American Weasels :: DISTRIBUTION AND SPECIATION

DISTRIBUTION AND SPECIATION

Weasels of the subgenus Mustela are known from the Pleistocene but not from deposits laid down at an earlier time (see page 10). The Pleistocene weasels from Rancho La Brea of southern California and from Potter Creek Cave and Samwel Cave, both of northern California, are subspecifically indistinguishable from the weasels living in those same localities today. The other notable occurrence of weasels in the Pleistocene is in the Conard Fissure of Arkansas. Brown (1908:181, 182, pl. 17) names two kinds from the Fissure. One is an extinct subspecies (Mustela frenata gracilis) possibly of the species which occurs in the same region today and the other, Mustela erminea? angustidens, is an extinct subspecies of a species which occurs only farther north today. M. erminea came south, probably in front of one of the ice sheets, as did several other species of American mammals, now of more northern distribution, that left their remains in Conard Fissure. Mustela rixosa is not recorded as a fossil in America although it is known from the "Diluvial" deposits of the Old World; see Woldrich (1884:1000), who employs the name "Foetorius minutus n. sp.," and see also Zimmerman (1943:295-296).

The ermine, Mustela erminea, is the most generalized of the full species. For example, the number of teeth is as large as in any other species and greater than in certain species. The teeth are sharp-pointed, uncrowded, and individually less specialized than in any other American weasel. M1 has the inner half, or lobe, of approximately the same size as the outer lobe instead of much larger than the outer lobe (the outer lobe is the larger in several other species). The tympanic bullae are less inflated and less protruded from the braincase. The skull is rounded, and has no marked crests and ridges whereas the skulls of the other species are more pronouncedly modeled and sculptured. Therefore, it is possible to think of these other species as derived from M. erminea. A derivation in the reverse direction would be more difficult. From the foot soles of an ermine, or a weasel closely resembling an ermine, the more complex soles of Mustela africana could have been derived by a decrease in hairiness, although it would be necessary to suppose that the thenar pad has been retained in africana and has been lost in the living erminea. The alternate possibility, namely, that the thenar pad was a relatively recent acquisition in the africana line seems less probable. The tail of erminea is of "average" length and in size of entire animal erminea is intermediate between the other American weasels. Structurally, Mustela erminea appears to be nearest the stem form from which all of the living weasels ascended. Its present holarctic distribution is in harmony with the view that it is a direct descendant from the stem form because the stem forms of most of the known kinds of mustelids appear to have lived in the holarctic region. To be sure, Mustela erminea is regarded as having undergone some progressive change in structure, but less than the other weasels, in the period of time when the weasels were evolving from the stem form.

The least weasel, Mustela rixosa, seems to be an ancient type and to judge from the size and proportions of its parts, was differentiated from the erminea stem at a time earlier than were the other American Recent species of weasels. In size, in reduction of the tail, and in proportions of the skull, M. rixosa is, in each instance, the most aberrant of all the weasels, Mustela nivalis of Europe and western Asia included. This aberrancy results from the retention of certain primitive features, in the teeth and basicranial region, and from specialization in proportions of the skull. The skull is long, deep, and narrow. These proportions probably are adaptations permitting the animal to follow the smaller kinds of mice into their burrows. In most of that part of North America where erminea and rixosa occur together, erminea is a much larger animal and takes as prey almost all kinds of land vertebrates that it is powerful enough to kill. These include varying hares and ptarmigans. The least weasel, rixosa, can hardly manage such large prey and lives on the smaller rodents. Mustela rixosa may eat numbers of insects (see page 176 beyond),—a kind of food which Mustela erminea is not known to eat. Apparently the two species are able to live in the same areas because each eats a somewhat different kind of food than does the other and hence they do not compete to the point where one is crowded out by the other. This is the case in the latitudes where the two species of weasels are of different bodily size, but in the southernmost latitudes where these two species occur, erminea becomes almost as small as rixosa and only one of the species, to the exclusion of the other, occurs in a given area. All through the Rocky Mountains, south of Montana and in the territory west of these mountains all the way to the Pacific Coast, only the small subspecies of erminea is to be found. In the Alleghenies of the eastern United States only rixosa occurs. In New England where erminea approaches the size of rixosa, the latter is unknown. Probably this exclusiveness results from competition for food, although competition for dens, safe breeding places and other requirements of life may be involved.

The species erminea invaded the western United States and in the process of invasion probably developed there the small size appropriate to permit erminea to live in that latitude before it could do the same thing in the Appalachian region. Later than erminea, the least weasel, Mustela rixosa, which was small to begin with, also spread southward from the holarctic region, stopped short in the western United States at the northern boundary of the area in which erminea was of small size, but in the Appalachian region of the eastern United States continued on southward to the limits of temperature tolerant for it because erminea had not yet penetrated into that region and no other small carnivore was there to offer competition.

