Part 9

The absence of water, or scarcity of it to a degree that closely approaches absence, in any large area appears to prevent weasels from living there. At any rate, the one sizeable region of North America from which weasels are unknown is the desert of the southwestern United States and adjoining part of northwestern México. More precisely, in western Arizona, the Mohave Desert and the desert of northwestern Sonora, collectors of mammals have repeatedly sought small carnivores without ever finding any weasels.

Degree of moisture is closely correlated with color in weasels. Humidity and cloudiness as well as actual precipitation seem to be involved. Even if we take into account average annual rainfall alone, the darkest-colored weasels are found in the areas of heaviest rainfall and the lightest-colored weasels in areas of lightest rainfall (extreme type of desert where no weasels occur being excepted). In any large region where there is a geographic gradient in rainfall, the transition from light to dark color almost exactly parallels the increase in amount of rainfall. Within a given species the same color reappears in widely separated areas that have the same amount and seasonal distribution of rainfall. This correlation is repeated so often that one can almost certainly say that heavy rainfall, or the associated phenomena of high humidity and cloudiness, acting separately or together, causes an increase in intensity of color. Relative extent of the color of the upper parts and underparts and presence and absence of light facial markings seem also to be correlated, in a more general way, with differences in rainfall. A fuller discussion of the nature and amount of the variation in color is given on page 51.

Temperature seems not to be an important factor in directly limiting the distribution of weasels, since M. frenata occurs from the hottest to some of the coldest parts of the Americas. Do M. erminea and M. rixosa range no farther south, than they do at present, because high temperatures constitute a barrier? No evidence is known to me which provides an answer, one way or the other, to this question. Granting that temperature is unimportant in limiting the distribution of weasels, it seems to cause geographic variation. Increase in mean annual temperature is correlated with decreased size in M. erminea and with increased size in M. rixosa. Temperature, it seems, causes the hair to vary; the pelage is harsher and sparser in weasels from tropical regions than in those from boreal regions. Difference in number of hairs is especially well shown on the soles of the feet. In the weasels from the far north, the pads are concealed by hair and in the weasels from the tropical regions the soles are mostly bare. Also, the hair on the soles of the feet is longer in northern than in southern weasels. Furthermore there is seasonal change in length of the hair on the soles of the feet; at a given locality in southern Canada the hair of the white winter coat is so long on the soles of the feet as to obscure completely the palmar and plantar pads whereas the hair of the brown summer coat is shorter and leaves these pads boldly exposed to view. This seasonal change, as would be expected, is most marked in animals of northern regions and is not perceptible in those from the tropics; it is correlated with increase in seasonal change as the distance from the equator increases.

Temperature and moisture acting together may cause extensive white 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.