stabilized gene-complexes to account for the persistence of subspecies.
The persistence of subspecies as genetic units has been best explained,
I feel, by Sumner (1932: 84-86) who theorizes as follows:
1. The number of young produced by a subspecies is greater than the
carrying capacity of the land they occupy, at least at certain
times or in some years.
2. Population pressure results, with a tendency of individuals to
emigrate outwards, to the border of the range of the subspecies,
where the population pressure is less.
3. The outward moving tendency keeps the center of the range of
the subspecies genetically "pure."
4. The peripheral wave continues, as long as favorable habitat is
encountered, until an oppositely directed wave of another race is
encountered.
5. Areas of intergradation represent local mingling of genetic
factors and do not affect the "pure" individuals of the central
part of the range of the subspecies.
Certain aspects of this hypothesis are strongly supported by the
distribution of mammals in Washington. Witness the rapid invasion of
_Citellus beecheyi_ and _Lepus californicus_ in Washington, and the
eastern cottontail in western Washington and the domestic rabbit in the
San Juan Islands. The volume of the "wave of population pressure" where
no opposing force is met, is scarcely believable. In seven years the
eastern cottontails released in southwestern Washington multiplied from
a maximum of 12 individuals to a minimum of 40,000.
Competition between subspecies where their ranges come into contact
seems to be exceptional. _Peromyscus maniculatus oreas_ and _P. m.
austerus_ seem to afford an example of this. However, in a few cases
subspecies seem to be determined in part by adaptation to restricted
environments; each race lives only where local conditions favor its
respective adaptations.
In the pocket gophers, where restricted habitat and fossorial habits
cause numerous microgeographic races, these microgeographic races may be
potential subspecies. This is especially true in the Puget Sound area,
where six races occur in a small area. These races meet all the
requirements of subspecies and are recognized as such. It should be
pointed out, however, that these races and probably many other races
produced by isolation, may represent degenerative mutations of the type
mentioned by Wright (in Huxley, 1940). The principal differences of such
races seem to have resulted from the loss of factors of original
multiple factor series, with resultant homogeneity of the race. Inherent
variability is another thing that has to be taken into account when
considering the differentiation of the mammals of Washington into
subspecies. The pocket gopher is an extremely plastic species,
especially in Washington, whereas the Douglas squirrel is less so. The
flying squirrels, the yellow-pine chipmunk and the snowshoe rabbit are
the other plastic species. These species are not so likely to break up
into numerous subspecies over all of their ranges as they are in
Washington where in a small area the topography is highly varied. The
range of the one subspecies, _Tamias amoenus amoenus_, to the southeast
of Washington is larger than the combined ranges of all six races
occurring in Washington but, so far as I can see, the topography and
environment are no more varied in Washington than in the mentioned area
to the southeast of it. The range of one subspecies, _Lepus americanus
americanus_, in Canada is several times larger than the entire state of
Washington, in which four races are found.
The shrews are poor subjects for a study of differentiation, principally
because their small size makes it difficult to see morphological
variations that may be present. The difficulty is increased because
cranial sutures become ossified at an early age. Although it is
difficult to evaluate the differentiation in them, there is some. The
bats, especially the _Myotis_, are less restricted by geographic
barriers than are terrestial mammals. Nevertheless, obvious
differentiation exists. The larger predatory mammals and the
artiodactyls are able to move over large areas, at least in the breeding
season, but in these animals also, some differentiation has occurred.
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.
SOREX CINEREUS.--It is reasonable to suppose that the cinereous shrew
had a continuous range across the forested area of British Columbia in
pre-Wisconsin time. Without having been isolated, the dark coastal race
(_streatori_) may have developed from the wider-ranging inland
_cinereus_, as a response to the denser, humid, coastal forest-habitat,
after having been forced southward to Washington by the Vashon
Glaciation. Since that time it is presumed to have reoccupied the coast
of British Columbia and southern Alaska. This coastal race might have
developed in Vashon time, while isolated in southwestern Washington. The
Cascades are populated by a race of the Rocky Mountain Fauna, _S. c.
cinereus_, which probably entered the Cascades from northeastern
Washington or British Columbia in Recent Time. The absence of the
species in western Oregon, its rarity in western Washington, and its
abundance farther north suggest a northern origin and northward rather
than southward postglacial movement. Had the full species _cinereus_
been a preglacial resident of western Washington we would expect
_streatori_ or a race related to it to occur in the Cascades.
