524 
comes more and more evident and may tip the balance either in favor of 
forest or of prairie. Similar conditions exist between prairie and plains 
vegetation on the western edge of the prairie. But as we approach the 
center of the prairie region edaphic factors become less prominent, and 
it is here that we find prairie on the greatest number of edaphic habitats. 
Transeau (25,26) was the first to point out the characteristics of 
the centers of distribution of the vegetation of bogs and forests of North 
Map showing ratio of rainfall to evaporation expressed in percentages. 
(After Transeau. ) 
America and to call attention to the meaning and use of the term “center 
of distribution”. As he used it the term does not imply that the plants 
have necessarily spread from the present centers, but that the complex 
of climatic factors most favorable to the development of the type of 
vegetation characteristic of each center is localized there, and that as we 
depart from such centers we find conditions more and more unfavorable 
and resulting in the elimination of such species as are most rigidly 
