4 BIG SPRING PRAIRIE. 
The climatic factors of irregular and scanty rain- 
fall and the general low humidity of the atmosphere are 
potent causes of the western type of prairie. ‘These 
factors in combination with the occurrence of frequent 
prairie fires are sufficient to account for the absence of 
trees upon this type of prairie. 
The origin, development, and future of the eastern 
type of prairie must be considered apart from the west- 
ern type, as each possesses its own peculiar factors and 
specific characteristics. Prairies may be studied by a 
comparative survey of the likenesses and differences of 
the various types of prairie areas, and the causes which 
led to them. Again we may study each type compara- 
tively with the topographic features and vegetative 
characteristics immediately surrounding it. This latter 
survey should be conducted along the border line of 
prairie and forest, where the struggle can be said to be 
fairly on, and where the effects and results of this rather 
uneven struggle can be most advantageously observed. 
A life-time is frequently long enough to observe the 
forest encroachment upon the eastern prairie, ages 
might be required to note the same degree of encroach- 
ment upon the western type of prairie. 
Many theories have been advanced to account for 
the absence of trees on prairies; as the prairie fire theory; 
the fine soil theory; the ulmic acid theory; ete. 
Lesquereux states that prairies are formed from old 
lakes, or bays extending from them with the marsh as 
the intermediate formation, and he furthermore asserts 
in regards to these: ‘‘Trees never invade them, never 
grow upon them, even when drained.” This latter 
statement may need some modification, as observation 
does not bear out the assertion “even when drained.” 
In the West, the prairie is the natural formation, 
the presence of the forest is especially to be explained. 
In the East, the forest formation is the natural one, 
the presence of the prairie is to be explained. The 
