or 
ras) 
or 
dependent upon climatic conditions. He also showed that the species 
comprising the vegetation of any center are more abundant, attain a 
greater size, and have a wider range of habitats within the limits of the 
center of distribution than elsewhere. These same conditions apply to 
the prairie. In addition to the climatic, edaphic, and biotic factors men- 
tioned above, prairie fires are also important locally and have been va- 
riously emphasized from time to time.* 
To sum up, climatic factors are important in determining the gen- 
eral boundaries of distribution of the prairie, while edaphic factors are 
important in determining the origin and character of the prairie asso- 
ciations within these boundaries. The edaphic factors become more and 
more prominent toward the edges of the prairie, as in eastern Illinois. 
When a prairie association is once established, biotic factors and prairie 
fires are important in checking invasion by forest vegetation. 
The location of Illinois at the eastern boundary of the prairie makes 
it a region of considerably more than local ecological interest. As a 
prairie state it has awakened interest since the time of the first settlers, 
several of whom have recorded brief but glowing descriptions of the early 
prairies. Of these older descriptions, one of the most important was 
written in 1857 by Frederic Gerhard (7), who greatly enriched his pub- 
lication by appending a forest and prairie map of the state compiled 
by Dr. Frederick Brendel of Peoria. This map shows the relative dis- 
tribution of prairie and forest at that time, and a reproduction of it by 
Barrows (1) is included in the present article (page 526). 
Unfortunately these pioneer writers, accustomed to life in a forested 
region, were generally unfamiliar with the plants of the prairie and 
failed to leave us an adequate account of the natural prairie flora. Further- 
more, the importance of a knowledge of the relationship of plants to 
the environment in which they live was not recognized until a later date. 
Indeed the dynamic and genetic relationship of plant associations was not 
fully appreciated until Cowles (4) pointed out its significance in 1899. 
As a result of this belated appearance of modern ecological conceptions 
the earlier accounts of the prairie are notably lacking in a record of the 
different associations of prairie plants and of their composition and suc- 
cessional relationships. The published accounts deal chiefly with asso- 
ciations on the broken soil of slopes or with those along forest borders 
conspicuous for their great display of coarse herbs, and leave the errone- 
ous impression that the original prairie flora was a mixture of a great 
variety of species. This impression, however, is offset by the occa- 
sional and significant reference to the prairie as a ‘‘sea of grasses” and by 
the statement of C. W. Short in 1845 that “Its leading feature is rather 
the unbounded profusion with which a few species occur in certain 
localities than the mixed variety of different species occurring every- 
where”. 
* For an historical résumé of the theories of the prairie see Shimek (24), ‘The 
Prairies.” 
