lyo INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



of the North Pole in the North Pacific, the imphed change in the 

 earth's axis bringing the South Pole into the South Pacific. 



If the theory of polar oscillations were accepted, it would in turn 

 make unnecessary any separate theory of land-bridges, as changes 

 in level of the sea induced by oscillations of the earth (that are pre- 

 supposed to have caused those of the poles) would suffice to cause 

 the joining together or separation of different parts of the earth's 

 land surface. However, in spite of some recent attempts at resur- 

 rection, and continuing discussion of the possible instability of the 

 earth's axis and of the phytogeographical implications this might 

 have,^ this theory of polar oscillations is said to possess little geo- 

 physical foundation or geological support. Moreover, if it were 

 made to explain some anomalies of plant distribution, it would 

 apparently merely precipitate others ! 



The theory of land-bridges has long been popular in some quarters, 

 and indeed there are few seas or even deep oceans that have not 

 been hypothetically bridged by one or another over-enthusiastic 

 author to explain present-day anomalies in plant or animal distribu- 

 tion. And certainly the simplest way to explain similarities (some- 

 times only supposed) of plant and animal life between such areas 

 as Europe and eastern North America, or Australia and South 

 America, is to assume that they were once connected by a bridge 

 of land or a ' lost continent ', though the bridging and hence pos- 

 sibility of migration across the present-day ocean need not necessarily 

 have been continuous at any one time. But altogether the theory 

 strikes the sceptic as being too artificial and ' convenient ' for reality, 

 and again there are contrary geophysical and geological arguments. 

 It was advanced chiefly as a concession to some aspects of bio- 

 geography, while leaving others unexplained. Thus it leaves 

 unclarified why plants in former geological ages flourished in regions 

 whose climates are now far removed from those to which their 

 modern counterparts are accustomed, and it also gives no satisfactory 

 explanation of discontinuous areas of distribution. For if the 

 floristic similarities of two distant areas having alike plants were to 

 be explained by their having once been joined by a ' bridge ', it 

 would be necessary to assume that the entire extent of that bridge 

 oflFered similar ecological conditions suitable for the migration of 

 these plants. This requires more imagination than to visualize 

 recent dispersal by the means discussed in Chapter IV, or, perhaps, 



^ See, for example, Nature, vol. 175, p. 526, 1955, vol. 176, p. 349, 1955, and 

 vol. 176, p. 422, 1955. 



