definitely for _frenata_. For convenience of handling, the population is referred to _frenata_, providing that the average of specimens from a nearby locality to the westward is not in favor of _arthuri_. In event the average of specimens from a locality next adjacent to the westward is in favor of _M. f. arthuri_, the total evidence from the two localities may be weighed together and appropriate decision as to subspecific status of weasels from the area is made according to what the average is for the area as a whole. The three individual animals of an intermediate sort are ordinarily termed _intergrades_. This implies that their characters are the result of mixed parentage--perhaps a female of _M. f. arthuri_ and a male of _M. f. frenata_ but probably each parent itself was an intergrade and the offspring, of which we examined three, owe their characters to reproductive processes operating in obedience to Mendelian laws of inheritance. The two kinds of animals, _Mustela frenata arthuri_ and _Mustela frenata frenata_, are identified as subspecies because of the intergradation between them. If at this and all other places where the geographic ranges of _arthuri_ and _frenata_ met there was no crossbreeding (no intergrades), the two kinds would be treated as distinct species. Intergradation, and the lack of it, are accepted as the criteria of subspecies and species, respectively. These criteria suffice for animals, in this instance weasels, which have a continuous geographic distribution. Some kinds of weasels are confined to islands, as for example the islands off the coast of Alaska and British Columbia. Because weasels are land animals, crossbreeding in nature between the weasels of two islands is, of course, impossible. A modified test (used in the study here reported upon) in deciding on specific versus subspecific status in these instances can be made as follows: On the adjacent mainland, ascertain the degree of difference between two subspecies whose geographic ranges meet (for example, _M. e. richardsonii_ and _M. e. alascensis_). Next ascertain the degree of difference between the insular kind of animal and the kind on the mainland. If the degree of difference is greater when the insular kind is compared than when only the kinds of the mainland are compared, the insular kind is to be regarded as a species. If the degree of difference is no greater between the insular kind and the mainland kind than it is between the two adjacent mainland kinds, the insular kind is to be regarded as a subspecies. In short, for insular kinds, the criterion is degree of difference, with the limitation of geographic adjacency, rather than intergradation.