closely related species of plants or animals that are distributed in various areas (geographic vicarious species) or are found in the same area of distribution but under different ecological conditions (ecological vicarious species). In the first instance, the areas of distribution of the vicarious species may or may not be adjacent. Examples of such species are the little suslik, which inhabits the steppes in the south, and the spotted suslik, found in the steppes and forest steppes but farther north; the Norway spruce peculiar to the western part of the USSR east of the Onega River-Southern Urals line, which is replaced by the Siberian spruce; and the European beech, which in the Caucasus is replaced by the Oriental beech (in the Crimea both these species are replaced by the Crimean beech). Examples of vicarious species that occupy the same area of distribution but are ecologically isolated (for example, by soil conditions) are the Northern three-toed jerboa, which inhabits sandy deserts, and the small fivetoed jerboa, which lives in clayey deserts, and “chernozem” and “sandy” plants, which replace one another—steppe thyme and sandy thyme, Koeleria gracilis and K. glauca, and others.