232 THE EVOLUTION OF PLANTS 



far adjusted to them that no great changes 

 were called for. Under such circumstances Nat- 

 ural Selection would simply maintain the existing 

 standard of adaptation, without, on the whole, 

 favouring any new forms that might arise. It is 

 probable that we are now living in a stable period 

 of this kind, so far as our own part of the world 

 is concerned. It appears that there has been 

 very little change in European plants since 

 glacial or even pre-glacial times. There has 

 been a good deal of migration, but the species 

 themselves have, it seems, altered very little. 



The geological record shows, on a greater scale, 

 that times of comparative constancy have alter- 

 nated with intervals of apparently rapid change. 

 Thus, throughout all the middle part of the 

 Mesozoic period, from the Upper Triassic to the 

 beginning of the Cretaceous, the Flora remained 

 much the same; after that the Angiosperms 

 appeared and a complete transformation soon took 

 place. During the Upper Carboniferous age 

 the changes in the Flora were small compared with 

 those that ensued about late Permian times. 



It is evident that at any period when species 

 remained nearly constant for a long while, they 

 must have been quite well adapted to the condi- 

 tions of life that then prevailed; otherwise 

 new variations would have gained the upper hand, 

 and species would have changed. We may there- 



