THE TIME FACTOR AND MIGRATION 45 



at a distance far behind other more mobile forms. Thus in the slow 

 extinction of a pond by the deposition of peat, there is no break in 

 the time-continuity of the contracting zones of vegetation around it, 

 since mobility exceeds the rate of environmental change. But if a 

 pond is drained artificially, producing in a single year an effect similar 

 to that of a century of peat formation, the more mobile species enter 

 first on the freshly exposed area and some years may elapse before all 

 the remainder have appeared. So also the sudden cessation of prairie 

 fires on parts of the prairies led to the immediate advance of the 

 forest, and the more mobile species composed the bulk of the early 

 invaders. In general, migration connected with physiographic suc- 

 cession keeps pace with the environmental changes, so that associations 

 are well marked and the correlation between zonation and succession 

 is prominent. Even such a slow environmental change as the advance 

 or retreat of a continental glacier may have been too rapid for some 

 species, while certainly slow enough for others. Numerous species 

 were doubtless destroyed completely with the advance of the glaciers 

 and others during their retreat, from inability to keep pace in their 

 migration with the changing conditions of the environment. 



The most rapid rate of migration is found in the anthropochorous 

 species which utilize human activities for their dispersal. Such not 

 only move with extreme rapidity, but by the nature of their dispersal 

 are able to cross barriers quite impassable for other species. Our own 

 country has been occupied during the past century by hundreds of 

 such species which have crossed an ocean to reach us and have spread 

 hundreds of miles over the land in a very short time. Yet most of 

 these same species are stopped completely by an area of undisturbed 

 natural vegetation, which they are unable to colonize. 13 



The Time Factor and Migration. — The present status of migra- 

 tion depends upon its rate, as discussed in the preceding paragraph, 

 and on the elapsed time since migration began. The more rapidly 

 moving species may be at the limit of their present potential range and 

 wait upon further environmental change, while the slower ones may be 

 far from their actual limit, although progressing toward it at their 

 best speed. The condition is well illustrated in the belts of forest 

 which parallel the rivers of Kansas and Nebraska, in which, in general, 

 the more mobile species have extended farthest to the west, 14 It is 

 also prominent throughout the prairie region of the Middle West, where 



is Gleason, H. A. & F. T. McFarland. The introduced vegetation of the vicinity 

 of Douglas Lake, Michigan. Bull. Torrey Club 41: 511-521. 1914. 



i* Kellogg, R. S. Forest belts of western Kansas and Nebraska. U. S. Dep. 

 Agr. Forest Bull. No. 66. 1905. 



