facial markings, that neither alone would cause. In _Mustela frenata_
these markings occur where there is heavy rainfall and high mean annual
temperature. Where there is heavy rainfall and a low mean annual
temperature they do not occur and where there is high mean annual
temperature and light rainfall the markings are not pure white but are
of the same color as the underparts. Plate I and the description of
color on page 51 may be consulted in this connection. Extremely high
mean annual temperature together with extremely heavy rainfall may
inhibit the development of light facial markings. _M. f. meridana_,
_panamensis_ and _costaricensis_ are cases in point. In either
direction, north or south, from the territory inhabited by these three
subspecies a similar combination of temperature and rainfall is found
and similar light facial markings appear there.
Considering the delicate response of structure to climate, a person
naturally questions whether or not natural selection accounts for all
of the differences between subspecies. To show that natural selection
determines the color of _Mustela frenata_, it would be necessary to
assume that climate, color, and utility of color are positively
correlated. Although climate (rainfall) and color are correlated in
such a manner that three subspecies of weasel in places as far apart as
New England, Perú, and the state of Washington are colored alike, other
features of the three environments are unlike. Kinds of animals which
the weasel catches for food, and flora in which the weasel finds
concealment, are dissimiliar. If natural selection alone determined the
color, some difference in color would be expected between the weasel
which needed to be obliteratively colored, that is camouflaged, the
better to catch a _Phyllotis_ in Perú and the weasel in Washington
which needed nature's aid in catching _Microtus_. _Mustela frenata
goldmani_ of the highlands of southern México, which is known to attack
the huge pocket gophers, _Orthogeomys_ and _Cratogeomys_, has a weaker
dental armature than _Mustela frenata texensis_ which does not have to
overcome prey so formidable as does _goldmani_. Equally formidable
enemies endanger _M. f. goldmani_ and _texensis_. Examples of this
nature could be multiplied. Without actually proving anything
concerning selection, these examples give reason for us to suppose that
some characters are not determined by natural selection.
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.
Sumner (1932:84) suggests that homogeneity is prevented by population
pressure. Applying his suggestion to the species _Mustela frenata_ we
could say that the subspecies _longicauda_ pressing westward meets
strong pressure from the subspecies _nevadensis_ pressing eastward and
that the width of the zone of intergradation between the two subspecies
varies inversely with the strength of the population pressure from the
two sides. Sumner recognizes that according to his hypothesis the two
contiguous races would remain distinct only so long as there was a
preponderance of centrifugal movement from both of the centers of
dispersal. Sumner (_op. cit._:85) recognizes that an abrupt change of
environmental conditions could account in part for the boundaries of
the ranges of the two subspecies and finally that his hypothesis does
not certainly answer the question of why crossbreeding does not result
in homogeneity between two subspecies with contiguous geographic
ranges.
The hypothesis of harmoniously stabilized complexes of genes was
offered by Timofeeff-Ressovsky (1940:124) to explain why the swamping
effect of crossbreeding does not obliterate subspecies. The hypothesis
takes into account that any one of several characters of a subspecies
may be caused by several genes. Some characters of this kind may be
favored by natural selection more than others. In the belt of
intergradation between two subspecies, where two of these favored
characters meet, a "biological tension" as Huxley (1939:415) terms it
"will result, which will produce _partial discontinuity_ between the
two groups. Each group will evolve a gene-complex which is not only
broadly adapted to the external environment of the central area of its
range, but is also harmoniously stabilized, in adaptation to the
internal genetic environment, by the selection of modifiers." Crosses,
that is to say intergrades, between the two subspecies will lack this
stabilization and will therefore be at a selective disadvantage. The
zone of intergradation will therefore remain narrow; intermediates are
constantly being brought into existence there by crossing but are as
constantly being extinguished by selection.
These two hypotheses are the best that geneticists yet have offered.
Neither has been tested and both, as originally proposed, would hardly
apply everywhere because there are some contradictions.
I can offer no better explanation--in fact no original one as good--but
would emphasize that under similar climate, weasels remain constant in
character, or at most do not vary beyond certain limits. Crossing at
the margins of ranges of two subspecies does not result in homogeneity
of weasels. There is, therefore, some stabilizing influence, or
influences, that maintain, and even develop, structural characteristics
of weasels in opposition to the contrary tendency of crossing.
