Chapter IX —143— On Migration 



As a second indirect proof of his theory Good {I.e., p. i68) cites the 

 relation of plants to edaphic conditions: — 



"Many plants are markedly calcifuge and it is usually impossible to find individuals 

 of the same species which do not conform to this character. Nor will cultivation alter this 

 relation unless it results in the production of new forms with new tolerances, so that the 

 original tolerance and species is hidden or lost. Here there is indeed a distinction between 

 cultivated and wild plants, since the efifect of cultivation may cause, in a comparatively 

 short time, an evolutionary change which, in nature, would have been much longer de- 

 layed". 



Good's theory of the migration of floras, though providing a plausi- 

 ble explanation of the movements of floras from one geographical zone 

 to another, leaves out of account two exceptionally important factors: 

 first, the process of divergence which a species undergoes during the 

 course of its migrations and, second, the movements of floras on moun- 

 tain slopes from one altitudinal belt to another. When a flora moves 

 into a more southern zone, its original place is occupied not only by a 

 flora moving southward from a more northern zone but also by a flora 

 descending from the mountains situated within the limits of the initial 

 zone. These latter elements, as they increase in number, occupy a 

 dominant position. This exceptionally important factor in floral suc- 

 cession is entirely disregarded by Good and, in general, is accorded 

 very Httle attention. Vegetation belts on mountain slopes have un- 

 doubtedly existed during all geological periods and within the limits 

 of all cUmatic zones of the earth, just as they now exist on mountains in 

 the frigid, temperate, and torrid zones. In our opinion the replacement 

 of floras of lowlands by species of mountain slopes, whenever cold 

 climatic conditions have extended their sway over wider territories, has 

 played a very important role in the formation of present-day floras, a 

 circumstance which makes necessary considerable alterations in the 

 migration theory of floral succession. 



Another important point. It has usually been assumed that migra- 

 tions of floras took place only from north to south or from south to 

 north. However, it is now considered definitely established that dur- 

 ing the entire Tertiary period and also later there occurred migrations 

 of species from east to west, from eastern Asia into Europe, at first 

 along the mountain systems and later, as the climate became still 

 cooler, also over the plains, replacing the Tertiary flora of Europe that 

 had become extinct. 



We may supplement the foregoing by a number of arguments of a 

 different kind. We have seen that, in case of species migration induced 

 by changes in habitat conditions, the plants occup3dng that part of the 

 area most subjected to the action of these changes find themselves in 

 disharmony with the new conditions of life. They are forced to mi- 

 grate, following the direction of the climatic change. Such migration 

 will be possible only for the new generation, the seeds of the parent 

 plants, which latter are condemned to remain stationary. What be- 

 comes of these plants? Though we know of numerous instances of the 

 dying out of such plants, we also know of a considerable number of 

 cases of their preservation in the most favorable spots of their original 

 area. In the latter cases relic habitats are created, and there arise 

 isolated fragments of an area, to which phenomenon we have already 

 devoted considerable attention. 



