Variations of taxonomic worth usually are referred to as characters. For example, shortness of the tympanic bulla is a character, and the opposite condition, long tympanic bulla, is another character. Specific variations, that is to say specific characters, are provided by the color-pattern, length of tail, number of premolar teeth, shape of the tympanic bullae, and length of the braincase in relation to the length of the tooth-bearing parts of the skull. Subspecific characters are provided by color-pattern, color itself, size as measured by weight of the animal, and its linear measurements, size of the skull, and size and shape of parts of the skull. The characters distinguishing subspecies from one another are not of a different nature from those distinguishing species from one another. Given any one of the above structural features, say, dorsal outline of the skull, several characters may be provided by it. For example, weasels of the species _Mustela frenata_ have the dorsal outline of the skull convex in southern Louisiana, straight in Missouri and concave in North Dakota, thus providing three characters. This is geographic variation. These variations, characters in zoölogical parlance, when plotted on maps, reveal the geographic occurrence of, say, the convex shape of the skull. In combination with other characters, for example, dark color and short tail, basis is provided for recognizing a subspecies, in this instance _Mustela frenata arthuri_ of Louisiana. Because the change from convex to flat skull takes place geographically at about the same place (in eastern Texas) as does the change from short tail to long tail, and the change from dark color to light color, it is easy to draw a line there marking the western geographic limit of occurrence of the _M. f. arthuri_. This same line marks also the eastern margin of the geographic range of the subspecies _Mustela frenata frenata_, the subspecies next adjacent to the westward. On this line and for several miles to either side of it weasels show varying combinations of these three characters or an intermediate condition as regards one or more of the characters, or both. For example, from a locality in eastern Texas a weasel may have (1) a facial pattern exactly intermediate between that of the unicolored face of _arthuri_ and that of the bicolored face of _frenata_, (2) the long tail of _frenata_ and (3) the convex skull of _arthuri_. In the sum of its characters this specimen is exactly intermediate between typical _arthuri_ and typical _frenata_. Another specimen from the same place may differ from the first specimen only in having the tail slightly shorter. The total "score" for the two specimens is, therefore, by a very slight margin in favor of _arthuri_. Let us suppose that we obtain a third specimen from the same place and that it has the face marked like that of _arthuri_ but the tail fully as long, and the skull as lacking in dorsal convexity, as in _frenata_. Now the score is