SOREX MERRIAMI.--The periphery of the range of this member of the Great
Basin Fauna may have been in southeastern Washington since pre-Wisconsin
time.
SOREX TROWBRIDGII.--This shrew is a typical Pacific coastal species with
an extensive range along the Pacific Coast south of Washington. The
Washington population may have been isolated in southwestern Washington
during Vashon time or may have crossed the Columbia into Washington from
western Oregon early in the Recent. Since the retreat of the ice it has
extended northward to southern British Columbia and eastward to the
eastern side of the Cascades. Save for crossing the Cascades its
postglacial movements have been slight, as is typical of Pacific Coastal
species. The race _destructioni_ probably has been isolated on
Destruction Island for several thousand years.
SOREX VAGRANS.--This species probably has had a continuous range over
the western United States since the late Pleistocene. The dark coastal
race (_vagrans_) probably was differentiated from the paler races of the
Great Basin in response to the more humid climate along the coast.
SOREX OBSCURUS.--The history of this shrew of alpine predilection
probably corresponded closely to that of _Sorex cinereus_. The
derivation of the dark, long-tailed, coastal race (_S. o. setosus_) from
the smaller, paler, inland race (_obscurus_) probably occurred before
Vashon-Wisconsin Time. _Sorex o. setosus_ is one of a complex of races
distributed along the Pacific Coast from Alaska to California.
SOREX PALUSTRIS.--This species has a wide range in North America and
extends southward in the Cascade-Sierra Nevada Chain to southern
California. Its extensive range at present in this mountain chain
suggests that it was resident in the Cascades previous to Wisconsin
time. Mountain water shrews probably reached the Olympic Mountains from
the Cascades by way of the Puget Bridge in early Recent Time.
SOREX BENDIRII.--This Pacific Coastal species probably had a history
very similar to that of _Neurotrichus_ and _Scapanus orarius_.
The difference between the Bendire water shrews of western Washington
and western Oregon indicates that the Washington population was
separated from the shrews of western Oregon during Vashon Time. The
white-bellied race of the Olympic Peninsula is probably of local
origin.
MICROSOREX HOYI.--The Washington record of this shrew at Loon Lake,
Stevens County, is in an area where mammals typical of the Rocky
Mountain Fauna occur.
MYOTIS LUCIFUGUS.--The dark race of this species (_alascensis_) may have
persisted through the glacial period in southwestern Washington. The
race _carissima_, of the Great Basin Fauna, may have entered the state
since the glacial period, from the south, of course. Habitat selection
determines their range at present.
MYOTIS YUMANENSIS.--The dark, coastal race (_saturatus_) seems to be an
established member of the Pacific Coastal Fauna. Unlike _lucifugus_, the
coastal race is not found east of the Cascades. The race _sociabilis_,
of the Great Basin, has doubtless entered the desert of eastern
Washington from eastern Oregon.
MYOTIS KEENII.--The southernmost record station for this north coastal
species is on the Olympic Peninsula of Washington. It probably developed
in the humid, northern part of the Pacific Coastal area previous to the
last Pleistocene glaciation and extended its range to the south in
Vashon-Wisconsin Time. The range of tolerance in _M. k. keenii_ seems to
be more restricted than that of _M. lucifugus alascensis_.
MYOTIS EVOTIS.--In Washington, the distribution of this bat is similar
to that of _Myotis lucifugus_. The dark, forest race probably originated
in the north-coastal region. The paler race, that developed in the
southwest, entered eastern Washington from Oregon.
MYOTIS THYSANODES.--In Washington this species has been recorded only in
the southeastern part where the Great Basin Fauna occurs. It probably
originated in the southwestern United States, and a point in British
Columbia a little way north of Washington marks the northern edge of its
natural range.
MYOTIS VOLANS and MYOTIS CALIFORNICUS.--Remarks made about _Myotis
lucifugus_ apply also to these two species.
MYOTIS SUBULATUS.--The northwestern periphery of the range of this
species seems to be in eastern Washington.
LASIONYCTERIS NOCTIVAGANS.--Undifferentiated subspecifically from coast
to coast, no basis is provided for judging the route by which this
species entered the state.
PIPISTRELLUS HESPERUS.--The northwestern periphery of the range of this
bat, also, lies in eastern Washington.
EPTESICUS FUSCUS.--Big brown bats from both eastern and western
Washington seem to have been derived from the Pacific Coastal race of
the species. Presumably it extended its range westward across the
Cascades in early post-Pleistocene Time.
LASIURUS CINEREUS.--No speculation as to the distributional history of
the hoary bat seems justified at present.
