﻿Theories 
  of 
  Evolution 
  7 
  

  

  only 
  a 
  sieve, 
  which 
  decides 
  what 
  is 
  to 
  live, 
  and 
  

   what 
  is 
  to 
  die. 
  But 
  evolutionary 
  lines 
  are 
  of 
  

   great 
  length, 
  and 
  the 
  evolution 
  of 
  a 
  flower, 
  or 
  of 
  

   an 
  insectivorous 
  plant 
  is 
  a 
  way 
  with 
  many 
  side- 
  

   paths. 
  It 
  is 
  the 
  sieve 
  that 
  keeps 
  evolution 
  on 
  

   the 
  main 
  line, 
  killing 
  all, 
  or 
  nearly 
  all 
  that 
  try 
  

   to 
  go 
  in 
  other 
  directions. 
  By 
  this 
  means 
  nat- 
  

   ural 
  selection 
  is 
  the 
  one 
  directing 
  cause 
  of 
  the 
  

   broad 
  lines 
  of 
  evolution. 
  

  

  Of 
  course, 
  with 
  the 
  single 
  steps 
  of 
  evolution 
  

   it 
  has 
  nothing 
  to 
  do. 
  Only 
  after 
  the 
  step 
  has 
  

   been 
  taken, 
  the 
  sieve 
  acts, 
  eliminating 
  the 
  unfit. 
  

   The 
  problem, 
  as 
  to 
  the 
  manner 
  in 
  which 
  the 
  in- 
  

   dividual 
  steps 
  are 
  brought 
  about, 
  is 
  quite 
  an- 
  

   other 
  side 
  of 
  the 
  question. 
  

  

  On 
  this 
  point 
  Darwin 
  has 
  recognized 
  two 
  pos- 
  

   sibilities. 
  One 
  means 
  of 
  change 
  lies 
  in 
  the 
  sud- 
  

   den 
  and 
  spontaneous 
  production 
  of 
  new 
  forms 
  

   from 
  the 
  old 
  stock. 
  The 
  other 
  method 
  is 
  the 
  

   gradual 
  accumulation 
  of 
  those 
  always 
  present 
  

   and 
  ever 
  fluctuating 
  variations 
  which 
  are 
  in- 
  

   dicated 
  by 
  the 
  common 
  assertion 
  that 
  no 
  two 
  

   individuals 
  of 
  a 
  given 
  race 
  are 
  exactly 
  alike. 
  

   The 
  first 
  changes 
  are 
  what 
  we 
  now 
  call 
  * 
  ' 
  muta- 
  

   tions, 
  ' 
  the 
  second 
  are 
  designated 
  as 
  ' 
  ' 
  individ- 
  

   ual 
  variations, 
  " 
  or 
  as 
  this 
  term 
  is 
  often 
  used 
  in 
  

   another 
  sense, 
  as 
  " 
  fluctuations." 
  Darwin 
  rec- 
  

   ognized 
  both 
  lines 
  of 
  evolution; 
  Wallace 
  disre- 
  

   garded 
  the 
  sudden 
  changes 
  and 
  propo.sed 
  fluctu- 
  

  

  