Variation

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,...