squirrels of adjacent British Columbia southward into the Rocky
Mountains. These squirrels were probably closely related to
_fuliginosus_, or to _bangsi_, which latter race already may have been
established farther south in the Rocky Mountains. The Blue Mountains of
southeastern Washington were probably inhabited by _bangsi_ in Wisconsin
times, or even earlier. The retreat of the Vashon-Wisconsin glaciers
allowed the flying squirrels to extend their ranges northward. In
western Washington _oregonensis_ moved to southern British Columbia. In
the Cascade Mountains the more boreal _fuliginosus_ moved much farther
northward and, north of the Okanogan Valley, spread eastward to the
arid, interior valleys of British Columbia. Subsequent differentiation
in the population of the arid, interior valleys developed the slightly
differentiated race _columbiensis_. Farther east, flying squirrels from
the northern Rocky Mountains moved northward. Northeastern Washington
and adjacent British Columbia were occupied by _latipes_, derived from
_bangsi_.
THOMOMYS TALPOIDES.--Views as to the probable historical distribution of
this plastic group have been presented in an earlier report (Dalquest
and Scheffer, 1944: 308-333). This may be briefly summarized as follows.
Previous to Vashon-Wisconsin Times, pocket gophers occupied at least the
Cascade Mountains and the Columbian Plateau of Washington. The race
occupying the Columbian Plateau, _devexus_, was probably racially
distinct in pre-Wisconsin time. The descent of the Vashon-Wisconsin
glaciers isolated gophers in the southern Cascades. Three racial stocks
developed there: _shawi_ in the Mount Rainier area; _limosus_ in the
Columbia River Valley south of the Cascades; and _immunis_ in the
mountainous area between the other two races. At the maximum extent of
the Wisconsin ice, gophers from the Columbia River Valley (_limosus_)
were able to cross the Simcoe Bridge and reach the Blue Mountains. With
the retreat of the Wisconsin ice, the Simcoe Bridge was closed. Gophers
isolated in the Blue Mountains developed the racial characters of
_acqualidens_ and those between the Blue Mountains and the Columbia
River intergraded with the desert race, _devexus_. These intergrades,
which have, also, some characters of their own, bear the name
_columbianus_. Gophers in the southern Cascades (_limosus_) moved
westward on prairie-like river terraces to Clark County where, isolated,
they became racially distinct (_douglasii_).
Gophers from the Mount Rainier area (_shawi_) moved westward on glacial
outwash trains to the extensive outwash aprons of the Vashon glaciers in
the area about Puget Sound. Here they multiplied and spread to the
Olympic Mountains. Growth of forest on the original outwash apron broke
the area into numerous isolated prairies. Gophers in the Olympic
Mountains (_melanops_) were isolated from those in the area about Puget
Sound. Six distinct races originated on the isolated prairie
(_glacialis_, _tacomensis_, _pugetensis_, _yelmensis_, _tumuli_,
_couchi_).
Following the retreat of the glacial ice from eastern Washington, pocket
gophers from the Blue Mountains of Oregon (_wallowa_) moved northward
into Washington and gophers from the Rocky Mountain Fauna of Idaho moved
onto the deglaciated part of northeastern Washington. From northeastern
Washington they spread westward to the Cascades and thence southward to
meet the native gophers of the Cascades in the Yakima Valley Area. No
racial differentiation in these gophers occurred; all are referable to
_fuscus_. Where _fuscus_ and the native gophers came together in the
Yakima Valley, a new race, _yakimensis_, developed.
PEROGNATHUS PARVUS.--Three races of the pocket mouse occur in
Washington. Two of these (_lordi_ and _columbianus_) occur on the
Columbian Plateau. Like many desert species that occur on the Columbian
Plateau, the pocket mice are rather different than their relatives in
eastern Oregon. Presumably they have been isolated on the plateau since
before Vashon-Wisconsin Times.
The range of the pocket mouse of southeastern Washington, _Perognathus
parvus parvus_, is continuous with the range of the race in Oregon.
This same race occurs in the Yakima Valley, whence it probably arrived
from Oregon in relatively recent time.
