410 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’? 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, Leverett,” 
Salisbury,75 Upham,” 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. Rept. Brit. Ass. 
Adv. Sci. rg00: 334. 
“CHAMBERLIN, T. c, oe " American glacial sy yes Jour. Geol. 
8: 270; rth America, Geikie’s /ce Age, 3d ed. p. 274. 1894- 
- ees F. , Changes of climate indicated by interglacial beds. Proc. Bost. 
Soc. Nat. Hist. 455. 1890.—The Illinois glacial lobe. Mon. 38, U. S. G. S.—The 
glacial Gansidiesle ‘and drainage features of the Erie and Ohio Basins. Mon. 41, 
U.S.G.S 
5 SALISBURY, R. D., and ATwoop, W. W., The geography of the region about 
Devils 9 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. Je 5 
1902. 
** UPHAM, W., The Glacial Lake Agassiz. Mon. 25. U. S. G. S. 1896. 
