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 

 trillium and the erect trillium ("Benjamin," or "squaw 

 root ") are the commonest species, while far south, in simi- 

 lar localities, the sessile trillium. Underwood's trillium, 

 and the large-flowered trilUum 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. 



