Commonly chapters by paleontologists are included in books about a particular group otherwise written by neontologists; it is much rarer that the situation is reversed, and I am conscious of the heavy responsibility imposed by my charge to suggest why paleontologists should pay some attention to living mammals. Most of us accept that the past is the key to the present, but here I must justify inverting the aphorism. A few years ago such an attempt, particularly as it related to the phylogeny of mammals, would have seemed arrogant; and while a number of molecular evolutionists might privately have held the view expressed in the epigraph above, I think few paleontologists would have agreed.