CONCLUSION 231 



selves at home in almost any climate, and are 

 consequently spread all over the world. This is 

 the case, for example, with various Ferns; the 

 common Bracken extends through all temperate 

 and many tropical regions, in both hemispheres. 

 The Bladder-fern (Cystopteris fragilis) also has 

 an extremely wide distribution, extending from 

 the Arctic regions to New Zealand and Tas- 

 mania. It may well be that in former ages this 

 adaptability and comparative indifference to 

 climate, now limited to a few cosmopolitan 

 species, was common to most plants, and may 

 account for the world-wide distribution of so many 

 Palaeozoic and Mesozoic forms. If so, the 

 special accommodation of particular species to 

 particular climates and environments may be a 

 characteristic of the later Floras, and so far there 

 may have been an advance in exact but limited 

 adaptation. Plants, like their investigators, have 

 become specialists! 



It was held by Darwin that "the period during 

 which each species underwent modification, though 

 long as measured by years, was probably short in 

 comparison with that during which it remained 

 without undergoing any change " (Origin of 

 Species, p. 279). 



On this view there have been long periods 

 of stability, when the conditions of life remained 

 fairly constant, and plants or animals were so 