CORYNORHINUS RAFINESQUII.--The dark, coastal race of this bat probably
persisted in southwestern Washington and western Oregon through Vashon
Time and moved northward in the Recent. The paler _intermedius_ probably
invaded eastern Washington from eastern Oregon in the Recent.
ANTROZOUS PALLIDUS.--This species strays into eastern Washington from
Oregon as part of the Great Basin Fauna.
URSUS AMERICANUS.--The dark, western race of the black bear
(_altifrontalis_) and the paler, inland race (_cinnamomum_) were
probably separated by a glacial divergence. The inland race has entered
northeastern Washington in the Recent with other members of the Rocky
Mountain Fauna.
URSUS CHELAN, etc.--The apparent past distribution of _chelan_ indicates
it to have invaded Washington from British Columbia since the
Pleistocene.
The apparent absence of grizzly bears from the southern Cascades and
western Washington may indicate their absence from these areas
immediately before pre-Vashon time, or their extermination in or shortly
after that period.
PROCYON LOTOR.--The raccoon of western Washington seems to be the
Pacific Coastal race which occurs also in western Oregon and
northwestern California. This indicates that the coastal race (_psora_)
was confined to the coastal area south of Washington during Vashon Time
and has only recently reinvaded western Washington. It is possible,
though less likely, that raccoons existed in southwestern Washington
during Vashon Time but did not develop racial characters, or that the
Columbia was crossed so frequently that genetic differences were
dispersed throughout the entire population.
Reasons why the second hypothesis is inadequate are: (1) Raccoons range
but little north of the state of Washington, both east and west of the
Cascades. (2) Raccoons of western Washington and the area about San
Francisco Bay, California, are as much alike as are raccoons from
southwestern Washington and northwestern Oregon. It is thought that
raccoons, if resident in western Washington since interglacial time,
would have developed strong racial characters, and the fact that they
have not indicates that they have entered the state at a relatively
recent date.
The raccoon of eastern Washington (_excelsus_) is a member of the Great
Basin Fauna and has probably included southeastern Washington in its
natural range for a long period of time. The raccoon has not extended
its normal range into northeastern Washington, although it is seemingly
ideal raccoon habitat; only an occasional vagrant occurs there. A stock
of raccoons from which emigrants might come has existed in southeastern
Washington and the Yakima Valley for some time. The Columbia River might
serve as a highway by which emigrants could reach northeastern
Washington.
MARTES CAURINA.--The earlier distributional history of the western
marten has been postulated by Davis (1939: 131-132), who stated: "When
the ancestral stock split into the two groups, the one that gave rise to
_americana_ may have pushed eastward across Canada to the Atlantic
Coast; the other, giving rise to _caurina_, may have migrated southward
along the Sierra Nevada-Cascade and Rocky mountains. Perhaps the great
ice sheet was instrumental in pushing _americana_ eastward and
separating it geographically from _caurina_." The present occurrence of
_americana_ in Alaska and British Columbia is thought to have been by
invasion from the east in postglacial time.
Davis' theory seems basically correct but subject to correction in
detail. The presence of _caurina_ in the southern Rocky Mountains
suggests that it is not a Pacific Coastal species in the common sense.
Had _americana_ occupied northern British Columbia in pre-Wisconsin
Time, it and not _caurina_ would be expected to occur in the southern
Rocky Mountains today, for the form found in British Columbia almost
certainly would have been forced into the Rockies. The range now
occupied by _caurina_ in the Rocky Mountains is so extensive as to
suggest that martens could not have migrated into all of it from the
Pacific Coast since Vashon Time, even had the region been unoccupied by
any species of marten. The presence of _americana_ in Alaska and British
Columbia suggests that it arrived in those areas before _caurina_ and
that had the Rocky Mountains been unoccupied by martens in pre-Wisconsin
time, _americana_ and not _caurina_ would have reached the Rockies
first. It appears that _caurina_ occupied much of western North America
in pre-Wisconsin Time and was forced southward into the southern Rocky
Mountains and along the Pacific Coast by Vashon-Wisconsin ice.
The separation of _americana_ and _caurina_ may be supposed to have
occurred before the pre-Vashon-Wisconsin interglacial interval, perhaps
by a glacier similar to but antedating the Vashon-Wisconsin glaciation.
