Chapter IX 



THE MIGRATIONS OF SPECIES AND FLORAS AND 



THEIR CAUSES 



As we have shown in the preceding chapter, plant dispersal in most 

 cases is not "saltatory", i.e., it is not achieved as a result of the 

 chance transport of seeds over great distances. Plants usually extend 

 their range slowly, gradually, step by step, as a result of the dissemina- 

 tion of their seeds by natural factors over territories located near the 

 mother plants and not extending far beyond the boundaries of the 

 latter's area. Such slow extensions of area may, moreover, affect not 

 only separate forms or varieties but also species and even whole plant 

 communities. The fact that the progeny of any plant — by means of 

 various autochoric processes and such natural agencies as wind, water, 

 and animals — usually takes root at some distance (but not a great 

 distance) from the mother plant results in the slow, gradual advance of 

 a species, in an expansion of its area, continuing until the given species 

 encounters some obstacle — physiographic, climatic, edaphic, or biotic — 

 hindering its further spread. Changes within the plants themselves 

 occurring, as a result of natural selection, during the process of their 

 dispersal often enable them to overcome some of these obstacles and 

 so to continue their spread. Nevertheless, such extensions of area lack 

 any definite direction and are inadequate to explain those moments in 

 the history of our globe when entire floras were in a state of motion, 

 changing their areas of distribution in one and the same direction. The 

 question arises: What are the causes underlying such mass movements 

 of floras and their component species? 



Migration Theory: — The distribution of plants and their grouping 

 in floras are primarily controlled, as we know, by climatic conditions, 

 which determine the geographical zones and altitudinal belts in which 

 their areas are located. Within these climatic limits plant distribution 

 is further controlled by edaphic conditions. The latter are of secondary 

 significance, being indefinite in character and not subject to any regular 

 zonation as are climatic conditions. If, however, climatic zonation 

 were unalterable, as was until recently presumed, the distribution of 

 plants over the globe would be fixed and the species composition of 

 floras would not be characterized by such diversity as exists. Paleo- 

 botanic data and analyses of the areas of species in present-day floras 

 provide clear evidence that time and again during the history of our 

 earth — in all geological epochs down to the last post-glacial period — 

 there have occurred great movements of floras. That such movements 

 have actually occurred is Hkewise testified to by the fact that plant 

 fossils differ in their geographical and altitudinal distribution from the 

 living representatives of the same genera and by the mixed character 

 of the species composition of floras. 



In view of the fact that the dispersal of species takes place as the 



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