Darwin discusses variation in the natural world, asking whether variations in species in nature can be considered similar to variations in domesticated species. Monstrosities are variations occurring in nature that are unhelpful, and in some cases harmful, to individual plants or animals. Other variations are spurred by environmental conditions, such as climate and temperature, but these are generally not heritable from parents to offspring. In these two cases, the particular variations seen in individuals would not perpetuate themselves through offspring and therefore would not be among the catalogued variations for a species. In a third case, however, slight variations can be passed from parents to offspring, and these can then accumulate in the species as a whole.
To consider variations in nature, Darwin discusses the problem of differentiating variations within a species from variations that signal the existence of two separate species. Overall, the distinction between species and subspecies varieties is nearly impossible to define. No good criterion exists in scientific categorization to distinguish between the two. Darwin notes that he observed “vague and arbitrary” variations in species of birds between one Galapagos island and the next. He also notes that many naturalists disagree about the number of existing plant species in Britain, simply because there is no definitive way to differentiate separate varieties from separate species. A naturalist attempting to categorize a group of unknown organisms might become perplexed, because the line dividing species from varieties is virtually invisible. In the end, Darwin defines the difference with as much clarity as he can: The amount of difference between species must be greater than the amount of difference between varieties within species. Darwin finds that this distinction, while vague, is clear enough.
Darwin distinguishes between dominant species, which exist on a wide scale, and lesser-known species, which are fewer in number and exist only in particular areas. Generally, larger groups of genera (a classification that groups species together as a family of related species) will have a greater and more diverse number of subspecies than smaller groups, as variation is more likely to occur in a larger sample. He recounts an experiment in which he grew two plant species—one from the more abundant genera, the other from the smaller genera—on separate sides of a plot. Sure enough, he found more varieties in the larger genera group than in the smaller one. Darwin uses this finding to refute directly the theory of the separate creation of species, which theologians believed happened by an act of God. If every species were formed by an individual act of creation, Darwin argues, there would be no difference in the amount of variety between the smaller and larger groups of genera. Darwin also hints that the differences between larger and smaller genera affect each group’s struggle for survival: Already-dominant groups have more opportunities to form variations, giving them a better chance to survive over smaller species.
As Darwin discusses the distinction between species and varieties, he highlights uncertainties in science’s systems of classification. Not until 1942, eighty-three years after the publication of Origin of the Species, was a clear criterion established to differentiate species from varieties. It was in 1942 that German biologist Ernst Mayr proved the modern definition of species as a group that could reproduce among its members but not with individuals of other species. In Darwin’s era, many naturalists did look to reproductive abilities to differentiate species from varieties, but they disagreed over the validity of this method. Darwin exposes conflicts in the scientific community by citing the names of particular naturalists who disagree about the number of existing species. His language is strong in this section. He sounds incredulous, almost angry, about the uncertainty and arbitrariness of the species classification.
Darwin’s conclusions about variations also contradict the idea, widely held at the time, that an act of God created species independently. Darwin’s use of the term creation is ambiguous here, but it could easily be interpreted as a reference to creationism (the idea that God created each species). Darwin’s reference to creationism is an implied critique of the teachings of Christianity—a daring move, given the influence of Christianity in Darwin’s time. Darwin suggests that the range of variations within species and between the larger genera and smaller species proves that the theory of divine creation is flawed. If each species formed independently of one another, Darwin argues, there would be no reason or explanation for the existence of so many variations. To the contrary, as Darwin’s findings show, variations within a species develop as a result of unique circumstances, proving that nature and heredity are crucial factors in species development. Darwin’s criticism bolsters his theory that different species must have descended from one parent species.
In this chapter, Darwin introduces his major theories about evolution, places them in the larger context of natural selection, and connects his concept of variation to the broader theory of evolution. He presents the notion of hereditary variation, noting that not all variations are inheritable and reminding his readers that heredity is a component of the perpetuation of variety in species. Darwin also explicitly discusses natural selection, explaining that individual variations are sometimes selected to reappear in later generations. In reflecting on his trip to the Galapagos Islands, he introduces the argument that geographical isolation influences variations within species: The isolated environments of the islands bring about variations in species of birds that diverge from the original parent species and will be advantageous in their specific physical conditions. Geographical isolation also hints at Darwin’s theory of the struggle for existence, the idea that not all species can survive equally well in their environments and that, therefore, species better equipped to propagate will be naturally selected over others. Darwin exemplifies this theory in his discussion of large genera versus lesser-known species: Genera have a greater chance of survival because they stand a greater chance of acquiring variations that will help them adapt to their environment.