﻿Periodic 
  Mutations 
  689 
  

  

  testing 
  of 
  native 
  plants 
  on 
  the 
  hypothesis 
  of 
  

   unit-characters 
  as 
  deduced 
  from 
  Darwin's 
  

   Pangenesis. 
  This 
  conception 
  led 
  to 
  the 
  expec- 
  

   tation 
  of 
  two 
  different 
  kinds 
  of 
  variability, 
  one 
  

   slow 
  and 
  one 
  sudden. 
  The 
  sudden 
  ones 
  known 
  

   at 
  the 
  time 
  were 
  considered 
  as 
  sports, 
  and 
  

   seemed 
  limited 
  to 
  retrograde 
  changes, 
  or 
  to 
  

   cases 
  of 
  minor 
  importance. 
  The 
  idea 
  that 
  

   sudden 
  steps 
  might 
  be 
  taken 
  as 
  the 
  principal 
  

   method 
  of 
  evolution 
  could 
  be 
  derived 
  from 
  the 
  

   hypothesis 
  of 
  unit-characters, 
  but 
  the 
  evidence 
  

   might 
  be 
  too 
  remote 
  for 
  a 
  starting 
  point 
  for 
  ex- 
  

   perimental 
  investigation. 
  

  

  The 
  success 
  of 
  my 
  test 
  has 
  given 
  proof 
  to 
  the 
  

   contrary. 
  Hence 
  the 
  assertion 
  that 
  no 
  evidence 
  

   is 
  to 
  be 
  considered 
  as 
  inadequate 
  for 
  the 
  pur- 
  

   pose 
  under 
  discussion. 
  Sometime 
  a 
  method 
  of 
  

   discovering, 
  or 
  of 
  producing, 
  mutable 
  plants 
  

   may 
  be 
  found, 
  but 
  until 
  this 
  is 
  done, 
  all 
  facts 
  of 
  

   whatever 
  nature 
  or 
  direction 
  must 
  be 
  made 
  use 
  

   of. 
  A 
  very 
  slight 
  indication 
  may 
  change 
  for- 
  

   ever 
  the 
  whole 
  aspect 
  of 
  the 
  problem. 
  

  

  The 
  probabilities 
  are 
  now 
  greatly 
  in 
  favor 
  of 
  

   our 
  finding 
  out 
  the 
  causes 
  of 
  evolution 
  by 
  

   a 
  close 
  scrutiny 
  of 
  what 
  really 
  happens 
  in 
  na- 
  

   ture. 
  A 
  persistent 
  study 
  of 
  the 
  physiologic 
  

   factors 
  of 
  this 
  evolution 
  is 
  the 
  chief 
  condition 
  

   of 
  success. 
  To 
  this 
  study 
  field-observations 
  

   may 
  contribute 
  as 
  well 
  as 
  direct 
  experiments, 
  

  

  