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 



