On the Origin of Species | Study Guide

Charles Darwin

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On the Origin of Species | Chapter 6 : Difficulties of the Theory | Summary



Chapter 6 is focused on addressing the potential objections to Darwin's theory of natural selection. The chapter deals specifically with three issues: the lack of transitional forms; difficulties of the emergence of unique forms (such as the bat); and the selection for complex organs (such as the eye).

Regarding the lack of transitional variations of species, Darwin argues the time depth (period in which genetic development is at play) involved with natural selection, the incompleteness of the geological record, the fact that natural selection acts on variation by selecting beneficial adaptations, and the wide geographical ranges of species result in relatively few transitional forms. He notes while the examples of transitional forms are few because of some of the issues he discussed in the previous section (such as the extended timeframe by which natural selection operates), it is not difficult to conceptualize the links that formally existed, such as connecting mammals with similar structures—the flight membranes of the flying squirrel, the flying lemur, and the bat. He notes these variations could arise in species "following habits different from those proper to their species."

Darwin argues that "organs of little apparent importance" could represent structures that were originally important to the species, but have since become unimportant. These organisms would then represent "transitional" forms, or at least "transitional" structures. He then addresses the argument certain structures of species are the results of the selection for beauty. He argues the definition of beauty is inherently subjective. In cases where a structure can be considered beautiful—such as in a strawberry, or in the plumage of a male bird—it is the case these structures have evolved to serve a specific function. In the case of the fruit, it is to spread its seeds, and in the case of the bird it is to attract a mate.


In this chapter Darwin is attempting to address some of the issues that have already been raised regarding his theory of natural selection.

Even while Darwin was writing On the Origin of Species, researchers such as Fritz Müller and Édouard Claparède were providing independent evidence for Darwin's theory. Müller was a German naturalist who began corresponding with Darwin after sending him a book titled Für Darwin (1865). His work focused on crustaceans. Similarly, Édouard Claparède was a Swiss anatomist who focused on parasitic mites. In both cases these individuals argued the variations they were seeing could only be due to natural selection.

At the end of the chapter, Darwin states it is only by natural selection the phrase "Natura non facit saltum" has truth. This phrase translates to, "nature makes no leap," a long-held canon in the studies of natural history. The term was originally used by English naturalist John Ray (1682) to refer to the idea species existed in gradations. Here, Darwin uses the phrase slightly differently. He does not see species as existing in gradations—one transitioning into another—but rather as distinct groups. He is instead referring to the idea natural selection occurs slowly, and only over long periods of time are new species formed.

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