Nationality, cultural origin, or even the sport itself is irrelevant. You are simply seeing the best of what that population has to offer. Unless you are actually watching an amateur club in Fiji, any Fijian you are likely to see playing rugby is a professional athlete. All professional athletes are genetic freaks, with some combination of size, agility and pace which makes them utterly extraordinary when compared to the general population. You can say the same of Fijian rugby players, Canadian ice hockey players, English football players, American gridiron players, Indian cricket players, or whatever. Or -- in scientific terms -- you have to realize (and accept) that your sample selection is not random.
Come on, African Americans and Jamaicans make them look slow as snails!
