D.T. Max, "Two Cheers for Darwin", The American Scholar Spring 2003, p. 69.

Tautology is the core of Darwinism, the result of its postulate that whatever is must be. This is compounded by Darwin’s research technique, which was in essence to go where he knew selection occurred and analogize from it. So he based most of his observations not on nature but on domesticated animals—not on the barnacles but on the pigeons. He had seen how breeding influenced their physical appearance. He had confirmed his intuitions with other pigeon breeders. From this fount came his data on natural selection. The long history of animal husbandry, not nature red in tooth and claws; provided his examples of improved species. “It’s a beautiful part of my theory that domesticated races of organisms are made by precisely the same means as species,” he wrote in one of his notebooks, forgetting the circular reasoning by which he had come to know this. The process leaves a gap in Origin itself. When it comes time to cite examples from nature, Darwin has none. He begins one passage: “In order to make it clear how, as I believe, natural selection acts, I must beg permission to give one or two imaginary illustrations.” But in a sense, what he has been doing all along is imaginative. He takes facts from farm life and uses them as analogies for nature.

D. T. Max,  "Two Cheers for Darwin", The American Scholar Spring, Washington: Spring 2003. Vol. 72, Iss. 2; p. 63 (1 page)