The long-tailed weasel, Mustela frenata, occurs mostly south of the regions inhabited by the ermine, and mostly south of the region inhabited by the least weasel which appears to live as well with frenata as with erminea. It is true that erminea and frenata occur in the same region, but this is a relatively narrow belt across the United States; and from within it a person cannot go far either north or south without reaching a region in which only one of the two species occurs. Exception has to be made for the Rocky Mountains and the Sierra Nevada, where erminea is of exceptionally small size. In these mountains and in the boreal mountainous parts of the intervening region of the United States, erminea and the large-sized frenata occur together over a wide area. Presumably the two occupy different ecologic niches, much as rixosa and frenata probably do where they occur together.

Most of the geographic range of the long-tailed weasel, M. frenata, is in the temperate region. Structurally, this species is the most advanced of the American weasels. Its dentition is the most highly specialized for cutting. M1 is relatively small and the inner lobe is slightly larger than the outer lobe. The skull, throughout, is more modeled than in the other species; the rostrum, the lower jaws and the teeth—all parts of the offensive equipment—are well developed relative to the corresponding structures in other weasels; the basicranial region exhibits an advanced stage of development in that the tympanic bullae show the maximum degree of inflation. Also, they are thrust far out of the braincase, thereby providing more room for the relatively larger brain which is protected by a more solidly built braincase than in erminea.

Several subspecies of Mustela frenata occur in the tropics, that is to say, south of the Mexican tableland and on the coastal plain to the east of it. Each is structurally more primitive than subspecies of the temperate region. As compared with Mustela frenata frenata of the temperate Mexican tableland the size in these tropical subspecies is smaller; the tail is shorter; the braincase and entire skull are less modeled; the postorbital breadth is more; the teeth are smaller; the deuterocone of P4 is not so far anterior to the protocone; the tympanic bullae are less inflated, are farther removed from the foramen ovale, and a larger proportion of each bulla is contained within the braincase. These features serve to set off from northern races of frenata all those subspecies of frenata which occur from southern MÉxico southward to the northern and western limits of the Amazon drainage of South America. The Amazon Basin is inhabited by another species, Mustela africana, having more primitive characters.

In the species frenata, the explanation for this abrupt change in characters between the animals of the temperate highlands and those of the tropical lowlands may be this: In the early Pleistocene, after the emergence of much or all of Central America took place, weasels distributed themselves over the Isthmus and into South America. These weasels were more generalized in structure than those now inhabiting the uplands of MÉxico. Failure of this stock of weasels often to cross some still-persisting water barrier, or failure of this stock to cross some water barrier that was widened or reformed because of a rise in sea level in some one of the interglacial periods of the Pleistocene cut the frenata stock into two or more parts. After the land connection was established or re-established and when the necessary precedent plants and rodents again had established themselves, the two groups of weasels, one from the northern tableland of MÉxico, and the other from the southern area of tropical complexion, met. The weasels of the frenata stock that reinvaded the area from the north probably did so by following along the chain of high volcanic cones and narrow uplifts. If and when a subsequent inundation occurred in some part of Central America, weasels were stranded on the adjacent mountains—converted into islands—only the higher parts of which were above water. Mustela frenata costaricensis and Mustela frenata goldmani may be examples of a northern stock of weasel that pushed southward in the highlands and became stranded for a short time. Following the latest emergence of land to provide a continuous highway between the two continents, weasels from the south and the insular populations, as for example, M. f. costaricensis, were the first to invade the low tropical areas most recently under water. When the Pleistocene history of Central America is better known, the facts will provide a useful means of testing the hypothesis that has been outlined immediately above.

As explained above, fossil specimens of M. frenata from deposits of the last half of Pleistocene time show that no appreciable change occurred in some areas, for example, in the vicinity of Hawver Cave and Samwel Cave of California, and that but slight change occurred in other areas, for example, in southern California (fossils from Rancho La Brea) and probably in the central United States (fossil from Conard Fissure). It is possible to imagine, therefore, that the two groups of weasels, one occurring southward only as far as the highlands of Central America and the other occurring in northern South America, had not differentiated sufficiently in the period of their isolation to prevent crossbreeding when they last came into contact. If the separation of the two groups had been maintained for a longer period, the two groups, tropical weasels and austral weasels, probably would have been so different when the two met as to prevent crossbreeding and they would have constituted two full species instead of only one.

Mustela africana is the most primitive of the American weasels. Some of the most important structural features that mark it as such are in the basicranial region. The tympanic bullae are less inflated than in other weasels, are pointed anteriorly and posteriorly, and do not have the lateral margins carried outward to the outer margins of the braincase. The mastoid sinus is not involved, by inflation or marked modification in the production of the auditory complex. Between the alisphenoid and the squamosal there is a clear demarcation posteriorly from a point directly lateral to the foramen ovale. This demarcation permits a transverse rounding of the alisphenoid to form a longitudinal ridge between the anterior margin of each bulla and the base of the pterygoid of the same side. Nevertheless, there is no such specialization of this primitive, structural feature such as occurs in some African and Asiatic mustelids in which the tympano-pterygoid part of the alisphenoid fuses with the tip of the hamulus of the pterygoid. However, the tympano-pterygoid eminence has not been obliterated in M. africana as it has in the other American weasels. Another primitive feature in the basicranial region of M. africana is the tendency toward separation of the paroccipital processes from the tympanic bullae. The thenar pad of the foot probably is an inheritance from a primitive ancestor since the pad is present in the viverrids and in a majority of mustelids judged to be more primitive than Mustela.