That this influence not only maintains uniform characters over areas of
large extent, but also permitted their development over large
geographic areas, must logically be supposed, for otherwise,
considering the swamping effect of crossing, such variations would not
have made their appearance in more than a few individuals. Also, if the
races had been formed in response to some kind of physiological
differentiation, or other non-climatic cause, the characters of the
population in the belt of intergradation probably would disappear in a
short time. In any event the close correlation between degree of change
in weasels and degree of change in climate, at once makes one suspect
that climate has been the deciding factor. Finally, when one recalls
that in certain parts of the animal, certain characters invariably
appear under similar climates and never under dissimilar climates, the
evidence is almost conclusive that, given long enough time, the animals
vary in response to climate. The variations (characters) may be induced
indirectly, but are no less exactly reproduced than if they can be
shown to be induced directly.
In considering how the species and subspecies of American weasels were
formed and in attempting to account for some of the individual
characters, it is profitable to view the facts in the light of some of
the theories of species-formation--theories that are accessory to that
of organic descent and that are concerned with the _modus operandi_ of
organic descent.
In any group of closely related species some of them, by the laws of
chance, are almost certain to be more primitive than others. _Mustela_
is no exception and the more primitive species closely match, in
several characters, ontogenetic stages passed through by more advanced
species. Jaeckel's (1902) theory of metakinesis, therefore, is to be
considered since it postulates that many cases of epistasis occur; that
is to say, that many sexually adult animals are arrested in
development in early otogenetic stages and undergo no further
development. Although this theory is appealing upon initial
consideration, it is less so when we recall that in _Mustela_ there is
a direct correlation of increasingly primitive structure with
decreasing latitude as one proceeds from the steppe of North America
southward to the equator. It follows that the conditions seen in
_Mustela_ can be explained even better than by metakinesis, by assuming
that the several species have differentiated from a stem form at
different times, have developed at different rates, have developed in
different directions and that ontogeny recapitulates phylogeny.
The theory of Age and Area (see Willis, 1922) holds that the species of
widest distribution are, on the average, the oldest, and that the
species which are distributed over small areas are, in general, of
recent origin. So far as the weasels are concerned, little support is
given to this theory. The same can be said of any one of the teological
theories, including the orthogenesis of post-Darwinian writers. All of
these imply a determinate line of variation controlled by the inherent
qualities of the organism. The idea that the several species of
_Mustela_ result from mutations of large degree and sudden appearance
is contrary to the evidence accumulated. In fact the evidence rather
clearly indicates that the mutations which may have occurred were of
small degree and in most instances owe their preservation to natural
selection.
The data obtained by the study of weasels accords almost exactly with
the theory of species-forming embodied in Matthew's (1915) "Climate and
Evolution." Although the essential features of this theory were made
out from a study of families and orders and therefore would not be
expected to apply to members of only a genus or subgenus, the facts
known about the present distribution of American _Mustela_,
nevertheless, are strikingly in accord with the ideas advanced by
Matthew. In the first place, climate is an important factor in the
evolution of the weasels. In the second place, the line of migration
seems to have been outward from the holarctic region. In the third
place, the geographic changes necessary to explain the present
distribution of the species of _Mustela_ are not extensive and do not
affect the permanency of oceans as defined by the continental shelf.
These three statements are, almost verbatim, those made in the first
three of the five points of Matthew's (1915:172-173) thesis. The
remaining two points of Matthew's thesis have to do with
generalizations based on evidence obtained from sources outside the
scope of the present study.
Furthermore, the relative degrees of specialization of the different
species and subspecies in relation to their geographic distribution are
in accord with the ideas elaborated by Matthew. For instance, the most
primitive species is farthest south from the probable center of
dispersal, the holarctic region. Also the full species become
progressively more primitive as one proceeds southward from the
holarctic, or at least from the northern half of the nearctic, region.
Although, in view of the known geological changes that have occurred in
the Caribbean region, we cannot say that the more primitive species owe
their positions entirely to having been pushed farther south from the
center of dispersal by actual and _continuous_ contact and competition
with the more advanced species, this seems to have been the case in a
general way. At any rate the more primitive kinds seem to have been
prevented from pushing northward by the more advanced kinds which
developed there and the latter have actually pushed southward.
Additionally and in review: There is strong indication that the
American species of weasels were formed by gradual and slow change.
