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  most 
  widely 
  known 
  instance, 
  although 
  other 
  

   cases 
  might 
  easily 
  be 
  cited. 
  

  

  For 
  our 
  special 
  purpose 
  we 
  may 
  take 
  up 
  

   only 
  the 
  more 
  simple 
  cases 
  that 
  may 
  be 
  avail- 
  

   able 
  for 
  experimental 
  work. 
  The 
  great 
  lines 
  of 
  

   evolution 
  of 
  whole 
  families 
  and 
  even 
  of 
  genera 
  

   and 
  of 
  many 
  larger 
  species 
  obviously 
  lie 
  outside 
  

   the 
  limits 
  of 
  experimental 
  observation. 
  They 
  

   are 
  the 
  outcome 
  of 
  the 
  history 
  of 
  the 
  ancestors 
  

   of 
  the 
  present 
  types, 
  and 
  a 
  repetition 
  of 
  their 
  

   history 
  is 
  far 
  beyond 
  human 
  powers. 
  We 
  must 
  

   limit 
  ourselves 
  to 
  the 
  most 
  recent 
  steps, 
  to 
  the 
  

   consideration 
  of 
  the 
  smallest 
  differences. 
  But 
  

   it 
  is 
  obvious 
  that 
  these 
  may 
  be 
  included 
  under 
  

   the 
  same 
  heads 
  as 
  the 
  larger 
  and 
  older 
  ones. 
  

   For 
  the 
  larger 
  movements 
  are 
  manifestly 
  to 
  be 
  

   considered 
  only 
  as 
  groups 
  of 
  smaller 
  steps, 
  

   going 
  in 
  the 
  same 
  direction. 
  

  

  Hence 
  we 
  conclude, 
  that 
  even 
  the 
  smallest 
  

   steps 
  in 
  the 
  evolution 
  of 
  plants 
  which 
  we 
  are 
  

   able 
  to 
  observe, 
  may 
  be 
  divided 
  into 
  progres- 
  

   sive, 
  retrogressive 
  and 
  degressive 
  ones. 
  The 
  

   acquisition 
  of 
  a 
  single 
  new 
  quality 
  is 
  the 
  most 
  

   simple 
  step 
  in 
  the 
  progressive 
  line, 
  the 
  becom- 
  

   ing 
  latent 
  and 
  the 
  reactivating 
  of 
  this 
  same 
  

   quality 
  are 
  the 
  prototypes 
  of 
  the 
  two 
  other 
  

   classes. 
  

  

  Having 
  taken 
  this 
  theoretical 
  point 
  of 
  view, 
  

   it 
  remains 
  to 
  inquire, 
  how 
  it 
  concurs 
  with 
  the 
  

  

  