As stated previously, the zones are inhabited by more or less definite groups of plants and animals, but there is no distinct line of demarcation between the various zones, and there is often considerable variation in the altitudinal distribution of plants. If temperature and moisture were uniform at a given altitude, the zones would probably be quite distinct. However, these conditions are obviously not uniform. On northern exposures, for example, there is less evaporation, consequently soil moisture is increased, and lack of sunshine results in lower temperatures. Plants which normally occur at 5,000 feet on a sunny southern exposure may be found at a lower elevation on northern slopes, and the reverse is true, of course, with a reversal of exposures. Such variation is even more marked in the distribution of mammals and birds. Many species are characteristic of one or more life zones, depending upon the season of the year, the scarcity or abundance of food, and other factors. For example, deer occupy the Transition or the extreme lower limits of the Canadian Zone in winter, but in summer range up to and occasionally beyond the limits of the Hudsonian Zone. Goats normally range within the upper limits of the Hudsonian and upward into the Arctic-alpine Zone in summer, but are most commonly found in the lower Hudsonian Zone in winter. The general characteristics of the zones are as follows: _Transition Zone_: This zone occupies that portion of the park which lies below 3,000 feet. For the most part it may be more adequately designated the Humid Transition Zone, although a limited area (roughly 4 to 6 square miles) on Stevens Creek and the Muddy Fork of Cowlitz River is characterized by a modified plant and animal population due to repeated fires in old Indian days. This burning favored the upward advance of low zone elements, the destruction of the original forest cover by fire opened the forest stand, accomplished a marked change in conditions of temperature and moisture, thus creating a drier, warmer site. The Humid Transition Zone proper is one of dense, sombre forest; magnificent trees rising from a mass of shade-loving plants with a great number of fallen trees of huge size. Even on bright, mid-summer days the evergreen canopy of interlaced branches permits only a little sunlight to penetrate to the forest floor, and semi-twilight conditions exist in the peaceful solitude of this cathedral-like serenity. [Illustration: The Humid Transition life zone is one of magnificent trees.] [Illustration: The forests of the Canadian life zone afford excellent cover for many mammals throughout the year. In summer such retreats are favored habitats for bear families.] Characteristic plants of this zone, though not confined to such association, include the Douglas fir, western red cedar, western hemlock, salal, Oregon grape, black cottonwood, bigleaf maple, and swordfern.