﻿Periodic 
  Mutations 
  695 
  

  

  species 
  has 
  never 
  been 
  denied. 
  It 
  is 
  observed 
  

   throughout 
  extended 
  localities, 
  and 
  during 
  long 
  

   series 
  of 
  years. 
  Other 
  proofs 
  are 
  afforded 
  by 
  

   those 
  plants 
  which 
  have 
  been 
  transported 
  to 
  dis- 
  

   tant 
  localities 
  some 
  time 
  since, 
  but 
  do 
  not 
  ex- 
  

   hibit 
  any 
  change 
  as 
  a 
  result 
  of 
  this 
  migration. 
  

   Widely 
  dispersed 
  plants 
  remain 
  the 
  same 
  

   throughout 
  their 
  range, 
  provided 
  that 
  they 
  be- 
  

   long 
  to 
  a 
  single 
  elementary 
  species. 
  Many 
  

   species 
  have 
  been 
  introduced 
  from 
  America 
  into 
  

   Europe 
  and 
  have 
  spread 
  rapidly 
  and 
  widely. 
  

   The 
  Canadian 
  horsetail 
  (Erigeron 
  canadensis) 
  , 
  

   the 
  evening-primrose 
  and 
  many 
  other 
  instances 
  

   could 
  be 
  given. 
  They 
  have 
  not 
  developed 
  any 
  

   special 
  European 
  features 
  after 
  their 
  introduc- 
  

   tion. 
  Though 
  exposed 
  to 
  other 
  environmental 
  

   conditions 
  and 
  to 
  competition 
  with 
  other 
  spe- 
  

   cies, 
  they 
  have 
  not 
  succeeded 
  in 
  developing 
  

   a 
  new 
  character. 
  Such 
  species 
  as 
  proved 
  ade- 
  

   quate 
  to 
  the 
  new 
  environment 
  have 
  succeeded, 
  

   while 
  those 
  which 
  did 
  not 
  have 
  succumbed. 
  

  

  Much 
  farther 
  back 
  is 
  the 
  separation 
  of 
  the 
  

   species 
  which 
  now 
  live 
  both 
  in 
  arctic 
  regions 
  

   and 
  on 
  the 
  summits 
  of 
  our 
  highest 
  mountain- 
  

   tops. 
  If 
  we 
  compare 
  the 
  alpine 
  flora 
  with 
  the 
  

   arctic 
  plants, 
  a 
  high 
  degree 
  of 
  similarity 
  at 
  once 
  

   strikes 
  us. 
  Some 
  forms 
  are 
  quite 
  identical; 
  

   others 
  are 
  slightly 
  different, 
  manifestly 
  repre- 
  

   senting 
  elementary 
  species 
  of 
  the 
  same 
  sys- 
  

  

  