208 Part III. Chapter 3. 
upon the time when the swamps came into existence as swamp habitats. If 
they have existed since the days of tundra conditions, they may show a bog 
flora today. If they are of recent origin, the plants will correspond to the 
normal swamp plants of the present conditions. We may say, that the chances 
of capturing newly exposed land areas at the present time are all in favor of 
the swamp plants, largely because of the greater production of seeds, more 
adequate means for seed dispersal and better adaptations to present climatic 
conditions‘). In early postglacial times, however, the conditions were far dif- 
erent. The climate being more boreal in its character favored the bog 
plants so that, they became practically the only competitors for the low- 
ground situations. 
The spreading northward of the coniferous trees, as the third great 
wave of vegetation, was much slower in the west than in the east, because 
the local glaciation lasted much longer in the west than in the cast for many 
of the mountains there today are glaciated. However, the conifers of the 
southwest suffered a gradual destruction with the disappearance of the inland 
sea and the recession of the glaciers due to increase in temperature, as com- 
pared with the rainfall. It is possible that the rainfall in Nebraska was never 
any greater than at the present time, and trees, such as Pinus ponderosa var. 
scopulorum, found in the central part of Nebraska*), far removed from the 
main area of their occurrence, seem to indicate that with the decrease in trans- 
piration accompanying decrease in temperature certain trees might have been 
more widely distributed during glacial times, which cannot live now in Ne- 
braska under present conditions. The bog plant associations perished with the 
coniferous forests and their southwestern boundaries today correspond with 
that of the forest. The delay in the northward migration of the coniferous 
forests of the Rocky Mountains and of the Pacific coast incident to the pres- 
ence of extensive glaciers, which poured their ice streams down into the valleys 
and on the Pacific coast into the ocean, established the difference which we 
find in the distribution of the forest trees in northern Canada (the Mackenzie 
Basin) and Alaska at the present day. The forests of central, most northern 
Canada, are of the Atlantic type consisting of such trees as black and white 
spruces, Picea nigra (= P. mariana), P. alba (= P. canadensis), balsam Aöxes. 4 
balsamea, tamarack Larix americana (= P. laricina) canoe birch Betula pay“ 
rifera, balsam poplar Populus balsamifera and aspen Populus tremuloides, 
trees of essentially eastern origin which migrated north after the retreat of the 
continental glaciers. In Alaska, the species which are capable of existing under 
the inclement conditions of Alaska, are eastern species ranging a : 
the continent, including the canoe birch, balsam poplar, aspen and white 
spruce. ' These reach the Pacific coast at Cook Inlet and thus head off, so 
1) TRANSEAU, E.N.: On the geographic Distribution and geological relations of the bog 
plant Societies of North America. Botanical Gazette KXXVI: 407-420. Dec. 1903. 
2) BESsEY, E.C.: The Forests and forest Trees of Nebraska. Annnal Report Nebraska Stalf 
Board Agriculture 1888: 93, 
