RELATIONSHIPS OF METASPERMAE. 605 



or to perish. Those for any reason well-fitted to migrate 

 were selected for re-establishment at successively lower lati- 

 tudes. Under the relentless overwhelming of the epoch large 

 numbers of plants were ejected forever from the Minnesota 

 valley, others were so modified in their movement south and 

 return to the north that they appear to-day in new specific 

 forms, while a large number of new forms, developed princip- 

 ally in the group of the Metachlamydeas, have been permitted to 

 gain a foot- hold. The palms and sequoias have been driven 

 out of all this central North American region, the palms to 

 maintain a precarious existence in tropical or insular regions, 

 the sequoias to their last stand in the limited area of the 

 Sierras, where they still continue their losing fight as the 

 remnants of an almost extinct race of vegetable gianis. The 

 enormous size of the "big trees" of Calaveras county, has, 

 however, one interesting word to tell us of the northern forests 

 that were once their home. The very fact of their spread- 

 ing their leaves to the light at a height of 300 feet above the 

 surface of the earth gives us a hint of the tremendous extent 

 to which solidarity of the Tertiary coniferous forests had 

 progressed and permits us to understand how stern had 

 become the competition for light in view of which such bulk 

 was necessary for the preservation of the species. Just as in 

 the formidable defensive armor of some extinct armadillo one 

 may read somewhat of its struggle with its enemies, so in the 

 one hundred meters of solid trunk and in the massive girth of 

 a living Sequoia gigantea one may learn of its struggles in the 

 ancient forest of Cretaceous and Tertiary times, when its allies 

 and competitors were alike more numerous. 



Of all the plants which went south before the first invasion 

 of the glacial sheet none showed greater capacity for variation 

 and improvement than the ancestral forms of the modern 

 dominant family of the Compositae. Such plants as, by per- 

 mitting their seeds to fly before the prevalent north winds or 

 to attach themselves to the fur of migrating bison, mastodons 

 or other animals, had achieved a lower latitude were of course 

 assisted upon their return by the same characters. During 

 interglacial time they doubtless established themselves upon 

 the till of the Minnesota valley and underwent comminglings 

 such as those of to-day. As calculated by Winchell from the 

 study of abandoned gorges of the Mississippi valley, the inter- 

 glacial period was approximately 9,750 years in length and 

 this period as stated by the investigator named, would have 



