246 BOTANICAL GAZETTE [OCTOBER 
cause of changed conditions which Alfred Russell Wallace 
emphasizes as of importance in the modification of species.® 
The cycle begins in a mountainous tract with least facility for 
migration of species, and ends in broad lowlands, which favor 
the easy migration and wide distribution of plants and animals.’ 
The distribution of plants in the region of the southern 
Appalachians demonstrates the above-mentioned facts. Kearney® 
has called attention to the presence of a Lower Austral (austro- 
riparian) element in the flora of the mountains of North Caro- 
lina. Over one hundred species, which are most abundant and 
most widely distributed in the austro-riparian area, are known 
to occur in the mountains at an elevation of 1,000 (300™) or 
more. Below that altitude the flora of the southern Appalachian 
region is mainly Carolinian, and the presence in its midst of 
numerous austro-riparian forms would be expected. The occur- 
rence, however, of Lower Austral species at higher elevations in 
the midst of a chiefly transition flora is the noteworthy fact in 
the distribution of plants in the southern Appalachians. In 
studying this flora, one soon reaches the conclusion that it com- 
prises two categories of species, which are markedly different, 
not only in their systematic relationships, present distribution, 
and past history, but even to a considerable degree in their 
ecologic constitution. Two types of plants may be distin- 
guished : those of neotropic origin, which have in all likelihood 
made their first appearance in the Appalachian region in geologi- 
cally very modern times, probably after the close of the glacial 
epoch; and those not of neotropic origin, which probably repre- 
sent the more or less modified descendants of the flora that in 
later Eocene or Miocene time extended to high northern lati- 
tudes, represented in eastern North America by two closely 
allied species, one in the coastal plain, and the other in the 
Appalachian region. 
In order to explain the facts in the case, botanists have had 
® WALLACE, A. R., Darwinism, chap. v. 
7WoopworTH, J. D., The relation between base leveling and organic evolution. 
The ace Geologist 14:217. 1894. 
8 KEARNEY, The lower austral element in the flora of the southern Appalachian 
region. Science N. S. x 
