On The Origin of Species by Charles Darwin Chapter 1 Page 23

structure, in such case, I grant that we could deduce nothing from domestic varieties in regard to species. But there is not a shadow of evidence in favour of this view: to assert that we could not breed our cart and race-horses, long and short-horned cattle, and poultry of various breeds, and esculent vegetables, for an unlimited number of generations, would be opposed to all experience.

3. Character Of Domestic Varieties; Difficulty Of Distinguishing Between Varieties And Species; Origin Of Domestic Varieties From One Or More Species

When we look to the hereditary varieties or races of our domestic animals and plants, and compare them with closely allied species, we generally perceive in each domestic race, as already remarked, less uniformity of character than in true species.