34 CONTRIBUTIONS FROM THE NATIONAL HERBARIUM. 
It is generally admitted that heat is the most potent factor in 
determining the distribution of plants, and that in general the old 
division into Arctic, Temperate, and Tropical zones approximates the 
real truth. Such zonation, depending on heat, is far more wide- 
reaching than one depending mainly on moisture. While the former 
gives rise primarily to what we in general know as Arctic or Trop- 
ical zones, the factor of moisture determines the opposed conditions 
we distinguish as arid and moist. Differences in the heat factor 
are universal, resulting in the whole earth being divided into more or 
less well-marked zones, corresponding in general with isothermal 
lines. Differences in the moisture factor are relatively local, so 
that deserts may occur in the midst of the most varied surroundings. 
While the larger zones depending on heat are strikingly different, 
yet each passes gradually into the contiguous ones. Determination 
of such zones is therefore more or less arbitrary. The scheme of life 
zones, so far as North America is concerned, that has resulted from 
the studies of Merriam has been generally adopted. It is as follows: 
Arctic or Arctic-Alpine zone. 
Boreal region --) Hudsonian zone. 
Canadian zone. 
Transition zone -.------.----- Arid Transition area, 
Pacific or Humid Transition area. 
fCarolinian area. 
(Upper Sonoran area. 
Lower Austral zone ......... {Austromiparian area, 
Lower Sonoran area, 
Tropicalregion.........--------------------- {Hume Tropical area, 
Arid Tropical area. 
[anit rans area. 
Austral region.) Jpper Austral zone 
In this scheme zones are based primarily on the distribution of 
plants and animals as determined by the heat factor. The subdi- 
vision of the zones or areas depend mainly on the differences due to 
the moisture factor. As may readily be imagined, all possible combi- 
nations of these two factors occur, so that regions of mixed character 
are found wherever zones or areas are contiguous. This overlapping 
of contiguous zones that are usually well marked is perhaps more 
pronounced in the Pacific northwest than elsewhere in North 
America. It has been ascribed to the very equable temperature of 
the region. In consequence of this peculiarity the determination of 
the life zones in Washington, so as to coordinate them with adjacent 
regions, presents unusual difficulties. Six life zones or life areas are 
represented, namely, the Arctic, the Hudsonian, the Canadian, the 
Humid Transition, the Arid Transition, and the Upper Sonoran. 
The first-mentioned zone, the Arctic, is sharply marked, consist- 
ing of the alpine flora above timber line. These alpine meadows 
