1903] BOG PLANT SOCIETIES 409 
migration this vast aggregate of bog societies has made since the 
glacial period. It also represents an order from the tallest 
forms to those raised but slightly above the wet substratum. 
PREGLACIAL DISTRIBUTION. 
Of these fifteen species, three, Dulichium, Sarracenia, and 
Kalmia, are endemic. The larch and birch are represented in 
the Old World by closely related forms, while the remaining ten 
occur in similar habitats in Europe and Asia. This natur- 
ally points to their origin, and certainly indicates their pre- 
glacial distribution to have been in the circumpolar regions of 
both continents. It also implies that these great land masses 
must have been connected for a long time during the Tertiary 
period, so that migration from one to the other was by no means 
difficult. Whether these forms originated in a single polar area 
is of little consequence. They may have arisen partly in 
America, partly in Eurasia, but they were essentially the 
products of similar conditions and by migration came to be 
associated. 
THE GLACIAL MIGRATIONS. 
With the coming on of the cold period, which closed the 
Tertiary and inaugurated such extremes of climate between the 
equator and the poles, the consequent accumulation of ice on 
these northern continents destroyed the ancient habits of these 
plant societies. At the same time semitropical species, which were 
common alike to high and low latitudes, were killed by the increas- 
ing cold, the ground they had covered affording new areas for 
Occupancy. By the reversal of the drainage lines and conse- 
quent destruction of low-ground vegetation, new habitats suited 
to these plants arose in advance of the ice invasion. Just as 
the zones of vegetation in a small lake move toward the center, 
because that is the only direction in which development is pos- 
sible, so these plants spread away from the centers of ice accu- 
mulation. Where this migration moved to the west the plants 
were later on destroyed, but their southward extension brought 
_ them into areas which were not within reach of the subsequent 
ice invasion. Their adaptations for rapid seed dispersal are not 
