Today's soundtrack is One Bad Pig: Swine Flew.
This afternoon, I'm reading the second chapter of Charles Darwin's The Illustrated Origin of Species, "Variations Under Nature."
We often see varieties within species of communal descent. We less often see "monstrosities" (p. 61), which Darwin says refers to "some sudden and considerable deviation of structure, generally injurious or not useful to the species" (p. 61).
Individual differences are "[t]he many slight differences which appear in the offspring from the same parents" (p. 61). They are usually hereditary, and thus they contribute to the process of natural selection.
There are some species that are very similar to other species that "a naturalist can unite by means of intermediate links" (p. 62). Naturalists will usually consider the more common of the two as a species, and the less common of the two he will call a variety. Once a variety has lived on its own for long enough, or if it has enough differences, it may be considered its own species.