The distribution of pocket mice on the Columbian Plateau, in eastern
Oregon and in the Yakima Valley resembles that of the least chipmunk in
those areas. It is also similar to, but of more recent origin than, that
of the ground squirrels, _Citellus washingtoni_, and _townsendii_.
DIPODOMYS ORDII.--This kangaroo rat enters the desert area of
southeastern Washington from Oregon. It may be expected eventually to
cross the Columbia River to the Yakima Valley and the Snake River to the
Columbian Plateau.
CASTOR CANADENSIS.--Two races of beavers occur in Washington. One, found
in southwestern Washington and northwestern Oregon, is dark with a
short, wide skull. The other, ranging over most of the state, is paler
with a longer, narrower skull.
The form now found in southwestern Washington and adjacent Oregon
(_idoneus_) was probably isolated there by the Vashon glaciation and
developed its characters while isolated. The other race, _leucodonta_,
was probably widely spread in Wisconsin Time. Beavers are present in
Moses Lake, in almost the center of the Columbian Plateau. Beavers might
well have lived in the streams of melt water that emerged from the
Wisconsin Glacier. The beavers of western Washington, save those in the
extreme southwest, are like the beavers of eastern Washington. It seems
likely that the race _leucodonta_ originated north of the state of
Washington and was forced southward by the Vashon-Wisconsin glaciers.
This northern race, adapted to boreal conditions, competed with the
resident coastal race, _idoneus_, and occupied much of its range. The
distribution of the races of muskrat in Washington closely resembles
that of the beavers.
ONYCHOMYS LEUCOGASTER.--The desert-dwelling grasshopper mouse has
doubtless entered eastern Washington and the Yakima Valley from eastern
Oregon at a relatively recent time.
REITHRODONTOMYS MEGALOTIS.--The harvest mouse, like the grasshopper
mouse, seems to have entered Washington from Oregon at a relatively
recent date. Within the last ten years it has extended its range into
the Okanogan Valley in British Columbia.
PEROMYSCUS MANICULATUS.--Six subspecies of _Peromyscus maniculatus_
occur in the state of Washington. The geographic range of one of these
(_rubidus_) lies mainly in the states of California and Oregon and
includes, so far as is known, a single small island in the Columbia
River that is politically within the state of Washington. Another
(_hollisteri_) is restricted to certain islands in northern Puget Sound
and obviously has become subspecifically differentiated in postglacial
time. The remaining four subspecies, namely _oreas_, _austerus_,
_artemisiae_ and _gambelii_, have extensive geographic ranges. These
subspecies are not confined to their geographic ranges by geographic
barriers. Deer mice occur in the deep forests and the open desert, on
high mountains and in low valleys, and are almost everywhere the
commonest species of mammal present.
The study of several populations of deer mice from any general area
usually shows small but constant differences between them. Dice (1939:
21) studied stocks of deer mice from nine localities in southeastern
Washington and found significant differences between several of them. A
statistical study of mice from the San Juan islands shows that the
population of almost every island is different in one or more respects
from the mice of any other island. Geographically separated populations
of "wild caught" mice of the subspecies _austerus_, of the mainland,
were statistically compared and significant differences were found
between these populations, too. Small, differentiated populations are to
be found in many parts of the state, and each subspecies appears to be
an assemblage of such tiny genetic units.
These genetic units probably are the microgeographic races of Debzhansky
(1937). They have been intensively studied by Sumner (1917 A, B) and
Dice. An especially important paper by the latter author (1940)
summarizes much of the available information on speciation in
_Peromyscus_ and clearly discusses the microgeographic races of
_Peromyscus_.
The numerous microgeographic races of _Peromyscus maniculatus_ in
Washington present diverse combinations of characters which could result
from the random fixation and elimination of genes (Wright. 1932:
360-362). Such a hypothesis, however, requires at least partial
isolation of the populations involved. The mechanism of such isolation,
in such populations of deer mice as we have studied, is not readily
apparent. Some microgeographic races are not separated by noticeable
geographic or ecologic barriers and the distance between their ranges is
not too great to be traversed by a deer mouse. The tendency to remain on
a home range may have the same effect as isolation would be supposed to
have.
