“I look at the term species, as one arbitrarily given for the sake of convenience to a set of individuals closely resembling each other… it does not essentially differ from the term variety, which is given to less distinct and more fluctuating forms.” — Charles Darwin
Having established that domesticated species vary enormously, Darwin now turns to nature itself. Do wild species also vary? The answer is crucial: if there’s no variation in nature, there’s nothing for natural selection to act upon. Darwin demonstrates that variation is ubiquitous—and raises a profound question about the very nature of species.
Darwin begins with the most basic form of variation: individual differences. No two organisms are exactly alike. Examine any population carefully and you’ll find variations in size, color, proportions, behavior, and countless other traits.
Every naturalist who has studied a group closely acknowledges individual variation. These differences may seem trivial, but they are the raw material from which natural selection builds. The slight advantage one variant has over another can mean the difference between survival and extinction.
Darwin notes that these individual differences are often heritable—offspring resemble their parents in their peculiarities as well as their general form. This heritability is essential: without it, selection could not accumulate changes over generations.
Here Darwin raises one of the most troubling questions in natural history: What exactly is a species? The question seems simple until you examine actual organisms.
Consider a continuum:
Where exactly does one category end and another begin?
Darwin documents extensive disagreements among expert naturalists. In Britain alone, different botanists recognize vastly different numbers of species in the same genera. What one calls a variety, another calls a species. This confusion, Darwin argues, is exactly what we’d expect if species are not fixed creations but evolving forms caught at different stages of divergence.
This is Darwin’s radical proposition: varieties are species in the making. What we call a “variety” today may, given enough time and continued divergence, become what future naturalists call a “species.”
Well-marked varieties that have diverged significantly from their parent form. They represent species “caught in the act” of forming—an intermediate stage in the branching of the tree of life.
If this view is correct, the difficulty naturalists have in distinguishing varieties from species is not a failure of their methods—it’s evidence that no sharp boundary exists. Species and varieties differ in degree, not in kind.
Darwin identifies several patterns that will become important for his theory:
Genera with many species also tend to have species with many varieties. This suggests that whatever causes species to multiply also causes varieties to form—the same process operating at different scales.
Species with large geographic ranges tend to show more variation than those confined to small areas. Exposure to diverse conditions and larger populations both contribute to greater variability.
Abundant species show more individual variation and produce more varieties. Their large populations provide more “attempts” at variation, more opportunities for natural selection to act.
The traditional view held that species were created separately and remained essentially unchanged—each species a distinct “type” with clear boundaries. Darwin’s observations challenge this fundamentally.