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  Elementary 
  Species 
  

  

  testing. 
  Indeed 
  it 
  may 
  be 
  surmised 
  that 
  this 
  

   has 
  been 
  its 
  chief 
  and 
  prominent 
  function. 
  

   Taking 
  up 
  again 
  our 
  metaphor 
  of 
  the 
  sieve 
  we 
  

   can 
  assert 
  that 
  in 
  such 
  cases 
  climate 
  and 
  soil 
  

   exercise 
  sifting 
  action 
  and 
  in 
  this 
  way 
  the 
  ap- 
  

   plication 
  of 
  the 
  metaphor 
  becomes 
  more 
  defi- 
  

   nite. 
  Of 
  course, 
  next 
  to 
  the 
  climate 
  and 
  soil 
  in 
  

   importance, 
  come 
  ecological 
  conditions, 
  the 
  veg- 
  

   etable 
  and 
  animal 
  enemies 
  of 
  the 
  plants 
  and 
  

   other 
  influences 
  of 
  the 
  same 
  nature. 
  

  

  In 
  conclusion 
  it 
  is 
  to 
  be 
  pointed 
  out 
  that 
  this 
  

   side 
  of 
  the 
  problem 
  of 
  natural 
  selection 
  and 
  the 
  

   struggle 
  for 
  life 
  appears 
  to 
  offer 
  the 
  best 
  pros- 
  

   pects 
  for 
  experimental, 
  or 
  for 
  continued 
  statis- 
  

   tical 
  inquiry. 
  Direct 
  observations 
  are 
  possible 
  

   and 
  any 
  comparison 
  of 
  numerical 
  proportions 
  

   of 
  species 
  in 
  succeeding 
  years 
  affords 
  clear 
  

   proof 
  of 
  the 
  part 
  it 
  plays. 
  And 
  above 
  all, 
  such 
  

   observations 
  can 
  be 
  made 
  quite 
  independently 
  

   of 
  doubtful 
  theoretical 
  considerations 
  about 
  

   presumed 
  changes 
  of 
  character. 
  

  

  The 
  fact 
  of 
  natural 
  selection 
  is 
  plain 
  and 
  

   it 
  should 
  be 
  studied 
  in 
  its 
  most 
  simple 
  condi- 
  

   tions. 
  

  

  