18 LINKS WITH THE PAST [CH. 



(ii) a small group of plants known as the Cycads, 

 whose existing members, now almost confined to 

 a few tropical regions, are the descendants of a 

 vigorous race represented by many species in the 

 floras of the Mesozoic epoch. A third sub-division 

 of the Gymnosperms, the Ginkgoales, is represented 

 by a single survivor, which is described in a later 

 chapter as one of the most remarkable links with the 

 past in the plant kingdom. 



The Gymnosperms are geologically very much 

 older than the Angiosperms. Members of this class 

 played a prominent part in the vegetation of the 

 Coal age and it is certain that they existed in the 

 still older Devonian period. The only other group 

 to which reference is made in later chapters is that 

 of the Ferns, one of the sub-divisions of a large class 

 known as the Vascular Cryptogams or Pteridophyta. 

 These plants, like the Gymnosperms, are represented 

 in the oldest floras of which recognisable remains 

 have been preserved. The main groups of the 

 vegetable kingdom, founded on existing plants, are 

 distinguished by well-defined differences ; they are 

 comparable with separate twigs of a tree springing 

 from larger branches and these again uniting below 

 in a common trunk. The vegetation of to-day re- 

 presents only the terminal portions of the upper 

 branches. As we descend the geological series, 

 records of extinct types are found which enable 



