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Chapter 7
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 found some creatures’ instincts so amazing that he predicted some readers would object to his theories based on instinct alone. Darwin believed that there was a way for habitual actions to be inherited, and moreover that some traits like the hive behavior of ants and bees was so coordinated that it had to be more than just habit. As with other characteristics, instinct only arises through natural selection due to the gradual accumulation of small but useful variations. Ultimately, Darwin believed that under natural selection, instinct was similar to physical characteristics and followed similar laws.
Instincts might not seem like something that can be affected by natural selection, since they are not as concrete as an organ or other body part. Nevertheless, over the course of this chapter, Darwin makes the argument that for the purposes of natural selection, instincts act like just about any other feature that gets modified through natural selection.
Themes
Inherited Changes of Habit or Instinct in Domesticated Animals. Darwin believed that there was already good evidence of how instinct could be inherited, based on data from domestic breeding. Cats, for example, vary as to whether they prefer to chase mice or rats or birds, and this habit was known to be passed down through generations. He makes the distinction, however, that simply teaching a domestic animal how to do a certain action does not mean it will be passed down to offspring as instinct. Tumbler-pigeons, for example, were never taught how to tumble.
As he did in previous chapters, Darwin shows that his seemingly strange ideas are in fact based on concepts that are already familiar to people. Again, he uses the example of domestic animals. Then, and now, people were familiar with how domestic animals like cats could seem to inherit unique instincts from their parents that were present from birth, but the cats wouldn’t inherit any habits or preferences they gained through training.
Themes
Some natural instincts are lost in domestication: for example, some fowls lose their instinct to sit on their eggs as they would in the wild. This provides further evidence that instincts are inherited and that in these cases, humans affected this process by breeding domestic animals for specific qualities.
The fact that instincts change under domestication is a convincing argument that instincts are determined hereditarily (and so there is opportunity for natural selection to act on them).
Themes
Special Instincts. Darwin identified three specific instincts that he felt were central to his argument: the instinct of the cuckoo to lay eggs in other birds’ nests, the instinct of some ants to essentially make others their slaves, and the instinct of bees to make complex cells as hives.
This is another case of Darwin approaching abstract ideas by breaking them down into real or hypothetical examples from nature in the present day.
Themes
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Instincts of the Cuckoo. The habits of cuckoos vary around the world, with some laying their eggs over the course of days and others laying them all at once—this complicates the question of what is motivated by instinct. In some varieties, a blind cuckoo that hatched just three days ago has been observed to kick its foster siblings out of the nest. Darwin believed that this behavior helped the young cuckoo get enough food and that there was every reason to believe that such an instinct could have arisen through gradual, successive variations, just like any other inherited trait.
Cuckoos are birds that are famous for being parasites and laying their eggs in the nests of other birds. Once in the nests, the newly hatched cuckoos will actually push a bird’s real offspring out of the nest, in order to get more food themselves. Birds as young as three days old know how to do this, so it is clearly instinct. This was widely known in Darwin’s time, but Darwin’s contribution is to suggest that the way this instinct first arose was gradually through natural selection.
Themes
Slave-making instinct. Naturalists observed that some ants were totally dependent on making other ants into their “slaves.” They can’t make nests or feed larvae, and they need to be physically carried by slaves in order to migrate. Not all ant species are slave-makers, but even ants that aren’t will sometimes carry off the pupae of other species. It is possible that some species like this essentially found it was more efficient to steal workers than to create them through reproduction and that this is how they gradually became slave-making.
Slave-making ants were fascinating to Darwin and other naturalists because they seemed to break the normal rules about relationships between species. To some it seemed impossible that such a strange relationship could have arisen gradually, but here Darwin suggests intermediate steps based on actions that modern ants have been observed doing.
Themes
Literary Devices
Cell-making instinct of the Hive-Bee. Honeycomb has a beautiful, even mathematical structure. Darwin investigated how such a thing could arise in nature through natural selection. Following the examples of his peers, he looked at the habits of different species of bees and conducted his own experiments, showing how bees reacted to different obstacles placed in their way when building honeycombs. Ultimately, Darwin reached the conclusion that bees likely began by making combs from spheres at equal distances from each other, before gradually improving on the process until the combs reached their famous hexagonal shape.
When viewed in isolation, a honeycomb seems so complex that it had to have been carefully designed. Darwin’s critics argued it wasn’t possible to build something so intricate gradually. Darwin, however, has already answered this question in various forms (notably for the human eye), and here he employs the familiar strategy of looking for possible intermediate behaviors being practiced by other species that currently exist (and which could give a hint about the behavior of progenitor species).
Themes
Objections to the Theory of Natural Selection as applied to Instincts: Neuter and Sterile Insects. As he promised in the previous chapter, Darwin returned to the issue of neuter and sterile insects. Though it was a broad issue, Darwin narrowed the focus to sterile worker ants. The idea that a worker is so different from its parents and sterile seems at first to go against natural selection.
Darwin spends a lot of time on sterile animals since the entire concept of sterility might seem opposed to his theories (which suggest that organisms want to survive and pass their traits on). While the issue can get complex, Darwin always has an answer for the objections of his critics.
Themes
Darwin answered the objection first by noting that many characteristics of creatures are associated with specific ages or sexes. Birds, for example, get specific plumage around mating season, so it’s possible that the mechanism behind sterile insects is similar. Darwin also noted that selection applies not just to the individual but to the whole species.
The problem with Darwin’s critics, in Darwin’s view, is that they often focus on the issues too narrowly. Zooming out often reveals that seemingly insurmountable problems can often be explained by analogy to a similar, better-understood issue.
Themes
Still, the difficulty with ants is that sometimes neuters differ not only from their parents but even from each other (for example, some are workers while others are soldiers). While Darwin acknowledged this is a serious challenge to natural selection, he believed there was an explanation. Within the worker ants are “castes” of wildly different sizes and shapes. This variation and the presence of ants with “intermediate” forms seems to show a way in which natural selection could have acted on these neuter ants through gradual changes. The different castes seem to be a useful adaptation for the ants.
Ant hives present a particular challenge to Darwin’s theories. Darwin doesn’t believe that hives disprove natural selection, but he does allow that ants are a difficult case to explain. Ultimately, Darwin’s theories look not at the survival of individuals but at the survival of a whole species, and from this angle, it makes sense that there might be neuter ants within a hive that fulfill a specific role that doesn’t require reproduction.
Themes
Summary. Darwin repeated how evidence from domestication helped bolster his case about instincts and natural selection. He also summarized how natural selection explains the seeming issues with cuckoos, slave-making insects, and neuter insects.
Darwin covered a wide variety of topics in the chapter, so he uses the summary to emphasize the connections between the topics.
Themes
