﻿Periodic 
  Mutations 
  705 
  

  

  perhaps 
  of 
  still 
  more 
  universal 
  occurrence. 
  Pro- 
  

   gression 
  and 
  regression 
  are 
  seen 
  to 
  go 
  hand 
  in 
  

   hand 
  everywhere. 
  No 
  large 
  group 
  and 
  proba- 
  

   bly 
  even 
  no 
  genus 
  or 
  large 
  species 
  has 
  been 
  

   evolved 
  without 
  the 
  joint 
  agency 
  of 
  these 
  two 
  

   great 
  principles. 
  In 
  the 
  mutation-period 
  of 
  the 
  

   evening-primroses 
  the 
  observed 
  facts 
  give 
  direct 
  

   support 
  to 
  this 
  conclusion, 
  since 
  some 
  of 
  the 
  

   new 
  species 
  proved, 
  on 
  closer 
  inspection, 
  to 
  be 
  

   retrograde 
  varieties, 
  while 
  others 
  manifestly 
  

   owe 
  their 
  origin 
  to 
  progressive 
  steps. 
  Such 
  

   steps 
  may 
  be 
  small 
  and 
  in 
  a 
  wrong 
  direction; 
  

   notwithstanding 
  this 
  they 
  may 
  be 
  due 
  to 
  the 
  

   acquisition 
  of 
  a 
  wholly 
  new 
  character 
  and 
  there- 
  

   fore 
  belong 
  to 
  the 
  process 
  of 
  progression 
  at 
  

   large. 
  

  

  Between 
  them 
  however, 
  there 
  is 
  a 
  definite 
  

   contrast, 
  which 
  possibly 
  is 
  in 
  intimate 
  connec- 
  

   tion 
  with 
  the 
  question 
  of 
  periodic 
  and 
  stray 
  mu- 
  

   tations. 
  Obviously 
  each 
  progressive 
  change 
  is 
  

   dependent 
  upon 
  the 
  production 
  of 
  a 
  new 
  charac- 
  

   ter, 
  for 
  whenever 
  this 
  is 
  lacking, 
  no 
  such 
  muta- 
  

   tion 
  is 
  possible. 
  Retrograde 
  changes, 
  on 
  the 
  

   other 
  hand, 
  do 
  not 
  require 
  such 
  elaborate 
  pre- 
  

   liminary 
  work. 
  Each 
  character 
  may 
  be 
  con- 
  

   verted 
  into 
  the 
  latent 
  condition, 
  and 
  for 
  all 
  we 
  

   know, 
  a 
  special 
  preparation 
  for 
  this 
  purpose 
  is 
  

   not 
  at 
  all 
  necessary. 
  It 
  is 
  readily 
  granted 
  that 
  

   such 
  special 
  preparation 
  may 
  occur, 
  because 
  the 
  

  

  