The work of Murie and Murie (1931: 200-209, 1932: 79) is enlightening in
this respect. These authors found that mice residing in a locality
tended to remain in that locality; individuals trapped and marked were
retaken in the same locality a year later. Individuals released some
distance from the point of capture remained where released or returned
to the point of capture. Transported individuals did not spread at
random. The home instinct was developed in young as well as in old mice.
Two mice in the gray pelage, four to eight weeks old, returned to their
home ranges from distances one and two miles away. The authors fix the
home range of an individual _Peromyscus m. artemisiae_ in Teton County,
Wyoming, at approximately one hundred yards in diameter.
This home-range instinct is essentially a lack of incentive for
individual mice to emigrate to new localities where mice of the same
species are already established. This may partly account for the
microgeographic races of deer mice in Washington.
Dice (1939: 21) pointed out that, except in color, the differences in
nine stocks of mice from southeastern Washington could not be correlated
with environmental factors. We have found this to be true of
microgeographic races throughout the state of Washington.
Of the four subspecies of deer mice that occupy extensive geographic
ranges in Washington, one, _oreas_, is a long-tailed form that seems not
to intergrade with _austerus_, a neighbor in western Washington that has
a tail of moderate length. These two and _gambelii_, a short-tailed form
with which _oreas_ intergrades, are easily distinguished. In eastern
Washington two short-tailed subspecies, _gambelii_ and _artemisiae_, are
currently recognized. The taxonomic relationships of these two
subspecies are complex. The subspecies _gambelii_ has an extensive
geographic range in Oregon and California. These mice, with short tails,
occur in the Wallula Water Gap of southeastern Washington and on the
Columbian Plateau. To the west the desert conditions of the Columbian
Plateau fade into the Transition Life-zone forests of the eastern
Cascade Mountains. The pale, short-tailed desert mice (_gambelii_)
gradually change to the dark, long-tailed subspecies, _oreas_, that
occupies the Cascade Mountains.
North of the Columbian Plateau, in northeastern Washington, the deer
mice are darker and relatively longer-tailed than on the Columbian
Plateau. Some populations are distinctly reddish, almost as reddish as
_oreas_. Although assigned to _artemisiae_, they are almost identical
with populations of deer mice from the eastern Cascade Mountains, known
to be intergrades between _oreas_ and _gambelii_. This fact, and the
presence of surprisingly _oreas_-like characters in some
microgeographic races in extreme northeastern Washington, may indicate
that the race called _artemisiae_ is a group of intergrades between
_gambelii_ and an _oreas_-like mouse that has become extinct.
Intergradation between _gambelii_ and _artemisiae_ is normal and takes
place gradually where the ranges of the two subspecies meet. The same is
true of intergradation between _oreas_ and _gambelii_ in the eastern
Cascade Mountains. West of the Cascade Mountains the range of _oreas_
meets the range of _austerus_. These two subspecies appear not to
interbreed in nature, since no intergrades were taken at any of the
numerous localities in western Washington where the mice were trapped.
Pure populations of the two subspecies exist within a few miles of each
other. In the valley of the Skykomish River, in the western Cascades
Mountains, from the town of Skykomish, King County, to the lowlands to
the west, only _austerus_ was found. In the coniferous forests of the
mountains above Skykomish, only _oreas_ was taken. Several pairs, each
an _oreas_ and an _austerus_ of the other sex, were kept from four to
six months, and one pair was kept for a year, but they did not
reproduce. The _oreas_ were from the upper Skykomish Valley and the
_austerus_ were from Seattle, King County. Along the border of the
ranges of the two subspecies in the Skykomish River Valley, a definite
habitat preference was noted. The coniferous forests were occupied by
_oreas_ and brush or deciduous forests by _austerus_. Within the range
of _austerus_ and within the range of _oreas_ only one subspecies is
found whether the habitat be coniferous forest or thickets of alder and
willow, but where the ranges of the two subspecies meet _austerus_
occurs only in the thickets of aspen and willow and _oreas_ occurs only
in the coniferous forest.
