v] FERNS 83 



Scotland, in the Wealden rocks of Sussex, as well as 

 from other European localities. It is clear that the 

 Gleichenia-family, no longer represented in north 

 temperate floras, was in the Jurassic period, and 

 especially in the early days of the Cretaceous period, 

 widely spread in Europe, extending well within the 

 Arctic circle. It may be that the original home of 

 Gleichenia was in the far North at a time when 

 climatic conditions were very different from those 

 which now prevail. Gleichenia, like many other 

 northern plants, retreated to more southern regions 

 where, in the warmer countries of the world, many 

 species still flourish widely separated in space and 

 time from the place of their birth. 



The ferns so far mentioned have a more or less 

 extended distribution at the present day. In the 

 case of Pteridium aquilhmm, the cosmopolitan 

 Bracken Fern, wide range Avould seem to be corre- 

 lated with comparatively recent origin ; on the 

 other hand, the facts of palaeobotany show that 

 the wide distribution of Osmunda, a type of fern 

 which differs in many important respects from 

 members of the family (Folypodiaceae) to which 

 the Bracken belongs, is not inconsistent with an 

 exceptionally ancient family-history. There are, 

 however, certain genera of ferns which afford 

 remarkable examples of restricted geographical 

 distribution associated with great antiquity. The 



6—2 



