﻿6 
  Descent 
  

  

  factors 
  in 
  the 
  general 
  process 
  of 
  variation. 
  Re- 
  

   peatedly 
  Darwin 
  tried 
  to 
  formulate 
  these 
  causes, 
  

   but 
  the 
  evidence 
  available 
  did 
  not 
  meet 
  his 
  re- 
  

   quirements. 
  

  

  Quetelet's 
  law 
  of 
  variation 
  had 
  not 
  yet 
  been 
  

   published. 
  Mendel's 
  claim 
  of 
  hereditary 
  units 
  

   for 
  the 
  explanation 
  of 
  certain 
  laws 
  of 
  hybrids 
  

   discovered 
  by 
  him, 
  was 
  not 
  yet 
  made. 
  The 
  

   clear 
  distinction 
  between 
  spontaneous 
  and 
  sud- 
  

   den 
  changes, 
  as 
  compared 
  with 
  the 
  ever-present 
  

   fluctuating 
  variations, 
  is 
  only 
  of 
  late 
  coming 
  into 
  

   recognition 
  by 
  agriculturists. 
  Innumerable 
  

   minor 
  points 
  which 
  go 
  to 
  elucidate 
  the 
  breeders' 
  

   experience, 
  and 
  with 
  which 
  we 
  are 
  now 
  quite 
  

   familiar, 
  were 
  unknown 
  in 
  Darwin's 
  time. 
  No 
  

   wonder 
  that 
  he 
  made 
  mistakes, 
  and 
  laid 
  stress 
  

   on 
  modes 
  of 
  descent, 
  which 
  have 
  since 
  been 
  

   proved 
  to 
  be 
  of 
  minor 
  importance 
  or 
  even 
  of 
  

   doubtful 
  validity. 
  

  

  Notwithstanding 
  all 
  these 
  apparently 
  unsur- 
  

   mountable 
  difficulties, 
  Darwin 
  discovered 
  the 
  

   great 
  principle 
  which 
  rules 
  the 
  evolution 
  of 
  or- 
  

   ganisms. 
  It 
  is 
  the 
  principle 
  of 
  natural 
  selec- 
  

   tion. 
  It 
  is 
  the 
  sifting 
  out 
  of 
  all 
  organisms 
  of 
  

   minor 
  worth 
  through 
  the 
  struggle 
  for 
  life. 
  It 
  

   is 
  only 
  a 
  sieve, 
  and 
  not 
  a 
  force 
  of 
  nature, 
  not 
  a 
  

   direct 
  cause 
  of 
  improvement, 
  as 
  many 
  of 
  Dar- 
  

   win's 
  adversaries, 
  and 
  unfortunately 
  many 
  of 
  

   his 
  followers 
  also, 
  have 
  so 
  often 
  asserted. 
  It 
  is 
  

  

  