ALTERNATION 2gI 
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 Lomatium, 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. Very 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- 