The martens of western Washington (_Martes caurina caurina_) are a
coastal race. Those of northeastern Washington belong to a race of the
Rocky Mountain Fauna, and are referred to _M. c. origenes_. Davis (1939:
132) refers the martens of Idaho to _Martes caurina caurina_. I have
compared specimens from Idaho with animals trapped for fur from the
Pacific Coast proper and feel that the animals from northeastern
Washington and those from Idaho are more like _origenes_ than _caurina_,
although perhaps not typical. The animals from the Pacific Coast proper
are _caurina_ and have darker heads and brown instead of yellow patches
on the throat.
MARTES PENNANTI.--Fishers are found throughout the Cascade Mountains and
probably were widely distributed over western North America in
pre-Wisconsin Time.
MUSTELA ERMINEA.--The distribution of ermines along the coast of
northern California and in the Cascade-Sierra Nevada of Oregon and
California indicates, as does their differentiation there, that they
ranged southward to these areas before and during Vashon-Wisconsin Time.
In immediate pre-Vashon-Wisconsin Time, the dark race _streatori_
probably occurred in western Washington. The race _murica_ probably
occurred in the Blue Mountains then, as it does today, but probably
occurred also in the Cascades of Washington. The descent of the Vashon
glaciers probably displaced _streatori_ from the northern part of its
range, at least temporarily. In the Cascades, _murica_ was likewise
forced southward. Ermines related to the northern _richardsonii_ were
forced into northern Washington and Idaho by the Wisconsin ice. They
probably were unable to live on the barren, unglaciated plains of
eastern Washington but persisted in Idaho.
The ranges of the three forms at the maximum extent of the
Vashon-Wisconsin may be reconstructed as follows: _streatori_ in
southwestern Washington; _murica_ in the southern Cascades and the Blue
Mountains; _invicta_ stock in northern Idaho. While so isolated, the
ermines of the southern Cascades probably mingled, to a certain extent,
with _streatori_ and developed the characters that now separate _gulosa_
from both _murica_ and _streatori_. The intermediate nature of _gulosa_
has been mentioned by Hall (1945: 85).
The retreat of the ice allowed _streatori_ to move north and _invicta_
to move north and east into Washington and the northeastern Cascades. To
a lesser extent, _gulosa_ may have moved north. The poorly-marked race
_olympica_ probably evolved from _streatori_ in the Recent. It is
difficult to account for the dark race _fallenda_.
It must have evolved from _streatori_ in the Recent but the origin of
such a strongly marked race in such a short time is surprising. It might
be mentioned that a similarly differentiated race of chipmunk, _Eutamias
amoenus felix_, occupies much the same range.
MUSTELA FRENATA.--The long-tailed weasels of the Pacific Coast behave as
a plastic group and clearly show the effect of the Vashon-Wisconsin
Divergence. The range of the coastal race, _altifrontalis_, indicates
that it was isolated in southwestern Washington during Vashon Time. In
that period, or shortly after, it extended its range southward but only
along the extreme, coastal area of Oregon (see Hall, 1936: 101).
Following the retreat of the ice it extended its range northward to the
deglaciated area of western Washington.
Also following the retreat of the ice, a Great Basin subspecies
(_nevadensis_) extended its range northward. This race seems to have
been more adaptable and successful than other kinds of Great Basin
mammals, for it extended its range farther northward, eastward and
westward than most.
A third race, _washingtoni_, was isolated in the southern Cascade
Mountains during Vashon Time and became differentiated from both
_altifrontalis_ and _nevadensis_. It is now found in the Cascades from
central Oregon north to Mount Rainier. It is difficult to see why it did
not extend its range to include the northern Cascades when the glacial
ice left, but it did not. Instead _altifrontalis_ entered the northern
Cascades from the west and _nevadensis_ did the same from the east.
Weasels obtained in habitats north of Mt. Rainier are intergrades
between _altifrontalis_ and _nevadensis_.
One is reminded here of the _douglasii_ group of _Thomomys talpoides_ in
which subspecies did not move north of Mt. Rainier in postglacial time.
The area north of Mt. Rainier was populated instead by gophers of the
_fuscus_ group, subspecies of which invaded the area from the east.
Perhaps Mt. Rainier itself served as a barrier to alpine mammals in the
immediate post-Pleistocene. Perhaps _Mustela f. washingtoni_ will
eventually extend its range northward, displacing the
_altifrontalis-nevadensis_ intergrades from the habitats to which
_washingtoni_ may be better adapted.
The Blue Mountains of southeastern Washington are occupied by a weasel
(_effera_) that has a more extensive range in eastern Oregon. The range
of this race has probably not changed materially for a long period of
time.