At its heart, Darwin’s theory of evolution is founded on the hypothesis that variations within the same species have resulted in the variations between different species. The different variations that we see throughout the natural world—colors, shapes, sizes, bodily organs, physical structures, wings, feet, fur, feathers—are apparent because natural selection has caused these traits to be perpetuated in the species that exist today. These variations do not exist randomly, but because they have proven advantageous to the survival of the species. Wings, for example, allow birds to migrate easily while also giving them the means of escaping environmental disasters and harsh climates. The variations that are perpetuated provide the advantages that allow certain species to survive over others.

Darwin argues that variations are hereditary, which is key to their role in natural selection. As traits are passed from parents to offspring, the advantageous traits that are exhibited in parents will also be exhibited in their children. Natural selection ensures that those with advantageous traits will win the struggle for existence, surviving and bearing more offspring that carry their advantageous trait. If traits were not passed on from parents to offspring, there would be no mechanism through which advantageous traits could reappear in subsequent generations and therefore no way for species to diverge from one another through their unique traits. While scientists in Darwin’s era had not yet discovered the mechanisms by which traits were passed on through heredity, evidence existed that traits in parents often appeared in their offspring, which led scientists to hypothesize that characteristics were somehow inherited. The heritability of variation, then, was a key component of Darwin’s theory.

At the time of Darwin’s writings, most scientists defined species as groups of organisms that were unable to breed successfully with one another, as opposed to varieties within a species that could interbreed. Darwin points to flaws in this reasoning and argues throughout the text that there is no real distinction between varieties of the same species and separate species. The variations that create varieties may eventually lead to the emergence of different species—in other words, variations create the descent with modification that leads to the development of new species. Darwin’s theory does not explain how the divergence of varieties into species creates infertility between these new species. Instead of defining species according to their fertility, he argues that the line between varieties and species is arbitrary. While varieties of the same species are generally more alike than are varieties of different species, only degrees of difference separate species from varieties, and Darwin argues that these distinctions are scientifically undefined. This argument, while contested, allows Darwin to explain how varieties can easily diverge into species through natural selection without having to explain in detail how barriers to fertility might develop.