1) The mechanism of natural selection among human beings:"the races or species of men, whichever term may be applied, encroach on and replace one another, so that some finally become extinct"2) The way natural selection operates:"Extinction follows chiefly from the competition of tribe with tribe,and race with race. (...) when of two adjoining tribes one becomes less numerous and less powerful than the other, the contest is soon settled by war, slaughter, cannibalism, slavery, and absorption."Make no mistake about it, all I did here was to distill the "formal" hypothesis from a book that's supposedly science. By the quotations above from this book, Darwin's version of Natural Selection theory stands or falls.