PLANT SOCIETIES 309 
exist because all the plants in each society are adapted to 
their special environment. Wherever such an environment 
occurs such a society will be found in it,! or, if not already 
there, will flourish when introduced. 
380. Similar Species replace Each Other. — Two sets of 
localities alike in many respects but unlike in some points 
are often inhabited by different species of the same genus. 
For instance, the pine barrens of New England and the 
adjacent states are commonly covered with the northern 
pitch pine, while far southward, in sandy soil, its place 
is taken by the long-leaved pine. Along streams and 
swamps northward the speckled alder is generally found, 
while southward the smooth alder is most common. In 
rich woods of the northeastern United States the painted 
trilium and the erect trillium (“ Benjamin,” or ‘“squaw 
root’) are the commonest species, while far south, in simi- 
lar localities, the sessile trilium, Underwood’s trillium, 
and the large-flowered trilium are abundant. 
In all such cases —and they are very numerous — we 
are to infer that the genus is peculiarly well adapted to 
some especial set of conditions, as sandy soil, brooksides, or 
the rich, shaded soil of woodlands. But the more northerly 
species are capable of enduring the severe winters and 
brief summers of their region, while the more southerly 
ones perhaps cannot do so. The relative warmth of the 
climates in which they live may not be the only reason, or 
even the principal reason, for the distribution of such 
plants as those just mentioned, but it is one factor at any 
rate. And it is certain that, on the whole, most of our 
1 That is, in localities not separated by such natural barriers as seas, high 
mountains, or deserts. 
