94 Distribution of Timber and the Origin of Prairies. | February, 
rains and snow with “occasional though rare seasons of excessive 
dryness” characterize the prairie regions. This seems to come 
nearer explaining general facts, but though the underlying cause 
may be correct, viz: constant moisture, the means. by which it is 
secured is certainly not that needed to explain the particular 
cases before us, for both timber and prairie, in the region under 
consideration, has, as before stated, the same rains and the same 
droughts. 
IV. 
From these theories, then, we have found nothing helpful for 
solving our problems, unless it be the idea of the constancy of 
moisture. Furthermore it seems quite clear that there is nothing 
very variable in the region observed, except the surface of the 
land and the distribution of streams. Can these factors secure 
the constancy of moisture required, in the circumstances where 
we have found trees, and the lack of such constancy where we 
do not find them? Let us see. 
(1.) This constancy of moisture must be in one or both the 
media, in which the trees are located, viz: the soil and the aif, 
and it is reasonable to presume that this constancy is to be looked 
for when the forces of vegetable life are in activity. Excess or 
lack of moisture at other times may be safely ignored. 
(2.) We can readily see that moisture of the soil will be more 
constant on northern slopes than southern, because they are not — 
so much exposed to the sun’s heat. In spring they are many 
days later in drying up, so also after showers, and at certain 
degrees of humidity of the air, the northern slope may act as 4 
condenser, thus collecting the moisture, which the opposite hill- 
side is giving off. These processes also affect the air, tending t° — 
secure a greater constancy on the northern slope. Moreover the 
same slope would be more protected, by its position, from the 
summer south wind, which occasionally is very hot and dry. 
These considerations may sufficiently account for the timber 
occurring on northern slopes and for its absence, in general, from 
southern slopes. 
(3.) The prevailing winds of the region are westerly in sprig 
and summer, This perhaps may sufficiently explain the Pf 
ponderance of timber areas on the east side of streams flow- 
ing south, especially where the streams are of considerable 
~ size, and affording better opportunity for evaporation. Ths 
