Fertility of Varieties when Crossed, and of their Mongrel Offspring, not universal. While Darwin argued that there was some truth to the popular idea that varieties produced fertile offspring when crossed but species didn’t, he also argued that the rule was not universal. Some varieties of dogs generally don’t produce fertile offspring with others. Gourds and maize provide even stronger evidence of the difficulty of creating fertile offspring from crossing varieties in some cases, proving that the rule isn’t universal.
Darwin keeps returning to the idea that classification systems often do not account for all possibilities. While there are broad general differences between species and varieties, the exceptions to the rule make it difficult to draw a clear boundary. Ultimately, Darwin is not advocating throwing out classification systems. He is simply suggesting that naturalists must take a flexible approach and be mindful of exceptions.
Themes
Mongrels compared, independently of their fertility. Darwin decided to compare the offspring of crossed species and crossed varieties on traits other than simply fertility. He drew a distinction between “mongrels” and “hybrids,” arguing that in the first generation of breeding, mongrels were more variable, but in species that have been domesticated for a while, the opposite might be true. Ultimately, he concluded that if you left fertility out of the equation, the offspring of varieties and the offspring of species were, in general, more or less the same.
As Darwin shows, reproduction can get messy, and there are a lot of different ways to categorize the offspring of any given crossing. On the one hand, he believes that this complexity is important and worth considering. On the other hand, however, he believes that the theory of natural selection is broadly applicable and that in the end, nothing about sterility in hybrids offers a serious challenge to natural selection.
Themes
Summary of Chapter. Darwin reiterated that the sterility of first-generation crossed offspring was not due to natural selection. He admitted that he didn’t know exactly why this sterility arose, but believed that this didn’t fundamentally change his ideas about how species and varieties lived in nature.
After getting into the small details of sterility in hybrids, Darwin returns to broad-level generalizations, emphasizing once again that this sterility does not provide a benefit that is naturally selected.
Themes