The morphological characters that separate _oreas_ from _austerus_,
namely, larger size, richer color, and longer tail, are all features
that might be associated with a more arboreal existence in dark,
coniferous forests. Our observations show that _oreas_ is, to a large
extent, arboreal. Traps nailed to tree trunks six to eight feet from the
ground and set for flying squirrels often caught _oreas_ in the
Skykomish River Valley. On one occasion I walked up a leaning tree trunk
to set a trap, fifty feet from the ground on the trunk of another tree
that was upright. An _oreas_ was found in this trap the next morning.
Svihla (1933: 13) relates how a specimen of _oreas_ that escaped from a
live trap took refuge in a tree when pursued. We have set numerous traps
for flying squirrels in the area about Puget Sound. As far as memory and
field notes serve, we have never taken _austerus_ in these traps. Our
observations on the habits of _austerus_ are much more abundant than
those on _oreas_, but for _austerus_ no evidence of arboreal activities
has been obtained.
There are, then, two subspecies that do not interbreed, each capable of
existing in any ecologic niche that will support deer mice. Where the
ranges of the two subspecies come together, they compete. The large
size, richer color, longer tail, and perhaps arboreal habits of _oreas_
give it an advantage in coniferous forests. The small size and dark
color of _austerus_ give it an advantage in other habitats, especially,
perhaps, in winter, when the deciduous trees have shed their leaves.
The differences separating _austerus_ from _oreas_ would be expected to
have developed under complete isolation. That _oreas_ developed from
_austerus_ or _austerus_ from _oreas_ seems impossible. A glance at the
distribution map (Fig. 107) shows that the range of _austerus_ clearly
separates the range of _oreas_ into two segments. The range of
_austerus_ tapers out to the north, east and west. On the south
_austerus_ intergrades with _rubidus_ from Oregon, a subspecies from
which it is but slightly differentiated.
The geographic behavior of the four subspecies of deer mice that occupy
extensive areas in Washington may be summarized as follows: _P. m.
gambelii_ occupies southeastern Washington and intergrades normally with
_oreas_ in the eastern Cascade Mountains and with _artemisiae_ at the
northern edge of the Columbian Plateau; _artemisiae_, occupying
northeastern Washington, closely resembles populations of mice that are
known to be intergrades between _oreas_ and _gambelii_ and itself
intergrades with both _oreas_ and _gambelii_; _oreas_ occupies most of
western Washington, intergrades with its neighbors _artemisiae_ and
_gambelii_ to the east, but does not intergrade with _austerus_, its
morphologically more similar neighbor in the west; _austerus_ occupies a
range in western Washington that is almost surrounded by the range of
_oreas_, a subspecies with which it apparently does not interbreed.
The relations of these four subspecies appear to be the result of
certain great changes in the range of deer mice in the Pacific Northwest
that occurred during and since the last Pleistocene glaciation. We
reconstruct these changes as follows. In the Puyallup interglacial
cycle, just previous to the last (Vashon-Wisconsin) continental
glaciation, clines, or gradual transitions existed in the ranges of the
deer mice along the Pacific Coast. The small, dark, short-tailed mice of
the coastal redwood forests of California and Oregon became gradually
larger, redder and longer-tailed to the north, climaxing in a large,
red, long-tailed form in the spruce forests of southern Alaska and
northern British Columbia. A similar cline existed inland. Small, pale,
short-tailed mice of the Great Basin became increasingly larger, darker,
and longer-tailed to the north, reaching a maximum in the spruce forests
of northern British Columbia.
The development and advance of the Vashon-Wisconsin ice sheet
exterminated mice over much of British Columbia and the northern United
States. Long-tailed northern mice were forced southward and contacted
the southern, short-tailed forms. The preglacial clines were thus
destroyed.
What might be the southern part of the western cline may be noted in the
deer mice of western Oregon today. From the southern coast of Oregon the
mice (_P. m. rubidus_) do become larger, redder and longer-tailed to the
north. The climax of this cline is now reached in _austerus_, of the
Puget Sound area of Washington. The cline is not continued farther
northward because the range of _oreas_ is encountered.
