﻿LECTURE 
  XXIV 
  

  

  THE 
  HYPOTHESIS 
  OF 
  PERIODIC 
  MUTATIONS 
  

  

  The 
  prevailing 
  belief 
  that 
  slow 
  and 
  gradual, 
  

   nearly 
  invisible 
  changes 
  constitute 
  the 
  process 
  

   of 
  evolution 
  in 
  the 
  animal 
  and 
  vegetable 
  king- 
  

   dom, 
  did 
  not 
  offer 
  a 
  strong 
  stimulus 
  for 
  experi- 
  

   mental 
  research. 
  No 
  appreciable 
  response 
  to 
  

   any 
  external 
  agency 
  was 
  of 
  course 
  to 
  be 
  ex- 
  

   pected. 
  Responses 
  were 
  supposed 
  to 
  be 
  pro- 
  

   duced, 
  but 
  the 
  corresponding 
  outward 
  changes 
  

   would 
  be 
  too 
  small 
  to 
  betray 
  themselves 
  to 
  the 
  

   investigator. 
  

  

  The 
  direct 
  observation 
  of 
  the 
  mutations 
  of 
  

   the 
  evening-primrose 
  has 
  changed 
  the 
  whole 
  

   aspect 
  of 
  the 
  problem 
  at 
  once. 
  It 
  is 
  no 
  longer 
  

   a 
  matter 
  dealing 
  with 
  purely 
  hypothetical 
  con- 
  

   ditions. 
  Instead 
  of 
  the 
  vague 
  notions, 
  uncer- 
  

   tain 
  hopes, 
  and 
  a 
  priori 
  conceptions, 
  that 
  have 
  

   hitherto 
  confused 
  the 
  investigator, 
  methods 
  of 
  

   observation 
  have 
  been 
  formulated, 
  suitable 
  for 
  

   the 
  attainment 
  of 
  definite 
  results, 
  the 
  general 
  

   nature 
  of 
  which 
  is 
  already 
  known. 
  

  

  To 
  my 
  mind 
  the 
  real 
  value 
  of 
  the 
  discovery 
  

  

  686 
  

  

  