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396 



C. 0. Rosendahl. 



The two main stocks of the genus which developed in tlie area of 

 origin also began moving southward in the mountains during pre-glacial 

 times and in their progress soon evolved several new species. 



The M. Breiveri — M. ovalis group branched off early and attained 

 the greatest southward range of the west American species. Of these two 

 M. Breiveri is much the older judging both by its greater geographical 

 distribution and by the greater number of variations or divergences from 

 the type. On the various outposts of its range several more or less 

 distinct forms can be differentiated, but these have as yet not diverged 

 far enough to be regarded as species or even as good varieties. 



The relationship of M. ovalis with M, Breiveri is so obvious and un- 

 mistakable that there is no possible doubt as to its origin. It represents 

 the lowland extension of the alpine ancester. Its distribution is confined 

 to a comparatively narrow strip of the Pacific coast from northern Cali- 

 fornia to Vancouver Island and it is undoubtedly one of the youngest of 

 all the species. 



The M. pentanclra group probably arose within the original centre 

 and the species has spread far southward to southern Colorado in the 

 Rocky Mountains and nearly to middle California in the Coast, Cascade 

 and Sierra Nevada Mountains. It is more uniform than many of the other 

 species and it is only on the isolated high mountains like Mt. Hanier that 

 occasionally diverging forms appear. From this stock a branch diverged 

 early and spread westward across to eastern Asia and wandered south- 

 ward to japan. It has subsequently become isolated in the southern half 

 of the Island Empire and has developed into three clearly related yet well 

 marked species. 



The most direct phylogenetic line is the one leading from M. nuda 

 to M. caidescens. It is probable that the latter species originated very 

 close to or probably within the area that it occupies today. Its somewhat 

 limited distribution would indicate a comparatively recent origin. 



The species evolved from the M. diphylla stock have held more to 

 the eastern parts of the Rockies than those developed from the M. nuda 

 stock although a branch has extended westward into the Cascade and 

 Coast ranges. 



M. trifida is probably the oldest of the species in this relationship 

 and is one of the most polymorphous of the whole genus. Numerous 

 forms could be differentiated and it is evident that the species is still in 

 a very active slate of evolution. Variety violacea forms the southward 

 extension of the species in the Rocky Mountains of Montana and appears 

 fairly constant. It occurs northward in British Columbia with the species. 

 M. stauropetaki is more southern in its range and seems to represent the 

 termination of the stock in this direction. It runs through the correspon- 



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