﻿708 
  Mutations 
  

  

  more 
  simple 
  plans. 
  But 
  the 
  changes 
  have 
  by 
  no 
  

   means 
  removed 
  the 
  doubts, 
  nor 
  have 
  they 
  been 
  

   able 
  to 
  supplant 
  the 
  general 
  impression 
  of 
  dis- 
  

   tinct 
  groups, 
  united 
  by 
  broad 
  lines. 
  This 
  

   feature 
  is 
  very 
  essential, 
  and 
  it 
  is 
  easily 
  seen 
  to 
  

   correspond 
  with 
  the 
  conception 
  of 
  swarms, 
  as 
  

   we 
  have 
  deduced 
  it 
  from 
  the 
  study 
  of 
  the 
  lesser 
  

   groups. 
  

  

  Genealogic 
  trees 
  are 
  the 
  result 
  of 
  comparative 
  

   studies 
  ; 
  they 
  are 
  far 
  removed 
  from 
  the 
  results 
  

   of 
  experimental 
  inquiry 
  concerning 
  the 
  origin 
  of 
  

   species. 
  What 
  are 
  the 
  links 
  which 
  bind 
  them 
  

   together? 
  Obviously 
  they 
  must 
  be 
  sought 
  in 
  

   the 
  mutative 
  periods, 
  which 
  have 
  immediately 
  

   preceded 
  the 
  present 
  one. 
  In 
  the 
  case 
  of 
  the 
  

   evening-primrose 
  the 
  systematic 
  arrangement 
  

   of 
  the 
  allied 
  species 
  readily 
  guides 
  us 
  in 
  the 
  de- 
  

   limitations 
  of 
  such 
  periods. 
  For 
  manifestly 
  

   the 
  species 
  of 
  the 
  large 
  genus 
  of 
  Oenothera 
  are 
  

   grouped 
  in 
  swarms, 
  the 
  youngest 
  or 
  most 
  recent 
  

   of 
  which 
  we 
  have 
  under 
  observation. 
  Its 
  imme- 
  

   diate 
  predecessor 
  must 
  have 
  been 
  the 
  subgenus 
  

   Onagra, 
  which 
  is 
  considered 
  by 
  some 
  authors 
  as 
  

   consisting 
  of 
  a 
  single 
  systematic 
  species, 
  Oenoth- 
  

   era 
  biennis. 
  Its 
  multifarious 
  forms 
  point 
  to 
  a 
  

   common 
  origin, 
  not 
  only 
  morphologically 
  but 
  

   also 
  historically. 
  Following 
  this 
  line 
  backward 
  

   or 
  downward 
  we 
  reach 
  another 
  apparent 
  imita- 
  

   tion-period, 
  which 
  includes 
  the 
  origin 
  of 
  

  

  