Chapter VI 



FOUNDATIONS OF MODERN DISTRIBUTIONS 



The distribution of each kind of plant making up a modern (and 

 presumably any other) flora is apt to depend upon (i) the history 

 of the plant in geological and recent times, (2) what may be called 

 its migrational ability, and (3) its adaptability in physiological and 

 other ways to the conditions of such new environments as it may 

 reach. To a considerable extent migrational ability depends upon 

 the efficiency of dispersal (as described in Chapter IV), and adapta- 

 bility on plasticity of form or function (as treated in Chapter III). 

 This leaves the ' fossil history ' and recent vicissitudes to be dealt 

 with next, at least in such aspects as are best known or most pertinent. 



Whereas we have seen that, apart from such periods of local or 

 regional change as occurred most notably in the Permian, the flora 

 and vegetation of the aerial parts of the globe tended to be widely 

 comparable in different regions during the earlier geological ages up 

 to and including the Mesozoic, this relative uniformity was not 

 maintained through the Cainozoic. There is plentiful evidence to 

 show that still later than the end of the Mesozoic, during early 

 Tertiary times, the forests tended to be more widespread and more 

 uniform than at present — practically throughout the land of the 

 northern hemisphere, including what are now high-arctic regions. 

 And although climatic belts doubtless existed during these and earlier 

 times, they can scarcely have been as marked in favourable periods 

 as those we know nowadays ; for, with luxuriant vegetation flourish- 

 ing within a few degrees of both the North and South Poles, the 

 tropics would have been too hot for normal life — at least if the land 

 of the world had at all the conformity it has today. But particularly 

 from the Miocene onwards there were marked local changes in 

 conditions, and so these latest fossil and sub-fossil floras are of great 

 significance to us when considering the problem of the origin of the 

 flora existing today. 



154 



