416 BOTANICAL GAZETTE [DECEMBER 



notable, except in the case of the Dulichium and cottongrass. 

 The larch and birch have winged seeds, while the remainder 

 would seem to be dependent upon transportation by birds and 

 water currents. But the fact that the plants have survived the 

 ice advances proves that they were easily able to establish them- 

 selves in new areas as rapidly as the climate changed. Not less 

 than five such geographic migrations of more or less latitude, 

 corresponding with the five glacial epochs, must have occurred. 

 Between them were intervals- when the temperature, as shown by 

 plant and animal remains 12 found in interglacial deposits, was 

 fully as high as at the present time. If we consider this proved, 

 then the only glaciation which could materially affect the distri- 

 bution of our boreal societies today is that of the last or Wis- 

 consin epoch. Through the work of Chamberlin, 13 Leverett, 14 

 Salisbury, 15 Upham, 16 and others, the limits of this ice invasion 

 have been definitely mapped. 



In order to get an idea of the distribution of the boreal plant 

 societies during the maximum glaciation, let us try to picture 

 what would become of these same societies if a similar period of 

 glaciation were to come upon them now. A sufficient time has 

 probably intervened since the last glacial epoch to allow of 

 almost perfect climatic adjustment on the part of the tundra and 

 conifer societies, so that the climate now most favorable for their 

 development may well have characterized a zone just beyond the 

 ice margin. This zone would gradually move with the increase 

 of the ice fields until it would come to occupy the position shown 

 in fig. 2. According to Chamberlin, the climatic conditions pre- 



"CoLEMAN, A. P., Glacial and interglacial beds near Toronto, Jour. Geol. 9: 285. 

 1901. PENHALLOW, D. P., The Pleistocene flora of the Don Valley. Kept. Brit. Ass. 

 Adv. Sci. 1900: 334- 



X 3 CHAMBERLIN, T. C., Classification of American glacial deposits. Jour. Geol. 

 8: 270; The glacial phenomena of North America, Geikie's Ice Age, 3d ed. p. 274. 1894. 



14 LEVERETT, F., Changes of climate indicated by interglacial beds. Proc. Bost. 

 Soc. Nat. Hist. 24 : 455- 1890. The Illinois glacial lobe. Mon. 38, U. S. G. S. The 

 glacial formations and drainage features of the Erie and Ohio Basins. Mon. 41, 

 U. S. G. S. 



^SALISBURY, R. D., and ATWOOD, W. W., The geography of the region about 

 Devils Lake and the Dalles of the Wisconsin. Bull. 5, Wis. Geol. and Nat. Hist. Sur. 

 SALISBURY, R. D., Glacial geology of New Jersey. Rep. State Geologist N. J. 5. 

 1902. 



16 UPHAM, W., The Glacial Lake Agassiz. Mon. 25. U. S. G. S. 1896. 



