THE PLEISTOCENE CLISERES AND COSERES. 375 



sequence, the original tundra was overwhelmed by the advancing ice, but not 

 before it had established itself as a new tundra zone on the site of the original 

 bog-scrub climax. In similar fashion, the latter replaced the conifers, and 

 these di^laced the deciduous trees in the northern part of their original zone, 

 and caused them to crowd into the subtropical belt. Each further invasion 

 of the ice b\iried the tundra vegetation in front of it, while the latter again 

 invaded the bogS, and the corresponding wave of invasion passed over the 

 entire series. As a final consequence, the original tundra, bog, and conifer 

 climaxes were all overwhelmed, while corresponding zones were now foimd 

 about the edge of the ice, though much compressed and perhaps interrupted 

 as well. It is not improbable that tundra, bog, and coniferous forest may- 

 have alternated with each to some extent near the edge of the ice, but it does 

 not seem possible that their general disposition was other than zonal (fig. 36). 



This general relation must have persisted as long as the ice remained at a 

 maximum, varied only by the climatic oscillations of minor cycles. With the 

 first permanent melting and the withdrawal of the edge of the ice, a bare 

 area for colonization and succession was produced all along its front. Since 

 this bare area lay between the ice and the tundra chiefly, the invasion into 

 it was primarily from the latter. Migrules from the scrub and conifer zones 

 doubtless entered it to some degree, but the prevalent arctic conditions made 

 their ecesis very diflScult, if not impossible. The direction for efficient migra- 

 tion at the same time became the reverse of that diuing the southward move- 

 ment. The most successful ecesis was now in the northward direction, and 

 the migration of the tundra flora into the bare zone was as much in response 

 to the increasingly imfavorable conditions in its own zone as to the ease of 

 invasion in the new and unoccupied zone. As an inevitable outcome, the scrub . 

 advanced into the tundra, the conifers into the scrub, and the hardwoods 

 regained a portion of their original home. This process continued with each 

 fiu-ther retreat of the ice, each zone broadening as it pushed northward, until 

 the close of a grand sun-spot cycle began to cause the ice to halt. The amelio- 

 ration of the climate during an interglacial phase, the length of the latter, and 

 the distance to which the ice retreated are too largely conjectural to warrant 

 discussion here. The successional phenomena were essentially the same in 

 any event, the difference being solely in the number of cUsere climaxes which 

 became organized into the sere of each new climax of the interglacial period, 

 and of the number of seres which may have developed to form a cosere. After 

 a pause of indefinite duration, the ice began to move forward again, and the 

 southward shifting of the clisere was renewed. The length of the interglacial 

 phase is thus a diminishing one; it is greatest at the original southern edge, 

 and least in the region where the retreat is halted and the advance resmned. 

 Indeed, it is not impossible that diiring one of the interglacial phases the ice 

 may have begim to advance without a measurable pause. Northward of the 

 region where the ice came to a halt it is obvious that there was no interglacial 

 period. 



The behavior of the mountain clisere in general must have been similar to 

 that described. The movement was necessarily downward and upward, as the 

 alpine glaciers alternately increased and decreased in extent, but its climatic 

 relations were the same as in the continental clisere. The shifting of zones 

 must have been pronounced in the northern Rocky Mountains, for example, 



