l66 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



Started breaking and drifting apart during the Mesozoic era and 

 went on doing so until they came to reach the positions they now 

 occupy. According to some advocates of the theory, this drifting 

 is still proceeding and actually demonstrable in the case of Green- 

 land. Such recent drifting might conceivably have left Europe and 

 North America in contact with one another until the early Quaternary, 

 and likewise Antarctica and South America may be presumed to 

 have remained long in contact. The theory also accounts for 

 considerable rearrangement of the climatic zones on land, as the 

 drifting of the continents was supposed to involve changes in their 

 several positions relative to the North and South Poles. This 

 in turn would allow marked changes in the distributions of living 

 organisms, and bring plants and even entire floras to regions whose 

 present climates are barely adequate for their growth. Still more 

 spectacularly, it would allow fossil remains to drift with their 

 enclosing land-masses to distant parts of the globe — and hence into 

 entirely diiTerent climatic belts — and this could explain, for ex- 

 ample, the presence of fossils of warmth-loving plants in the Far 

 North. 



Although it is rejected by some geologists and, especially, geo- 

 physicists, the theory of continental drift has won the ardent support 

 of others — even if only for earlier geological times than those which 

 allow most help to the biogeographer in theorizing about the anomalies 

 he finds. To many of the botanists among these biogeographers, it 

 seems to constitute the most plausible working hypothesis upon 

 which they can base their suppositions as to the history of plant 

 ranges. In one connection or another, the controversy rages back 

 and forth and probably will continue to do so for a long time to 

 come, though it may be noted meanwhile that the maps produced 

 by proponents of the theory are extraordinarily suggestive, and 

 would, one imagines, if analyzed statistically, indicate a very high 

 degree of probability {cf. Fig. 45, A). However, geologists and geo- 

 physicists are prone to put back the time of possible rift and major 

 displacement of continents well before the beginning of the Mesozoic, 

 when it would be of little if any help to plant geographers in their 

 search for explanations of striking similarities of flora in certain areas 

 that are now widely separated by oceans. For it is obvious that, 

 as differences between species commonly involve the establishment 

 of various genetically independent mutations (sudden changes), the 

 chances that two isolated populations will evolve in exactly the same 

 way are incalculably low, while convergence in every respect of 