Some specializations are obvious in Mustela africana. One is the reduction in number of premolars; p2 is absent whereas it is normally present in the other weasels; P2 has one instead of two roots; and, in relation to the other teeth, m2 is smaller. The shortness of the preorbital part of the skull in relation to the length of the skull as a whole may reflect the mentioned reduction of the premolars or retention of a primitive shape of skull, or both. Also, certain features which denote immaturity in other weasels are retained in adults of this species, as for example, sutures on the dorsal face of the preorbital region of the skull.

Figs. 17-22. Views of the feet of American weasels (subgenus Mustela) to show differences in number and arrangement of the pads and variation in degree of hairiness of the soles. × 1-1/2. In each figure, left-forefoot on left, and left hind foot on right.

Fig. 17. M. rixosa rixosa, Halifax, N.S.; juv., ?, 7425 U.S.N.M.

Fig. 18. M. erminea richardsonii, Ft. Chimo; ad. ?, 14866 U.S.N.M.

Fig. 19. M. frenata noveboracensis, Mich., July 7, 1913; ad. ?, 44689 M.Z.

Fig. 20. M. f. frenata, Brownsville, June 1, 1892; yg. ?, 34043 U.S.N.M.

Fig. 21. M. frenata panamensis, PanamÁ, February 17, 1911; sad. ?, type.

Fig. 22. M. a. africana, ParÁ, Brazil, Sept., 1908; yg. ?, 37475 A.M.N.H.

Figs. 17, 18 and 19. Drawn from specimens preserved in alcohol.

Figs. 20, 21 and 22. Drawn from relaxed feet of dried skins.

Mustela africana, all characters considered, is the most aberrant of the American weasels. That is to say, greater difference prevails between M. africana and any other American weasel than exists between any other two American weasels. The distinctive cranial and dental characters, excepting the reduction in number of premolars, are of a primitive nature. For example, the relatively wide postorbital region, the large braincase that is inflated anteriorly, and the flattened tympanic bullae are points of resemblance to the holarctic Mustela erminea, the species which is regarded as most closely resembling the stem form. Also, the mentioned characters in adults of M. africana resemble ontogenic stages passed through by other weasels. Consequently, it is thought that M. africana crossed the filter-barrier from North America to South America, remained isolated from the original stock for a length of time sufficient to permit africana to differentiate from North American weasels and vice versa to such a degree that crossbreeding with the frenata stock was prevented when frenata, at a later time, pushed southward over the, then zoÖlogically less-effective, water barrier, or continental bridge if it was by this time in existence.

Fig. 23. Diagram indicating probable relationships of the species of American weasels.

The four full species of American weasels may well be thought of as having the same stem form of which erminea is the most nearly direct descendant. Geographic and climatic changes may have operated to isolate, and then to foster morphologic differentiation of, first rixosa in Eurasia, next africana, third the tropicalis section of M. frenata, and finally M. frenata itself, leaving M. erminea as a modern version, somewhat altered to be sure, of the stem form. Some of these ideas are expressed in figure 16. The climate is different in the ranges of the several species and the climate has changed through time in the ranges of at least many subspecies. Natural selection of morphological features best adapted to a particular kind of climate probably has altered some species more than others. M. erminea in almost every one of its characteristics is generalized and potentially progressive whereas africana retains more characters which are truly primitive along with a few which are specializations. M. africana is potentially the least progressive of any of the American weasels. The most specialized weasels are the North American races of Mustela frenata. A progressive series of increasing specialization is comprised in (1) M. africana, (2) the M. tropicalis (Central American, lowland) section of M. frenata, and (3) the races of M. frenata in North America.

Considering now features of the environment which have obviously influenced the distribution and speciation of weasels, water barriers are important. Bering Strait, Carquinez Strait (along with San Francisco Bay) which opens through the Golden Gate, and the channels between the islands of southeastern Alaska, have contributed to the formation of subspecies. The difference is really slight on the two sides of Bering Strait and San Francisco Bay and is slightly more on two sides of each of several of the channels between the islands of southeastern Alaska. The differences between the weasels on the two sides of one of these water barriers supposedly result from the preservation in animals on one side, or on one island, of small mutations, which would be swamped by crossbreeding if the water barrier were not present. The effect of this isolation is easily seen if ermines from the Queen Charlotte Islands are compared with those of the opposite mainland. The degree of morphological difference is great. Isolationwise, the Queen Charlotte Islands are the seaward end of a chain, beginning with Admiralty Island in southeastern Alaska, and are farther from the mainland, zoÖlogically, than the distance in actual miles across the water channel would suggest. Between any two islands that are geographically consecutive, however, and between the mainland and the first island of the chain, the difference in the ermines is small. In other places, water barriers of equal or greater width have contributed little if anything to the differentiation from one another of weasels on the two sides of the water barrier. The strait between eastern Canada and Newfoundland is an example.

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 1 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.

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