Another question upon which data obtained from a study of _Mustela_ has some bearing, is this: Where the geographic ranges of two subspecies meet, why does not the swamping effect of crossbreeding cause one subspecies to disappear? Although swamping may have occurred in some instances, it does not occur in the majority of instances. Witness the long-continued existence of the living subspecies _Mustela frenata nevadensis_ of which skulls are available from Pleistocene deposits. Therefore, its distinctive characters, cranially at least, have been maintained for a long time. Furthermore, these characters are maintained over a large geographic region more than a thousand miles across. On the eastern margin of its range, at the eastern base of the Rocky Mountains in Colorado, _M. f. nevadensis_ intergrades in a relatively narrow belt with the lighter-colored, longer-tailed and cranially different _Mustela frenata longicauda_, which has a geographic range almost equally extensive. _M. f. longicauda_ also is uniform in its characters over a large area but at approximately 400 miles east of the base of the Rocky Mountains, it begins to intergrade with the darker-colored, shorter-tailed and cranially different _Mustela frenata primulina_ and does so over a belt of 100 miles or more in width. At any given locality within this wide belt of intergradation the range of individual variation ordinarily does not exceed that in animals from a given locality well within the geographic range of _M. f. longicauda_. In the narrow belt of intergradation along the eastern base of the Rocky Mountains, the range of individual variation at several places is greater than in animals from a given locality well within the geographic range of _M. f. longicauda_ or for that matter from well within the geographic range of _M. f. nevadensis_. Considering the dominance and recessiveness of genes and the genetic mechanism in general by which characteristics of offspring are inherited from their parents, it would seem that _M. f. longicauda_ and for that matter _M. f. nevadensis_ and _M. f. primulina_ would lose their distinctive characteristics because of the crossbreeding that is every year going on between _longicauda_ and _nevadensis_ on the one hand and between _longicauda_ and _primulina_ on the other hand.