﻿570 
  Mutations 
  

  

  Fluctuating 
  variability, 
  as 
  a 
  rule, 
  is 
  subject 
  to 
  

   reversion. 
  The 
  seeds 
  of 
  the 
  extremes 
  do 
  not 
  

   produce 
  an 
  offspring 
  which 
  fluctuates 
  around 
  

   their 
  parents 
  as 
  a 
  center, 
  but 
  around 
  some 
  point 
  

   on 
  the 
  line 
  which 
  combines 
  their 
  attributes 
  with 
  

   the 
  corresponding 
  characteristic 
  of 
  their 
  ances- 
  

   tors, 
  as 
  Vilmorin 
  has 
  put 
  it. 
  No 
  reversion 
  ac- 
  

   companies 
  mutation, 
  and 
  this 
  fact 
  is 
  perhaps 
  the 
  

   completest 
  contrast 
  in 
  which 
  these 
  two 
  great 
  

   types 
  of 
  variability 
  are 
  opposed 
  to 
  each 
  other. 
  

  

  The 
  offspring 
  of 
  my 
  mutants 
  are, 
  of 
  course, 
  

   subject 
  to 
  the 
  general 
  laws 
  of 
  fluctuating 
  varia- 
  

   bility. 
  They 
  vary, 
  however, 
  around 
  their 
  own 
  

   mean, 
  and 
  this 
  mean 
  is 
  simply 
  the 
  type 
  of 
  the 
  

   new 
  elementary 
  species. 
  

  

  VII. 
  The 
  mutations 
  take 
  place 
  in 
  nearly 
  all 
  

   directions. 
  

  

  Many 
  authors 
  assume 
  that 
  the 
  origin 
  of 
  spe- 
  

   cies 
  is 
  directed 
  by 
  unknown 
  causes. 
  These 
  

   causes 
  are 
  assumed 
  to 
  work 
  in 
  each 
  single 
  case 
  

   for 
  the 
  improvement 
  of 
  the 
  animals 
  and 
  plants, 
  

   changing 
  them 
  in 
  a 
  manner 
  corresponding 
  

   in 
  a 
  useful 
  way 
  to 
  the 
  changes 
  that 
  take 
  

   place 
  in 
  their 
  environment. 
  It 
  is 
  not 
  easy 
  to 
  

   imagine 
  the 
  nature 
  of 
  these 
  influences 
  nor 
  how 
  

   they 
  would 
  bring 
  about 
  the 
  desired 
  effect. 
  

  

  This 
  difficulty 
  was 
  strongly 
  felt 
  by 
  Darwin, 
  

   and 
  one 
  of 
  the 
  chief 
  purposes 
  of 
  his 
  selection- 
  

   theory 
  may 
  be 
  said 
  to 
  have 
  been 
  the 
  at- 
  

  

  