﻿694 
  Mutations 
  

  

  Many 
  facts 
  plead 
  in 
  favor 
  of 
  the 
  constancy 
  

   of 
  species. 
  This 
  principle 
  has 
  always 
  been 
  

   recognized 
  by 
  systernatists. 
  Temporarily 
  the 
  

   current 
  form 
  of 
  the 
  theory 
  of 
  natural 
  selec- 
  

   tion 
  has 
  assumed 
  species 
  to 
  be 
  inconstant, 
  ever 
  

   changing 
  and 
  continuously 
  being 
  improved 
  and 
  

   adapted 
  to 
  the 
  requirements 
  of 
  the 
  life-condi- 
  

   tions. 
  The 
  followers 
  of 
  the 
  theory 
  of 
  descent 
  

   believed 
  that 
  this 
  conclusion 
  was 
  unavoidable, 
  

   and 
  were 
  induced 
  to 
  deny 
  the 
  manifest 
  fact 
  that 
  

   species 
  are 
  constant 
  entities. 
  The 
  mutation 
  

   theory 
  gives 
  a 
  clew 
  to 
  the 
  final 
  combination 
  of 
  

   the 
  two 
  contending 
  ideas. 
  Reducing 
  the 
  change- 
  

   ability 
  of 
  the 
  species 
  to 
  distinct 
  and 
  probably 
  

   short 
  periods, 
  it 
  at 
  once 
  explains 
  how 
  the 
  

   stability 
  of 
  species 
  perfectly 
  agrees 
  with 
  the 
  

   principle 
  of 
  descent 
  through 
  modification. 
  

  

  On 
  the 
  other 
  hand, 
  the 
  hypothesis 
  of 
  imitative 
  

   periods 
  is 
  by 
  no 
  means 
  irreconcilable 
  with 
  the 
  

   observed 
  facts 
  of 
  constancy. 
  Such 
  casual 
  

   changes 
  can 
  be 
  proved 
  by 
  observations 
  such 
  as 
  

   those 
  upon 
  the 
  evening-primrose, 
  but 
  it 
  is 
  ob- 
  

   vious 
  that 
  a 
  disproof 
  can 
  never 
  be 
  given. 
  The 
  

   principle 
  grants 
  the 
  present 
  constancy 
  of 
  the 
  

   vast 
  majority 
  of 
  living 
  forms, 
  and 
  only 
  claims 
  

   the 
  exceptional 
  occurrence 
  of 
  definite 
  changes. 
  

  

  Proofs 
  of 
  the 
  constancy 
  of 
  species 
  have 
  been 
  

   given 
  in 
  different 
  ways. 
  The 
  high 
  degree 
  of 
  

   similarity 
  of 
  the 
  individuals 
  of 
  most 
  of 
  our 
  

  

  