In The Origin of the Species, Darwin uses artificial selection (in which the fitness of a trait is determined by its usefulness to humans) to illustrate the plausibility of natural selection. Darwin, for whatever reason, decided to use pigeon breeding as his example of artificial selection. He very easily could have used the wild cabbage Brassica oleracea (a member of the mustard family) as an exemplar species, but it would've been a definite trade off: less feathers and droppings, more cabbagey smell.
Humans artificially selected for various Brassica traits before the time of the Greeks, either intentionally or accidentally. This selection gave rise to a number of important cultivars, including cabbage, brussels sprouts, broccoli, kale, and cauliflower.
Darwin himself thought that these plants were interesting, because he spent hours watching Brassica seedlings sprouting in his seminal work The Power of Movement in Plants. Here's an excerpt from a part of the book where he's just winding up to talk about Brassica for pages and pages:
"THE following chapter is devoted to the circumnutating movements of the radicles, hypocotyls, and cotyledons of seedling plants; and, when the cotyledons do not rise above the ground, to the movements of the epicotyl. But in a future chapter we shall have to recur to the movements of certain cotyledons which sleep at night.
'Brassica oleracea (Cruciferae)'.--Fuller details will be given with respect to the movements in this case than in any other, as space and time will thus ultimately be saved..."
Anyway, here are pictures of some modern Brassica cultivars: kale, broccoli, brussels sprouts, cabbage, and cauliflower.
all via Wikimedia.