The advance of the Vashon-Wisconsin ice from the north probably forced
species of mammals southward, ahead of it, including the long-tailed
northern deer mice which generation by generation encountered
progressively shorter-tailed forms of resident mice. Perhaps the
unfamiliar, and certainly the extremely frigid, conditions in the fore
of the glacier exterminated the short-tailed individuals but favored the
long-tailed mice, since the latter originally were adapted to a boreal
environment. The climax of the ice advance found the glaciers just
within the political limits of the United States and found the
long-tailed mice spread before the ice front.
In western Washington the Vashon glacier advanced as far south as the
southern edge of the Puget Sound area. Long-tailed northern mice
advancing before it reached the Columbia River. This glacially swollen
stream served as a natural barrier and prevented their southward
extension. At this time the northern mice had traversed more than half
the length of the original western cline. The northern mice, originating
in a boreal habitat a thousand miles away, were unable to interbreed
with the southern mice and such individuals as might have crossed the
Columbia River never became established in Oregon. During the existence
of the glacier in western Washington, the long-tailed northern emigrants
competed with the resident deer mice of western Washington to the total
elimination of the resident mice. The retreat of the Vashon Glacier
from Washington found the northern mice in complete control of that part
of the state from the Pacific Ocean to the Cascade Mountains.
In eastern Washington the Wisconsin Glacier advanced south to the
northern edge of the Columbian Plateau. Northern mice advancing before
it probably did not survive on the treeless plateau but existed in
forested areas of northern Idaho and driftless areas of northern
Washington. No natural barriers separated the northern mice from the
pale, short-tailed forms. The nonuniform topography perhaps allowed more
mingling of the two types where climatic conditions permitted.
Intergradation in some places as well as competition and elimination of
one form or another in other places occurred. Following the withdrawal
of the Vashon ice and the establishment of soil and forests on the
deglaciated land, the long-tailed mice of western Washington (_P. m.
oreas_) apparently spread northward, unhindered by competition, until
they reached southern Alaska. The deep coniferous forests of western
Washington presented conditions acceptable to _oreas_ and it persisted
there despite postglacial changes in climate.
Postglacial changes in climate did, however, permit the dark,
short-tailed mice (_rubidus_) to cross the Columbia River and to become
established in suitable habitats, namely the deciduous jungles of the
river valleys. From these points the mice spread northward through the
lowlands of western Washington, infiltrating the range of _oreas_,
competing with it, and driving it from the lowlands. This movement is in
progress today. By logging and clearing of lands for agriculture man has
considerably speeded the invasion of the southern mice. Slight
differentiation of the short-tailed mice north of the Columbia River
(_austerus_) separates them from their parent race, _rubidus_.
In the dense forests along the Pacific Coast of Washington, _austerus_
did not become established. This area includes most of the land west of
Puget Sound. There is a narrow band of _austerus_ that extends along the
eastern and part of the northern edge of the Olympic Peninsula, where
they have probably invaded in relatively recent time.
On the Oregon side of the Columbia River the range of _rubidus_ is
continuous from the Pacific to the Cascades. On the Washington side of
the river, _oreas_ extends from the Pacific to eastern Wahkiakum County,
where the range of _austerus_ starts. At the border of the ranges of
the two subspecies, ecological conditions determine the ranges; _oreas_
occurs in the Douglas fir and hemlock forests and _austerus_ occurs in
the jungles of alder and salmonberry in the stream valleys. The range of
_austerus_ extends from eastern Wahkiakum County east along the
Columbia, to western Klickitat County. In a north-south strip across the
Columbia, through the ranges of _rubidus_ in Oregon and _austerus_ in
Washington, normal intergradation is apparent. By gradual degrees
_rubidus_ changes to _austerus_. In a north-south strip farther west,
through the ranges of _rubidus_ and _oreas_, the same transition of the
Oregon _rubidus_ is seen, namely an approach toward _austerus_. The
cline is, of course, abruptly terminated by the range of the dissimilar
_oreas_.
On Puget Island, a small island lying in the Columbia River in western
Wahkiakum County and politically within the state of Washington (see
Scheffer, 1942) a population of deer mice occurs that resembles
_rubidus_ more closely than _austerus_. This island lies in the river
between the ranges of _rubidus_ and _oreas_. The lack of intergradation
between these two subspecies has probably kept the Puget Island
population pure _rubidus_. Puget Island is a junglelike lowland locally
known as "tideland."