Well, Wikipedia thinks they are so defined: Mammals (formally Mammalia) are a class of vertebrate, air-breathing animals whose females are characterized by the possession of mammary glands while both males and females are characterized by hair and/or fur, three middle ear bones used in hearing, and a neocortex region in the brain. Some mammals have sweat glands, but most do not. Ron Allen wrote: > > > Hi Neil & Martin: > > Well, here I am advocating analyticity, and the statement "all mammals > are vertebrates" struck me not as analytic, but rather synthetic, albeit > universally true on this planet. Maybe mammals are defined within > zoology as vertebrates whose female individuals possess mammary glands; > in that case, I agree. But, if the definition of mammal is an animal > with mammary glands and vertebrates have a spine, then, well, it could > be that someone will turn up a spineless animal, like an abalone, that > nurses its young. > > Sometimes informal or prescientific terminology gets converted for use > through formal definition. Water could be the stuff that Thales thought > it was, but now, it's two hydrogen and one oxygen atoms in a molecule. > > Thanks, > --Ron