Much of this change probably is the result of natural selection
operating on fortuitous variations of a minor nature, but, also,
particular features of the environment, especially climate, and more
especially amount of rainfall, seem to compel variations that
differentiate subspecies and that characterize full species--compel
some of them without the direct operation of natural selection, or at
least compel them within limits so wide that natural selection exerts
no exact control.
HISTORY OF CLASSIFICATION
In the earlier accounts of American weasels, from the time of Linnaeus
and before, up until 1890, names then in use for European weasels
frequently were applied also to those in North America. For the next 50
years, and almost without exception after 1896, the American weasels
were regarded as specifically distinct from those in the Old World. In
this 50-year period many new names were proposed, usually as full
species, although now that material from more localities has been
brought together and studied, geographic intergradation is evident
between many of the named kinds and most of these names now therefore
take only subspecific rank. In 1933 Glover M. Allen showed that
_Mustela rixosa_ occurred also in the Old World, and in 1943 I
emphasized that a second American species, _Mustela erminea_, was
circumpolar in distribution. In neither _rixosa_, nor _erminea_,
however, were the subspecies the same in the two continents. To this
general outline of the nomenclature, exception must be made for weasels
of the southwestern United States, México and Central America, and
South America, because as early as 1813 a distinctive name was given to
one of these and weasels from the three areas mentioned were, so far as
I know, never given names of Old World kinds.
The first paper that could be regarded as revisionary in nature was
"Remarks on the species of the genus Mustela" by the zoölogist and
world-traveler, Charles L. Bonaparte, in Charlesworth's Magazine of
Natural History, for 1838. In that paper three new names, _Mustela
cicognanii_, _M. richardsonii_ and _M. longicauda_, all still valid,
were proposed for American weasels.
Audubon and Bachman in their "Quadrupeds of North America," which
appeared in parts from 1845 to 1853, recognized 5 species. Actually
they were dealing with only 3 taxonomically valid kinds. For one of
these, _Mustela frenata noveboracensis_, they were misled by the
difference in size between males and females, and in the males by the
presence of a brown coat in some and a white coat in others. The male
that was white in winter they regarded as _Putorius ermineus_ of the
Old World; the male that was brown in winter they designated by their
earlier proposed name _P. fuscus_, and the female they named _P.
agilis_. The ermine, subspecies _M. erminea cicognanii_, they called
_P. pusillus_. Their fifth name, _P. frenatus_, included at least some
animals that today are assigned to the subspecies _M. frenata frenata_.
Each of three and perhaps four of the five names employed by Audubon
and Bachman embraced individuals of more than one species and in that
sense the names were composite.
Only five years later, in 1858, Professor Spencer Fullerton Baird's
great work, "The Mammals of North America," made it clear that no
American weasel was identical (in the modern subspecific sense) with
any Old World weasel, and he applied most of his names in a correct
zoölogical sense. It is true that he thought that the female weasel of
the eastern United States was specifically different from the male,
misapplied to it the name _richardsonii_, and did not correctly
allocate every one of the few poor specimens available to him of the
little ermine (_M. e. streatori_) of the Pacific Coast; but he did
recognize that the least weasel was a distinct kind and his treatment
in general was excellent.
After Baird came a period of great confusion in which most writers did
no better than had Audubon and Bachman, ordinarily confusing the two
sexes as different species, and, in 1877 in his "Fur-bearing Animals,"
Elliot Coues went rather to the other extreme and allowed only 4 kinds
to all of the Americas, regarding two of these, for purposes of
zoölogical nomenclature, as identical with the European species.
But, in 1896 Outram Bangs published "A Review of the Weasels of Eastern
North America" in which he correctly recognized eight kinds. Although
some of these were treated by him as full species, whereas the material
accumulated since 1896 has shown that subspecific status is in order,
his names, still in use, were correctly applied in every instance, save
probably one. This was his use of _Putorius richardsonii_ for the
animal now known as _M. e. arctica_. Unlike the earlier, excellent
treatment by Baird, this accurate one by Bangs was heeded and followed
by subsequent writers. For example, Dr. C. Hart Merriam in the same
year, 1896, accepted Bangs' conclusions except for correcting the
application of the name _richardsonii_. The principal contributions of
Merriam's paper "Synopsis of the Weasels of North America" were first,
the wider geographic scope and second, the naming as new of several
kinds outside the geographic area studied by Bangs. Otherwise the work
was not up to Dr. Merriam's usual standard and the internal evidence of
haste in its preparation and the superficial study of some of the
material at his disposal explain why the weasels of North America since
that time have been but little better understood than in 1896. Baird
and Bangs, then, unquestionably did the best systematic work on the
American weasels.
