On the Origin of Species | Study Guide

Charles Darwin

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On the Origin of Species | Chapter 2 : Variation Under Nature | Summary



In the second chapter, Darwin starts by defining some of the terms he introduced in the first chapter to set up his argument that species and varieties are ill defined. He explains numerous definitions exist for what is a "species," or a "variety." As a result the definitions are "vague and arbitrary." While in general he notes variations in species occur slowly as different traits are inherited, he is unclear if monstrosities—the sudden appearance of a trait—can be passed on. This could be considered what is today known as a mutation, or the sudden appearance of a trait that may or may not provide adaptive benefits, and thus may or may not be transferred to the following generation.

Darwin dedicates the remainder of the chapter to distinguishing between species and varieties. He begins by stating naturalists define these terms in numerous ways. In order to understand how these terms should be defined, it is necessary to study "individual differences" inherited within a population. Quantifying the degree of difference is thus important in distinguishing between varieties (with small degrees of difference) and species (with large degrees of difference).

In order to quantitatively describe the differences between species and varieties, Darwin evaluated "all the varieties in several well-work floras." From this data set, he is able to show genera with more "species" or "varieties" have a smaller degree of variation—and thus should be considered varieties—than genera with relatively fewer "species" or "varieties." These larger genera continue to diversify at a faster rate than smaller genera, and Darwin argues these patterns would not be present if "we look at each species as a special act of creation."


The second chapter moves from the larger-scale description of domesticated animals to a detailed look at what constitutes a species. Darwin is using this to set up his argument for describing natural selection in wild animal populations. He states while selection in domesticated animals makes more sense, the application of natural selection to wild animal populations requires more explanation.

The question of what constitutes a species is a question that continues to garner attention in scientific literature. While specific arguments have changed, the rationale remains relatively the same. Darwin's treatment of the term species is one of the first quantitative treatments aimed at defining it. He later notes in Chapter 14 other researchers were arbitrarily defining species as those who had been designed by the "Creator" and varieties as those that varied due to other processes. He argues these definitions should be treated more rigorously.

Darwin's evaluation of species-level variations within genera is not inherently different from work completed by previous naturalists. He notes the variation inherent in the categorization of different species into "species" or "variety" categories even when looking at the same data set. However, he is attempting to quantify these differences and base them on a more measureable standard than their creation by the "Creator."

What makes Darwin's work unique is his qualitative evaluation of what constitutes a species, along with his statement the patterns of variation he is seeing should not be expected if species were viewed as acts of creation. At the time On the Origin of Species was written, this would have been an extremely controversial statement. Even scientists who accepted the idea of evolution were reluctant to attribute species-level variations to a process other than creationism.

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