﻿Elementary 
  Species 
  in 
  Nature 
  37 
  

  

  respectively. 
  Other 
  authors 
  have 
  made 
  still 
  

   greater 
  numbers 
  of 
  species 
  in 
  the 
  same 
  groups. 
  

  

  It 
  is 
  very 
  difficult 
  to 
  estimate 
  systematic 
  dif- 
  

   ferences 
  on 
  the 
  ground 
  of 
  comparative 
  studies 
  

   alone. 
  All 
  sorts 
  of 
  variability 
  occur, 
  and 
  no 
  

   individual 
  or 
  small 
  group 
  of 
  specimens 
  can 
  

   really 
  be 
  considered 
  as 
  a 
  reliable 
  representa- 
  

   tive 
  of 
  the 
  supposed 
  type. 
  Many 
  original 
  diag- 
  

   noses 
  of 
  new 
  species 
  have 
  been 
  founded 
  on 
  di- 
  

   vergent 
  specimens 
  and 
  of 
  course, 
  the 
  type 
  can 
  

   afterwards 
  neither 
  be 
  derived 
  from 
  this 
  indi- 
  

   vidual, 
  nor 
  from 
  the 
  diagnosis 
  given. 
  

  

  This 
  chaotic 
  state 
  of 
  things 
  has 
  brought 
  some 
  

   botanists 
  to 
  the 
  conviction 
  that 
  even 
  in 
  syste- 
  

   matic 
  studies 
  only 
  direct 
  experimental 
  evidence 
  

   can 
  be 
  relied 
  upon. 
  This 
  conception 
  has 
  in- 
  

   duced 
  them 
  to 
  test 
  the 
  constancy 
  of 
  species 
  and 
  

   varieties, 
  and 
  to 
  admit 
  as 
  real 
  units 
  only 
  such 
  

   groups 
  of 
  individuals 
  as 
  prove 
  to 
  be 
  uniform 
  

   and 
  constant 
  throughout 
  succeeding 
  gener- 
  

   ations. 
  The 
  late 
  Alexis 
  Jordan, 
  of 
  Lyons 
  in 
  

   France, 
  made 
  extensive 
  cultures 
  in 
  this 
  direc- 
  

   tion. 
  In 
  doing 
  so, 
  he 
  discovered 
  that 
  syste- 
  

   matic 
  species, 
  as 
  a 
  rule, 
  comprise 
  some 
  lesser 
  

   forms, 
  which 
  often 
  cannot 
  easily 
  be 
  distin- 
  

   guished 
  when 
  grown 
  in 
  different 
  regions, 
  or 
  by 
  

   comparing 
  dried 
  material. 
  This 
  fact 
  was, 
  of 
  

   course, 
  most 
  distasteful 
  to 
  the 
  systematists 
  of 
  

   his 
  time 
  and 
  even 
  for 
  a 
  long 
  period 
  afterwards 
  

  

  