Chapter IX — 155 — On Migration 



Refuges of Species diiring Glacial Periods and Migrations of 

 Species: — The striking fact that the same species are found in the 

 Arctic and on mountain peaks, in localities separated from one another 

 by the lowlands of the temperate zone, where these species cannot 

 grow, was explained by Forbes and Darwin as an effect of climatic 

 conditions during the Ice Age. As the great ice-sheet advanced south- 

 ward, the arctic flora also migrated in the same direction, taking the 

 place of representatives of the flora of the temperate zone that had 

 died out or had likewise migrated. During the interglacial epochs, as 

 the climate became warmer, the arctic species retreated to the north 

 and to the mountain tops. 



During the glacial epochs the species of the north temperate zone 

 migrated southward, following chiefly the great mountain systems, into 

 southern Europe, southern Asia, Africa, South America, and that part 

 of North America south of the ice-sheet, i.e., to the unglaciated regions, 

 where pluvial periods coincided in time with the northern glaciations. 

 During the dry, warm, interglacial epochs, on the other hand, desert 

 xerophytes spread from Africa northward into the temperate zone. 



After the final retreat of the ice-sheets and the shrinking back of 

 the snow-line the mountain slopes and enormous expanses of territory, 

 on which the former vegetation had been destroyed, began gradually 

 to be re-stocked with plants, which spread northward following close 

 in the wake of the receding ice. The mixed flora, composed of arctic 

 species that had previously migrated southward and were growing on 

 the fringe of the ice-sheet and of alpine species that had descended 

 from the mountain tops, now returned to the north and to the alpine 

 heights, inhabiting the territories recently freed from ice. 



In mountainous regions located at more southern latitudes, where 

 the vegetation was affected to only a slight degree by glacial phenom- 

 ena, and also within the hmits of the Alps themselves, where un- 

 doubtedly even in regions covered by glaciers there were cliffs, 

 canyons, and sheltered spots that remained free of the ice cover- 

 ing, the pre-glacial flora was preserved (see Engler, 1916, and 

 Briquet, 1908) and there took place an intermingling of the new- 

 comers and the indigenes. An analysis of the present species composi- 

 tion of the flora of the Alps reveals with all clarity its mixed character 

 (Jerosch, 1903). 



As the climate became still warmer and the fringe of the retreating 

 ice-sheet withdrew farther north, the mixed arctic-alpine flora was 

 crowded farther and farther north by the forest flora, the species com- 

 position of which had changed, as we have seen, corresponding to the 

 climatic changes that had taken place. The freed territory was like- 

 wise colonized by herbaceous plants. Paleobotanic data give us 

 practically no aid in judging as to the composition and origin of the 

 elements of this flora. We needs must resurrect the history of its 

 formation on the basis of an analysis of the species composition of the 

 flora occupying this territory to-day. 



Whence came this vegetation on territory covered with ice for 

 thousands of years? There is no doubt that the re-stocking of this 

 territory did not occur all at once but at various times and that the 

 invading species came not from one but from many places, from many 



