228 TREES AND TREE-PLANTING. 
dant on the more sandy soils, and the post, black-jack, 
and laurel-leaved oaks have their favorite locality on 
formations of finely siliceous soils. In middle latitudes 
and northward, on rich lands, the burr oak will prevail. 
The white elm yields its finest results on humid lands, 
while the red elm prefers a drier and more porous but 
rich soil. The walnut is found in its grandest propor- 
tions only on the richest river alluvions, but the butternut 
finds its congenial home among the rocks of the north- 
ern valleys. The shellbark hickory prefers clay flats ; 
the pecan, rich river alluvions; while the large shellbark, 
with the pignut, are most abundant on fertile, rolling 
uplands. Of these great classes, we observe that nature 
usually groups certain species more or less exclusively 
together. “In one region, or on one area, there are pines, 
chiefly of single species; in another tract the spruces 
or the firs will prevail; and so, too, among the broad- 
leaved trees, made up of many genera and species, and 
apparently mingled rather promiscuously together, the 
willows and poplars will be more or less grouped by 
themselves; the oaks will prevail here, the maples and 
ash there, and the magnolias will prevail on one side. 
The various species of trees seem to have their prefer- 
ence for this or that locality, and appear more or less 
abundantly in this or that position. Independently of 
these results, that seem traceable to the influence of 
soils and elevation, in connection with latitude, the nat- 
ural grouping of species, either separately or combined, 
must often depend upon accidental circumstances. Wil- 
lows and cotton-woods shed their numerous light seeds 
at a season when they are floated upon the swollen wa- 
ters of our streams, and as the floods subside they are 
stranded upon the emerging sand-bars, where they find 
a favorable soil, and burst into growth in immense num- 
bers of a single species. The burned pine forests of 
mountain regions receive the seeds of the aspens that 
are often sown over wide tracts in the same way; and 
