[Illustration: 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.