84 PLANT LIFE OF ALABAMA. 
The opportunities for the dispersion of species by these means are 
more or less diminished by intervening extensive water and desert 
areas or high mountain ranges. 
These primary and secondary factors, iu their ever-varying combi- 
nations, are the conditions by which the various associations of species 
and their restriction within specific areas of greater or less extent are 
determined. Such areas constitute the floral regions; and the system- 
atic relationship of the different species, their numerical proportions, 
and their various assemblages impart to each region its floral character. 
Suitable environment, that is, a proper combination of conditions of 
moisture, sufficient room and light, proper exposure, etc., determines 
the place in which a plant finds all the requirements for its existence 
met, that is, its habitat. 
The conditions which outline its habitat, in combination with the 
greater factors of latitude, altitude, rainfall, etc., determine the distri- 
bution of plants over wider areas, in which the particular plant (species) 
may find few or many suitable localities, which areas constitute its 
range. Within this range the plant may be found in few or many 
places, isolated or gregarious, but outside of this range it does not 
occur. 
DISTRIBUTION OF SPECIES AS DEPENDING UPON GEOLOGICAL 
HISTORY. 
The distribution of plants can not always be explained on the ground 
of their dependence upon the atmospheric and terrestrial factors. 
The differences in climatic conditions become too insignificant to 
explain the confinement of many species within extremely narrow 
limits, and at the same time topographic and other conditions of 
environment offer no satisfactory account. Premising the theory 
that the existing plants are the descendants of similar types which 
flourished in past periods of the history of our globe, in most instances 
an explanation is easily found on geological grounds. Viewed in this 
light, the occurrence of the hemlock (Zsuga canadensis), for example, 
with its northern companion, the sweet birch, on the extreme southern 
extension of the Allegheny Mountains, in Winston County, Ala., in 
a completely isolated spot hundreds of miles distant from the range 
of its distribution, can be accounted for when they are regarded as 
the sole remnants of the northern arboreal flora which during the 
glacial period was pushed to lower latitudes and which on its recession 
to cooler zones left these trees behind in the narrow valley of the 
Sipsey River, where at present the former shades the cliff-bound 
banks. The Torreya (Zumzon taxifolium) and the Florida yew (Zaaus 
floridana) of the valley of the Apalachicola River in western Florida, 
the American smoketree or chittamwood (Cotinus cotinoides) in its 
isolated localities in north Alabama and southwestern Missouri, and 
