SIMILAR SPECIES REPLACE EACH OTHER 479 



buildings from Massachusetts to Missouri contain such weeds 

 as the dog fennel, the low mallow (" cheeses "), motherwort, 

 catnip, and some smartweeds. 



A little study of such cases soon leads one to the conclusion 

 that these plant associations and multitudes of others exist 

 because all the plants in each association are adapted to their 

 special environment. Wherever such an environment occurs 

 such an association will be found in it, 1 or, if not already there, 

 will nourish when introduced. 



457. Similar species replace each other. Two sets of locali- 

 ties 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, 2 while far 

 southward, in sandy soil, its place is taken by the long-leaved 

 pine. 3 Along streams and swamps northward the speckled alder 4 

 is generally found, while southward the smooth alder 5 is most 

 common. In rich w^oods of the northeastern United States the 

 painted trillium 6 and the erect trillium (" Benjamin," or " squaw 

 root ") 7 are the commonest species, while farther south, in sim- 

 ilar localities, the sessile trillium, 8 Underwood's trillium, 9 and 

 the large-flowered trillium 10 are abundant. 



In all such cases and they are very numerous we are 

 to infer that the genus is peculiarly well adapted to some spe- 

 cial set of conditions, as sandy soil, brooksides, or the rich, 

 shaded soil of woodlands. The more northerly species are capa- 

 ble 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 



1 That is, in localities not separated by such natural barriers as seas, 

 high mountains, or deserts. 



2 Pinus rigida. 5 A. serrulata. 8 T. sessile. 



3 P. palustris. 6 Trillium erythrocarpum. 9 T. Underwoodii. 



4 Alnus incana. 7 T. erectum. 10 T. grandiflorum. 



