INTRODUCTION 63 



recognised history or classification of present-day 

 plants, according to their supposed affinity or order 

 of development. 



The table on page 62 shows the distribution of 

 plant types in the rocks. 



18. Geographical Distribution. 



Another argument for the evolution of plants from 

 the lower forms to the higher types is derived from 

 the facts of geographical distribution. According to 

 this view plants have developed in different regions 

 or centres and have spread over the globe by various 

 agencies. Some plants, which are now of universal 

 occurrence, exhibit similar characteristics in all 

 regions, or variations according to the diversity of their 

 habitat. In other cases the plants that are charac- 

 teristic of certain regions, such as the Palms, for 

 instance, have been restricted to those regions owing 

 to limitations of climate or other barriers, such as 

 mountains, deserts, the existence of seas, oceans, or 

 because they have been evolved in insular regions 

 and have been cut off from other terrestrial regions. 

 In the last case, however, the fruits or seeds of 

 plants have been dispersed by currents, and have 

 germinated on islands and become established there. 

 Island floras may, when of oceanic type, be specialised, 

 or, when continental, resemble the flora of the 

 mainland. 



The existence of mountains may in some cases be 



