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A revision of the genus Milella etc. 395 



lo a 1 0-stamened ancestor. There is the possibility that the M. Breweri- 

 M. ovalis Hne could have arisen from M. caulescens but it seems more 

 reasonable to assume that the origin would have to be looked for in the 

 more general basic region than in the more specialized terminal one. 



The same tendency towards the reduction of the size of the flower, 

 the sinking of the ovary deeper into the hollow axis and the reduction in 

 the number of divisions of the petals that obtained in the other section, 

 characterize both these branches. M. ovalis of the Jf. Breiceri branch has 

 a completely inferior ovary and the petals have from 6 to 3 divisions. 11 

 marks the termination of this line of development. 



The M. Pentandra branch is characterized by having the stamens 

 placed opposite the petals and by the development of a prominent disk 

 which more or less covers the top of the ovary. This branch is perhaps 

 not as direct a series as our diagram would indicate, for it is probable 



M, paiiciflora and M, 



IS 



point. 



M. 



M, 



entire petals. 



1 



{ A consideration of the geographical distribution of the various species 



throws considerable additional light upon their relationship and a reference 



I to the accompanying chart will help to bring this out. 



The primary centre of development of the genus lies in the mountain 

 region of southern British Columbia, western Montana, Idaho, Washington, 

 Oregon and northern Cahfornia. A secondary centre has developed in 

 southern Japan. As far as present distribution can throw light on what 

 has gone before it seems most probable that the genus originated in Alaska 

 and that in Tertiary times it must have extended northward to beyond 

 the barrier of the Rocky Mountains. From the Alaskan place of origin it 

 spread out or wandered in three directions. The two oldest species M. 

 diphylla and M. nuda migrated southeastward through the forest country 

 of Canada in preglacial times as far as the Atlantic ocean. The present 

 isolation of M. diphylla in eastern North America must be ascribed to the 

 glacial period. This species, being adapted to temperate climates and low- 

 altitudes has subsequently not been able to penetrate farther northward 

 again than to about the 47" parallel of latitude. Furthermore being a 

 woodland species its westward progress has been determined by the limit 

 of the decidious forests. 



The North American distribution of M. nuda practically co-incides 

 with the geographical area of Picea canadensis. In Asia it extends west- 

 ward as far as the Yenisei River and south to the latitude of Lake Baikal. 

 The wide geographical range of this species is the more remarkable when 

 il is borne in mind that it has no special contrivances for seed distribution; 

 and it would indicate that the species is of great age. 





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