KINDS 173 



copious species of restricted adjustability. Sparse plants 

 likewise alternate, but they necessarily play a much less 

 conspicuous part. In habitats not too heterogeneous, a 

 large number of species are sufficiently adjustable to the 

 slight differences that they occur throughout the formation. 

 Often, to be sure, they show a characteristic response, ex- 

 pressed in the size or number. This is illustrated by the 

 facies and many of the principal species of the prairie for- 

 mation. Festuca, Koelera, Panicum, and Andropogon occur 

 throughout, except in the moist ravines which are prac- 

 tically meadows. Astragalus, Paoralea, Erigeron and Aster 

 grow everywhere on slopes and crests, but they are much 

 more abundant in certain situations. Other plants. 

 Lomatium, Meriolix, Anemone, Pentstemon, etc., recur in sim- 

 ilar or identical situations upon different hills. Lomatium 

 alternates between sandy or sandstone crests, Meriolix and 

 Pentstemon occur together upon dry upper slopes, while 

 Anemone alternates between dry slopes and crests. 



Owing to the accidents of migration and competition, 

 similar areas within a habitat are not occupied by the same 

 species, or group of species. A species found in one area 

 will be replaced in another by a different one of the same, 

 or a different genus. The controlling factors of the area 

 render imperative an essential ideality of vegetation and 

 habitat form, though in systematic position the plants may 

 be very diverse. Such genera and species may be termed 

 corresponding. The relation between such plants is essen- 

 tially alternation : it should, uerhaps, be distinguished from 

 alternation proper as corresponsive. The prairie formation 

 furnishes a good example of this on exposed sandy crests, 

 upon which Lomatium, Gomandra and Pentstemon alternate. 

 Formations exhibit a similar correspondence. 



All species that alternate show a variation in abundance 

 from one area to another. Frequently, the difference is 

 slight, and may be ignored, except in determining abund- 

 ance. Very often, however, the variation is so great that a 

 facies may be reduced, numerically, to the rank of a prin- 

 cipal species, or one of the latter to a secondary species. 



