﻿50 
  Elementary 
  Species 
  

  

  species 
  in 
  his 
  garden. 
  In 
  this 
  region 
  they 
  are 
  

   crowded 
  together 
  and 
  not 
  rarely 
  two 
  or 
  even 
  

   more 
  quite 
  distinct 
  forms 
  are 
  observed 
  to 
  grow 
  

   side 
  by 
  side 
  on 
  the 
  same 
  spot. 
  Farther 
  away 
  

   from 
  this 
  center 
  they 
  are 
  more 
  widely 
  dispersed, 
  

   each 
  holding 
  its 
  own 
  in 
  its 
  habitat. 
  In 
  all, 
  Jor- 
  

   dan 
  has 
  distinguished 
  about 
  two 
  hundred 
  spe- 
  

   cies 
  of 
  Draba 
  verna 
  from 
  Europe 
  and 
  western 
  

   Asia. 
  Subsequent 
  authors 
  have 
  added 
  new 
  

   types 
  to 
  the 
  already 
  existing 
  number 
  from 
  time 
  

   to 
  time. 
  

  

  The 
  constancy 
  of 
  these 
  elementary 
  species 
  is 
  

   directly 
  proven 
  by 
  the 
  experiments 
  quoted 
  

   above, 
  and 
  moreover 
  it 
  may 
  be 
  deduced 
  from 
  the 
  

   uniformity 
  of 
  each 
  type 
  within 
  its 
  own 
  domain. 
  

   These 
  are 
  so 
  large 
  that 
  most 
  of 
  the 
  localities 
  are 
  

   practically 
  isolated 
  from 
  one 
  another, 
  and 
  must 
  

   have 
  been 
  so 
  for 
  centuries. 
  If 
  the 
  types 
  were 
  

   slowly 
  changing 
  such 
  localities 
  would 
  often, 
  

   though 
  of 
  course 
  not 
  always, 
  exhibit 
  slighter 
  

   differences, 
  and 
  on 
  the 
  geographic 
  limits 
  of 
  

   neighboring 
  species 
  intermediates 
  would 
  be 
  

   found. 
  Such 
  however, 
  are 
  not 
  on 
  record. 
  

   Hence 
  the 
  elementary 
  species 
  must 
  be 
  regarded 
  

   as 
  old 
  and 
  constant 
  types. 
  

  

  The 
  question 
  naturally 
  arises 
  how 
  these 
  

   groups 
  of 
  nearly 
  allied 
  forms 
  may 
  originally 
  

   have 
  been 
  produced. 
  Granting 
  a 
  common 
  ori- 
  

   gin 
  for 
  all 
  of 
  them, 
  the 
  changes 
  may 
  have 
  been 
  

  

  