﻿602 
  Mutations 
  

  

  On 
  the 
  other 
  hand 
  occasional 
  specimens 
  with 
  a 
  

   single 
  variegated 
  leaf, 
  or 
  with 
  some 
  few 
  of 
  them, 
  

   are 
  actually 
  met 
  with, 
  and 
  if 
  attention 
  is 
  once 
  

   drawn 
  to 
  this 
  question, 
  perhaps 
  a 
  dozen 
  or 
  so 
  

   instances 
  might 
  be 
  brought 
  together 
  in 
  a 
  sum- 
  

   mer. 
  But 
  they 
  never 
  seem 
  to 
  be 
  capable 
  of 
  

   further 
  evolution, 
  or 
  of 
  reproducing 
  themselves 
  

   sufficiently 
  and 
  of 
  repeating 
  their 
  peculiarity 
  in 
  

   their 
  progeny. 
  They 
  make 
  their 
  appearance, 
  

   are 
  seen 
  during 
  a 
  season, 
  and 
  then 
  disappear. 
  

   Even 
  this 
  slight 
  incompleteness 
  of 
  some 
  spots 
  

   on 
  one 
  or 
  two 
  leaves 
  may 
  be 
  enough 
  to 
  be 
  their 
  

   doom. 
  

  

  It 
  is 
  a 
  common 
  belief 
  that 
  new 
  varieties 
  owe 
  

   their 
  origin 
  to 
  the 
  direct 
  action 
  of 
  external 
  

   conditions 
  and 
  moreover 
  it 
  is 
  often 
  assumed 
  that 
  

   similar 
  deviations 
  must 
  have 
  similar 
  causes, 
  and 
  

   that 
  these 
  causes 
  may 
  act 
  repeatedly 
  in 
  the 
  same 
  

   species, 
  or 
  in 
  allied, 
  or 
  even 
  systematically 
  dis- 
  

   tant 
  genera. 
  No 
  doubt 
  in 
  the 
  end 
  all 
  things 
  

   must 
  have 
  their 
  causes, 
  and 
  the 
  same 
  causes 
  

   will 
  lead 
  under 
  the 
  same 
  circumstances 
  to 
  the 
  

   same 
  results. 
  But 
  we 
  are 
  not 
  justified 
  in 
  deduc- 
  

   ing 
  a 
  direct 
  relation 
  between 
  the 
  external 
  con- 
  

   ditions 
  and 
  the 
  internal 
  changes 
  of 
  plants. 
  

   These 
  relations 
  may 
  be 
  of 
  so 
  remote 
  a 
  nature 
  

   that 
  they 
  cannot 
  as 
  yet 
  be 
  guessed 
  at. 
  There- 
  

   fore 
  only 
  direct 
  experience 
  may 
  be 
  our 
  guide. 
  

   Summing 
  up 
  the 
  result 
  of 
  our 
  facts 
  and 
  dis- 
  

  

  