230 CLIMAX FORMATIONS OF WESTERN NORTH AMERICA. 



DistribtUion of dominants 



The occurrence of the 13 dominant genera practically throughout the four 

 regions seems conclusive evidence of their formational unity. This is em- 

 phasized by the distribution of the genera of subdominants as well. The 

 preeminence of Carex is obvious, as well as the importance of Poa. Juncus 

 probably has a higher value than it deserves, owing to the fact that some sub- 

 cUmax species persist into the climax. The typical character of the Petran 

 region is shown by the fact that nearly 90 per cent of the dominants are found 

 in it. The number of endemic dominants in any one of the regions is so small 

 as to be negligible. The close relationship to the Eurasian arctalpine climax 

 is evident, as well as the fact that this is largely due to Carex, Juncodes, and 

 JunciLS. 



Relationship and contacts. The primary relationship of the arctalpine 

 climax is with the corresponding Eurasian formation. The number of domi- 

 nants and subdominants common to both is sufficiently large to suggest that 

 they should be regarded as associations of the same formation. In this re- 

 spect, however, their difference is greater than their similarity, as one who has 

 seen both must readily recognize. There can be Uttle question that the two 

 climaxes have originated from a common ancestral community. The arctal- 

 pine chmax is also related to a similar community on the high peaks of Mexico. 

 The two have many genera in common, but the species are nearly all different 

 and the r6le of the grasses is emphasized at the expense of Carex. In the pres- 

 ent state of our knowledge, it seems best to regard the alpine meadows of 

 northern Mexico as a transition between the arctalpine cUmax and an Andean 

 alpine climax. Finally, there are certain resemblances between the alpine 

 meadow and the short-grass plains which suggest a broader contact than exists 



