﻿Artificial 
  and 
  Natural 
  Selection 
  799 
  

  

  knowledged 
  as 
  the 
  first 
  step 
  in 
  the 
  way 
  

   of 
  selection, 
  and 
  is 
  designated 
  by 
  the 
  name 
  

   of 
  variety-testing, 
  applying 
  the 
  term 
  variety 
  to 
  

   all 
  the 
  subdivisions 
  of 
  systematic 
  species 
  indis- 
  

   criminately. 
  In 
  natural 
  processes 
  it 
  bears 
  the 
  

   title 
  of 
  survival 
  of 
  species. 
  The 
  fact 
  that 
  re- 
  

   cent 
  types 
  show 
  large 
  numbers, 
  and 
  in 
  some 
  in- 
  

   stances 
  even 
  hundreds 
  of 
  minor 
  constant 
  forms, 
  

   while 
  the 
  older 
  genera 
  are 
  considerably 
  reduced 
  

   in 
  this 
  respect, 
  is 
  commonly 
  explained 
  by 
  the 
  

   assumption 
  of 
  extinction 
  of 
  species 
  on 
  a 
  corre- 
  

   spondingly 
  large 
  scale. 
  This 
  extinction 
  is 
  con- 
  

   sidered 
  to 
  affect 
  the 
  unfit 
  in 
  a 
  higher 
  measure 
  

   than 
  the 
  fit. 
  Consequently 
  the 
  former 
  vanish, 
  

   often 
  without 
  leaving 
  any 
  trace 
  of 
  their 
  exist- 
  

   ence, 
  and 
  only 
  those 
  that 
  prove 
  to 
  be 
  sufficiently 
  

   adapted 
  to 
  the 
  surrounding 
  external 
  conditions, 
  

   resist 
  and 
  survive. 
  

  

  This 
  selection 
  exhibits 
  far-reaching 
  analo- 
  

   gies 
  between 
  the 
  artificial 
  and 
  the 
  natural 
  proc- 
  

   esses, 
  and 
  is 
  in 
  both 
  cases 
  of 
  the 
  very 
  highest 
  

   importance. 
  In 
  nature 
  the 
  dying 
  out 
  of 
  unfit 
  

   mutations 
  is 
  the 
  result 
  of 
  the 
  great 
  struggle 
  for 
  

   life. 
  In 
  a 
  previous 
  lecture 
  we 
  have 
  compared 
  

   its 
  agency 
  with 
  that 
  of 
  a 
  sieve. 
  All 
  elements 
  

   which 
  are 
  too 
  small 
  or 
  too 
  weak 
  fall 
  through, 
  

   and 
  only 
  those 
  are 
  preserved 
  which 
  resist 
  the 
  

   sifting 
  process. 
  Keduced 
  in 
  number 
  they 
  

   thrive 
  and 
  multiply 
  and 
  are 
  thus 
  enabled 
  to 
  

  

  