In western North America there are two species of water or marsh shrews: _Sorex palustris_ and _S. bendiri_. They have been placed in separate subgenera, but, as pointed out beyond, are closely related and here are placed in the same subgenus. The species _palustris_ is found throughout the Rocky Mountains, north into Alaska, across the Great Basin into the Sierra Nevada, and west to the Pacific coast in Washington. The species _bendiri_ is found from northwestern California north along the Pacific coast to southwestern British Columbia and east to the Cascades. Where the ranges of the two species overlap in western Washington they do not interbreed so far as is known, and are somewhat different in their ecology, _bendiri_ being a lowland, and _palustris_ being a montane, species. The two species probably were separated in a glacial period as seems to have been the case with the wandering shrews. Also, the water shrew of the Sierra Nevada is derived from that of the Great Basin and Rocky Mountains. _Sorex palustris_ is tied closely in its distribution to hydrosere communities and is not dependent upon the presence of forests. Red-backed mice, genus _Clethrionomys_, occur throughout the Rocky Mountains and west to the Cascades in Washington as the species _C. gapperi_. The species _C. californicus_ is found along the Pacific Coast from California north to the Olympic Peninsula. Where the ranges of the two species meet in Washington they seem not to intergrade. In some glacial interval these two species may have evolved in the same manner as has been described for the species of _Zapus_ and those of _Tamiasciurus_. No _Clethrionomys_ are found in the Sierra Nevada, nor are red-backed mice found in the boreal islands of the Great Basin. It is not known why _Clethrionomys californicus_ does not occur in the Sierra Nevada. Some boreal birds have distributional patterns similar to those of the mammalian examples cited above. One kind of sapsucker, _Sphyrapicus varius nuchalis_, occurs in the Rocky Mountains north into British Columbia and west to the Cascades and Sierra Nevada. A related kind, _S. varius ruber_, occurs along the Pacific Coast from California north into British Columbia. Recently Howell (1952) has shown that some intergradation takes place between _ruber_ and _nuchalis_ in Washington and British Columbia, although they do not intergrade freely. Previously the two kinds were thought not to intergrade and were regarded as two species. The two kinds intergrade also in northeastern California, although in that state _S. v. daggeti_, rather than _S. v. ruber_, is involved in the intergradation. Howell considered the two kinds to be conspecific with one another as well as with the eastern _S. varius_. He attributed a measure of the distinctness of _nuchalis_ and _ruber_ to their separation during a glacial period, but felt that the separation was much older than Wisconsinan. Whatever the time of separation, the