﻿Periodic 
  Mutations 
  691 
  

  

  agency 
  of 
  external 
  circumstances. 
  Should 
  this 
  

   be 
  granted 
  for 
  the 
  evening-primrose, 
  it 
  would 
  

   have 
  to 
  be 
  predicated 
  for 
  other 
  species 
  found 
  in 
  

   a 
  mutable 
  state. 
  Then, 
  of 
  course, 
  it 
  would 
  be 
  

   useless 
  to 
  investigate 
  the 
  causes 
  of 
  mutability 
  

   at 
  large, 
  and 
  we 
  should 
  have 
  to 
  limit 
  ourselves 
  

   to 
  the 
  testing 
  of 
  large 
  numbers 
  of 
  plants 
  in 
  

   order 
  to 
  ascertain 
  which 
  are 
  mutable 
  and 
  

   which 
  not. 
  

  

  If, 
  on 
  the 
  other 
  hand, 
  mutability 
  is 
  not 
  a 
  per- 
  

   manent 
  feature, 
  it 
  must 
  once 
  have 
  had 
  a 
  begin- 
  

   ning, 
  and 
  this 
  beginning 
  itself 
  must 
  have 
  had 
  an 
  

   external 
  cause. 
  The 
  amount 
  of 
  mutability 
  and 
  its 
  

   possible 
  directions 
  may 
  be 
  assumed 
  to 
  be 
  due 
  to 
  

   internal 
  causes. 
  The 
  determination 
  of 
  the 
  mo- 
  

   ment 
  at 
  which 
  they 
  will 
  become 
  active 
  can 
  never 
  

   be 
  the 
  result 
  of 
  internal 
  causes. 
  It 
  must 
  be 
  as- 
  

   signed 
  to 
  some 
  external 
  factor, 
  and 
  as 
  soon 
  as 
  

   this 
  is 
  discovered 
  the 
  way 
  for 
  experimental 
  in- 
  

   vestigation 
  is 
  open. 
  

  

  In 
  the 
  second 
  place 
  we 
  must 
  consider 
  the 
  past. 
  

   On 
  the 
  supposition 
  of 
  permanency 
  all 
  the 
  ances- 
  

   tors 
  of 
  the 
  evening-primrose 
  must 
  have 
  been 
  

   mutable. 
  By 
  the 
  alternative 
  view 
  mutability 
  

   must 
  have 
  been 
  a 
  periodic 
  phenomenon, 
  produc- 
  

   ing 
  at 
  times 
  new 
  qualities, 
  and 
  at 
  other 
  times 
  

   leaving 
  the 
  plants 
  unchanged 
  during 
  long 
  suc- 
  

   cessions 
  of 
  generations. 
  The 
  present 
  mutable 
  

   state 
  must 
  then 
  have 
  been 
  preceded 
  by 
  an 
  im- 
  

  

  