The Origin of Species: Chapter 2 Summary & Analysis

Darwin wrote about the structure of his book, and how before continuing the ideas of the first chapter, the second chapter would look at whether organic beings out in nature (as opposed to in domestication) were subject to a similar variation. Darwin acknowledged that there were reasons to doubt that variations would survive in wild populations, and promised to explore the issue in more detail in a later chapter.

Individual Differences. Darwin described how offspring from the same parents showed slight differences. Many naturalists considered these details as only affecting unimportant parts of organisms, and while Darwin acknowledged that this was often the case, he believed that his own experience and the work of other naturalists suggested that in fact, variations often affected important parts of animals, like interior organs and nerves. Even skeptics acknowledged that there was often wide variation between male and female offspring, with the different sexes having different forms.

Doubtful Species. Darwin wrote about the criteria for determining whether different forms of a particular animal or plant were a new species or just variation within a species. In many cases, this distinction was simply decided by a majority of naturalists, although many accepted classifications had dissenters, and some classification cases presented complicated difficulties. Botanists in Britain, France, and the U.S., for example, often disagreed on how many types of flora their countries contained, with counts differing by the dozens. 

When Darwin compared his experience with American birds versus the birds on the Galapagos Islands (which he saw during his time traveling with the Beagle), he found that the distinction between varieties and species often seemed arbitrary. Darwin gave an overview of many other organisms where the boundary between species and variety was contested by naturalists.

Darwin explained how young naturalists tended to prefer categorizing things as new species in order to emphasize and account for the differing traits. He believed, however, that naturalists with a wider range of experience, particularly with global specimens, seemed to be more inclined to recommend fewer species (which have several varieties within them).

While some naturalists might not see minor differences within a species as especially significant, Darwin argued that these little differences were extremely important to his theories, since he believed in adaptations that came from an accumulation of little changes over time. Not all little variations eventually lead to a new species: some become extinct or remain as simply varieties for a long time. Ultimately, Darwin found that terms like “species” and “variety” are somewhat arbitrary, and while they can be useful in indicating the degree of difference (with “species” signifying bigger differences), in essence the terms mean the same thing.

Wide-ranging, much-diffused, and common Species vary most. Darwin began this new section by writing that, although the topics deserve greater detail, he would attempt to briefly explore issues related to the “struggle for existence” and “divergence of character.” He found that with plants in a given country, the species that flourish the most are often also the ones that are the most widely distributed. This widely spread species often yields offspring that have slight modifications that help them survive in specific environments. This is why wide-ranging species have the most variety.

Species of the Larger Genera in each Country vary more Frequently than the Species of the Smaller Genera. Similar to the last section, genera with many species in them have more variety in them. Darwin admitted that this wasn’t the most surprising finding, but argued that it wasn’t a given and that it required a close look at the data to see this fact. Darwin found this out by comparing plants and tables of genera and species from his own collections.

Many of the Species included within the Larger Genera resemble Varieties in being very closely, but unequally, related to each other, and in having restricted ranges. Darwin noted that, as any naturalist would admit, the different species within a genus are not equally distinct. Amid all the variety, there are clusters and sub-groups of species that are actually quite similar to each other. One of the key points of difference between these similar species is that they often have limited geographic ranges.

Summary. Darwin concluded that it wasn’t possible to distinguish varieties from species, at least not without going through a complicated classification system.