Mycielski and Ulam have introduced three metrics on the set of all members of a species that reproduces by pairwise mating; their discussion concerns only a very orderly society in which the whole population is divided into disjoint generations, with random mating, universal monogamy and exactly two offspring of each mating. Here we discuss another metric, derived from certain probabilistic considerations, that is free of all such restrictions. It does not derive from Mendelian laws.