Farmer Brown raises border collies and enters them every year at sheep herding competitions. Following the practice of millennia of animal breeders before him, he mates his best males with his best females, hoping to produce a pup that will someday win the blue ribbon in the county trials. His neighbor, Farmer Jones, thinks Brown is wasting his time: all border collies have a herding instinct; that's what makes them border collies. Some are better at herding than others, and some are worse, but

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... they all average out. Is Farmer Brown a selectionist? He has never given any thought to the matter. He knows nothing of genetics, evolutionary theory, a science of learning, or the differential selection of cultural practices. But his actual practice of capitalizing on successive cycles of variation and selection to shift the typical value of a trait surely falls within a selectionist framework. (Darwin drew heavily from the observations of animal breeders like Farmer Brown.) In contrast, Farmer Jones attributes the behavior of his dogs to some quality that defines the breed; he sees any variability among dogs as mere chance deviations from the norm. Farmer Brown and Farmer Jones are elementary representatives of selectionism and essentialism, respectively, two contrasting views of nature whose threads implicitly pervade both philosophy and science even today (Palmer & Donahoe, 1992) . To a selectionist, variability is fundamental-there is no ideal type-but