In 1916 Dr. Joseph A. Allen published a valuable paper on the South
American weasels. The material available to him was inadequate and
prevented a thoroughly satisfactory treatment. There are too few
specimens even today to permit of a thorough treatment of the South
American weasels in the present paper; nevertheless the material today
is more nearly adequate than it was in 1916 and it is hoped that the
systematic arrangement is correspondingly improved.
Chronological List (annotated) of Specific and Subspecific Names
Applied to American Weasels
At least eighty-seven specific and subspecific names have been proposed
for American weasels. Of these sixty-nine are now regarded as valid
designations of recognizable subspecies. The average is 1.2 names per
subspecies. Some names in the following chronological list were a
second time applied wholly or in part to some other kind of weasel. In
general, mention of the second or any other later application is
omitted from the following list but two usages of _agilis_ (1844 and
1853) and of _americana_ (1865) are recorded.
1734. =javonica= (_Mustela_) Seba, Locupletissimi Rerum naturalium
Thesauri ..., 1:77, 78, pl. 48, fig. 4. The weasel to which this
name was applied was said to have come from Java. Since no animal
answering to the description has again been found in Java, and
because specimens from Central America or possibly some from
northern India, may do so, it is conceivable that Seba was the
first to distinguish by name an American weasel from those in the
Old World. My attempts to locate the specimen concerned in places
where it might have been preserved along with some of the other
specimens thought to have belonged to Seba have been fruitless.
Since it is impossible positively to link Seba's description with
any known weasel, no further use is made of the name _javonica_ in
the present account.
1772. =erminea= (_Mustela_) Forster [= _Mustela erminea
richardsonii_], Philos. Trans., London, 1772:373. Forster's use of
the name is one of the earliest applications of it to American
animals. The name dates from Linnaeus, Syst. Naturae, (10th ed.)
1:46, 1758, with type locality in Europe. In the subspecific sense
the name applies to the ermine which occurs over most of the
Scandinavian Peninsula, if Miller (1912:387) be followed in
regarding the type locality as Upsala, Sweden. If, instead,
Cabrera (1913A:394-396) be followed in regarding the type locality
as in Switzerland, the name, in the subspecific sense, will apply
to the ermine of continental Europe. As the earliest available
name applied to the circumpolar species concerned, it is used now
as the name of the species in the New World as well as in the Old
World. From the time of Forster until approximately 1890 the name
_erminea_ by many, but not by all, authors was applied to the
American weasels in the belief that they were zoölogically
indistinguishable from those in the Old World. From 1896 to 1943
the name was not used by American authors at all because the
ermine of America was in 1896 treated nomenclaturally by Merriam
as specifically distinct from the animal in the Old World. Since
1943 _erminea_ has been used in the specific sense for American
animals in recognition of the circumpolar distribution of the
species. Some of the early allocations of American specimens to
_erminea_ probably resulted in a composite use of the name in that
one or another subspecies of the American species _Mustela
frenata_ may also have been included with individuals truly of the
species _erminea_.
1772. =nivalis= (_Mustela_), Forster, Philos. Trans., London,
1772:373. This is one of the early applications of this name to
American weasels of small size, made in the belief that they were
taxonomically the same in America and Europe. Linnaeus, Syst. Nat.
(12th ed.) 1:69, 1766 is the authority for the name [_Mustela_]
_nivalis_, and the Province of Vesterbotten, Sweden, is regarded
as the type locality. The name is in use today for the common
weasel of Europe and parts of Asia. Animals of the species
_nivalis_ are intermediate in size between _Mustela erminea_ and
_Mustela rixosa_. The name as used for American animals by some
authors who wrote later than Forster did, probably was composite
in that these authors may have applied the name to the small
weasels of North America and thus may have intended it to apply
not only to _Mustela erminea cicognanii_ but also to females of
_Mustela frenata noveboracensis_, and conceivably to both sexes of
_Mustela rixosa_ of any American subspecies.