ALTERNATION 



291 



cies 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 imper- 

 ative an essential identity of vegetation and habitat form, though in system- 

 atic position the plants may be very diverse. Such genera and species may 

 be termed corresponding. The relation between such plants is essentially 

 alternation ; it should, perhaps, be distinguished from alternation proper as 

 corresponsive. The prairie formation furnishes a good example of this on 

 exposed sandy crests, upon which Lomaiinni, Comandra, and Pentstemon 

 alternate. Formations exhibit a similar correspondence. 



Fig. 74. Numerical alternation of Pinus and Pseudotsuga upon east 

 and west slopes. 



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 abundance. V.ery often, however, the variation is so great that 

 a facies may be reduced, numerically, to the rank of a principal species, or 

 one of the latter to a secondary species. This phenomenon is distinguished 

 as numerical alternation. It arises from the fact that the similar areas are 

 sufficiently different to affect the abundance, without producing complete 

 suppression. It is probable that this result is due almost entirely to compe- 

 tition. Astragalus crassicarpus grows on all the slopes of the prairie forma- 

 tion, but on some it has the abundance of a facies, while on others it is repre- 



