882 THE AMERICAN NATURALIST. | [Vor. XXXIX. 
ences of elevation and relative geological age tend to be strongly 
expressed in the vegetation (Pinchot and Ashe, '97, pp. 143-181 ; 
Mohr, :otr, pp. 107-133; Harshberger, :04, pp. 611-614). 
The Conifers probably represent the climax forest of this for- 
mation. The distribution of certain of its components is shown 
in Figs. 4 and 5. Several other associations of shrubs and 
trees might be similarly depicted and still further emphasize the 
identity of the Southeastern Conifer forest center. As shown 
by the accompanying maps and the descriptions of Smith, and 
Pinchot and Ashe, there is a mingling of societies of this center 
and of the Deciduous forest, in the region of the Piedmont 
plateau, which tends to obscure the recognition of the stages 
properly belonging to each. 
On the southern half of the Florida peninsula is a fourth 
formation, made up largely of xerophilous tropical species. It 
really represents the northern border of a center which domi- 
nates the West Indies and tropical America. It may be desig- 
nated the Insular Tropical forest center. 
There appear then to be four centers of distribution in eastern 
North America: (1) the Northeastern Conifer, (2) the Decid- 
uous, (3) the Southeastern Conifer, and (4) the Insular Tropi- 
cal. Each is made up of many societies, which bear a definite 
successional relationship to one another. 
With the exception of the tropical, each of these formations 
has its western border marked by gradation into the grasslands 
ofthe Great Plains. The local flora of any part of the interme- 
diate prairie region is composed of societies from the plains and 
the adjoining forest centers. For example, take the succession 
of plant societies on the bluffs of the Kansas River in eastern 
Kansas. The pioneer society is made up of Bouteloua hirsuta, 
Mentzelia oligosperma, Euphorbia marginata, E. dentata, E. 
petaloidea, Bebera papposa, Artemisia ludoviciana, Aster sericeus, 
A. fendleri, Megapterium missouriense, Tragia ramosa, Baptisia 
bracteata, B. australis, Lacinaria punctata, Croton texensis, Solt- 
dago missouriensis, and Silphium laciniatum. This society be- 
longs to the flora of the Great Plains and has its eastern limit 
in the prairie belt. The shrub stage following this is made up 
principally of Symphoricarpos symphoricarpos, Ceanothus ovatus, 
