(150) 
natural barriers and boundaries. For the United States and por- 
tions of Mexico these have been established with considerable 
detail, but for the West Indies, Central America and the greater 
portion of Mexico practically nothing is known. For the West 
Indies it is known that a definite migration route, for both animals 
and plants, extends northward from South America through the 
Lesser Antilles to Porto Rico, Hispaniola, Cuba, the Bahamas, 
and Florida. Minor centers of distribution among these islands are 
not so evident, but the number of species limited either to one island 
or to a few neighboring ones is so great that each may almost be 
regarded as a center in itself. The intervening water is certainly 
an effective barrier to the ready dissemination of many species. 
In Mexico and Central America a similar migration route extends 
from South America into the United States, and it is an easy mat- 
ter to trace the species of Vernonieae in the United States back to 
an origin in Mexico. But definitely circumscribed floral areas can 
not yet be located there because of lack of data. The work is still 
further hampered by the variation in altitude which is so great as to 
become of the highest importance for the distribution of plants in 
its relation to temperature and rainfall. 
There are in the eastern United States several well-marked centers 
of distribution from which the various species of the flora have 
spread andare still spreading. Some of these are entirely northern 
and consequently of post-glacial origin, and have nothing to do 
with a genus of such tropical origin as Vernonia. Four fairly 
well-marked centers, two in the southeast and two in the south- 
west, lie south of the glacial boundary, and have existed at least 
since inter-glacial time. ach of these is characterized by certain 
species of Vernonieae, some of which have migrated to the north- 
ward since the retreat of the glacial ice. 
The Appalachian center is the best known of these four, although 
it was not until 1902 that Adams * first recognized it as such and dis- 
cussed itin detail. It includes the whole of the lower Appalachian 
region, and is marked both by the extraordinary number of species and 
by the occurrence of numerous relics, endemic species, and monotypic 
genera. Migration routes from this center extend to the north and 
northwest, mainly along the uplands, and by far the largest part of 
*Southeastern United States as a center of geographical distribution of 
fiora and fauna. Biol. Bull. 3: 115-131. 1902. 
