In this chapter Darwin sets out to see if the principals set out in the last chapter regarding selection as it applies to domestic species applies to those in the wild. He notes the inherent difficulty of discussing this subject when terms like species and varieties found within species, are vaguely defined. He goes on to talk about “monstrosities” which are variations in the structure of a being that are often useless or harmful but can be helpful and passed down to the next generation. This variation when passed down through a few generations could be part of defining a new variety of species.
Individual differences are those that may arise between offspring of the same parents. They may seem insignificant but they are of great importance to Darwin as they can often be inherited. But where these difference constitute a different species or variation within a species can be vague and arbitrary. Amongst naturalists, who should be the authority on this subject, Darwin notes inconsistencies in their classifications of species even the number of plant species in Britain and Ireland.
Indeed our tendency to find most definition in that which is most familiar is illustrated by the fact that naturalists have defined the greatest number of varieties to species within the “best known countries” and within species of plants and animals most favored by man. This over defining of species leads to what Darwin calls “doubtful species” (p41). Darwin notes his own experience discerning differences between birds of the Galapagos and the South American mainland as well as disagreements between other well known zoologists and naturalists to illustrate that classifying species is still not an exact science. So that he may continue, Darwin sets aside these vagaries saying-
“I look to the term species as one arbitrarily given, for the sake of convenience, to a set of individuals closely resembling each other, and that it does not differ from the term variety, which is given to less distinct and more fluctuating forms.” (p46)
Darwin notes that in areas where there are more of a species one sees more variety than in areas where there are less. For him this casts doubt on the common view that God created all species at once. In his experiments with plants he sees process of variation is ongoing, active and the eventual variation of species slow.
Darwin closes the chapter noting that the difference of varieties, “when compared to each other or two their parent-species is much less than that between the species of the same genus” (p49). If the opposite were true than the variety would be the species.