﻿Double 
  Adaptations 
  435 
  

  

  out 
  any 
  relation 
  to 
  adaptation. 
  This 
  however 
  

   is 
  partly 
  due 
  to 
  our 
  lack 
  of 
  knowledge, 
  

   and 
  partly 
  to 
  the 
  general 
  rule 
  that 
  in 
  nature 
  

   only 
  such 
  sports 
  as 
  are 
  useful 
  are 
  spared 
  by 
  

   natural 
  selection, 
  and 
  what 
  is 
  useful 
  we 
  ordi- 
  

   narily 
  term 
  adaptive. 
  

  

  Another 
  side 
  of 
  the 
  question 
  remains 
  to 
  be 
  

   considered. 
  The 
  word 
  variety, 
  as 
  is 
  now 
  be- 
  

   coming 
  generally 
  recognized, 
  has 
  no 
  special 
  

   meaning 
  whatever. 
  But 
  here 
  it 
  is 
  assumed 
  in 
  

   the 
  clearly 
  defined 
  sense 
  of 
  a 
  systematic 
  va- 
  

   riety, 
  which 
  includes 
  all 
  subdivisions 
  of 
  spe- 
  

   cies. 
  Such 
  subdivisions 
  may 
  be, 
  from 
  a 
  

   biological 
  point 
  of 
  view, 
  elementary 
  species 
  and 
  

   also 
  be 
  eversporting 
  varieties. 
  They 
  may 
  be 
  

   retrograde 
  varieties, 
  and 
  the 
  two 
  alternating 
  

   types 
  may 
  be 
  described 
  as 
  separate 
  varieties. 
  

  

  It 
  is 
  readily 
  granted 
  that 
  many 
  writers 
  

   would 
  not 
  willingly 
  accept 
  this 
  conclusion. 
  But 
  

   it 
  is 
  simply 
  impossible 
  to 
  avoid 
  it. 
  The 
  two 
  

   forms 
  of 
  the 
  water-persicaria 
  must 
  remain 
  

   varieties, 
  though 
  they 
  are 
  only 
  types 
  of 
  the 
  dif- 
  

   ferent 
  branches 
  of 
  a 
  single 
  plant. 
  

  

  If 
  not, 
  hundreds 
  and 
  perhaps 
  thousands 
  of 
  

   analogous 
  cases 
  are 
  at 
  once 
  exposed 
  to 
  doubt, 
  

   and 
  the 
  whole 
  conception 
  of 
  systematic 
  varie- 
  

   ties 
  would 
  have 
  to 
  be 
  thrown 
  over. 
  Biologists 
  

   of 
  course 
  would 
  have 
  no 
  objection 
  to 
  this, 
  but 
  

   the 
  student 
  of 
  the 
  flora 
  of 
  any 
  given 
  country 
  

  

  