The greatest changes, other than the extinctions, to occur in the mammalian fauna of Washington since the late Pleistocene, are changes in distribution. The interglacial cycle preceding the Vashon-Wisconsin glaciation was of far greater duration than the Recent. Presumably the mammalian fauna had, from a distributional standpoint, reached a relatively stable condition. The descent of the Vashon-Wisconsin ice destroyed the stability and set parts of the fauna in motion. Probably no stability was reached before the ice began to recede, and when it did so the previous movements of the various species were, at least in part, reversed. Stability has not yet been reached by the mammalian fauna of Washington. Great changes have occurred in historic times and other changes probably are under way at present. In the following pages an attempt has been made to interpret the probable late Pleistocene and Recent distributional history of the species of mammals occurring in Washington. The interpretations are made in the light of what is known of the physical history of the state and are to be accepted as such rather than as evidence for the conclusions made concerning the physical history of the state of Washington and adjacent areas. SCAPANUS TOWNSENDII.--Probably this animal was confined to the Humid Transition Life-zone of the Pacific Coast since the Pliocene. SCAPANUS ORARIUS.--This species probably had a history similar to that of _townsendii_ up to the late Pleistocene. It seems slightly more adaptable than _townsendii_, and to be able to extend higher into the mountains. The distribution of the subspecies _orarius_ is almost exactly that of _S. townsendii_. In Oregon, _orarius_ extended eastward over the Cascades where the subspecies _schefferi_ developed. Perhaps this subspecies developed since the Pleistocene and since that time extended along the Columbia River Valley to southeastern Washington. The race _yakimensis_, in the Yakima Valley area, is closely related to _schefferi_, and seemingly could have been developed from a stock of _schefferi_ that migrated westward across the Simcoe Bridge. NEUROTRICHUS GIBBSII.--The history of _Neurotrichus_ in North America was probably similar to that of the two species of _Scapanus_. It tolerates environmental differences to about the same degree that _Scapanus orarius_ does but occurs much farther south (Monterey County, California) than _S. orarius_. This may be because _Neurotrichus_ has no counterpart to compete with it in the south, whereas _Scapanus orarius_ must compete in northern California with the morphologically similar _Scapanus latimanus_. _S. orarius_ stops short at this place and _S. latimanus_ occupies all the territory to the south. The shrew-mole of the lowland of Washington (_N. g. minor_) probably became distinct from the mountain subspecies (_gibbsii_) in Vashon-Wisconsin time.