 Preferential attachment is considered a possible candidate for, among other things, the distribution of the sizes of cities, the wealth of extremely wealthy individuals, the number of citations received by learned publications, and the number of links to pages on the World Wide Web.The general model described here includes many other specific models as special cases.In the species/genus example above, for instance, each genus starts out with a single species ("k"0 = 1) and gains new species in direct proportion to the number it already has ("a" = 0), and hence "P"("k") = B("k", "γ")/B("k"0, "γ" − 1) with "γ"=2 + 1/"m".