﻿Unbalanced 
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  249 
  

  

  manner 
  in 
  which 
  all 
  advance 
  in 
  the 
  animal 
  and 
  

   vegetable 
  kingdoms 
  must 
  have 
  taken 
  place 
  ; 
  con- 
  

   tinuously 
  adding 
  new 
  characters 
  to 
  the 
  already 
  

   existing 
  number. 
  Contrasted 
  with 
  this 
  method 
  

   of 
  growing 
  differentiation, 
  are 
  the 
  retrogres- 
  

   sive 
  modifications, 
  which 
  simply 
  retrace 
  a 
  

   step, 
  and 
  the 
  degressive 
  changes 
  in 
  which 
  

   a 
  backward 
  step 
  is 
  retraced 
  and 
  old 
  characters 
  

   revived. 
  No 
  doubt 
  both 
  of 
  these 
  methods 
  have 
  

   been 
  operative 
  on 
  a 
  large 
  scale, 
  but 
  they 
  are 
  evi- 
  

   dently 
  not 
  in 
  the 
  line 
  of 
  general 
  advancement. 
  

  

  In 
  all 
  of 
  these 
  directions 
  we 
  see 
  that 
  the 
  dif- 
  

   ferentiating 
  marks 
  show 
  more 
  or 
  less 
  clearly 
  

   that 
  they 
  are 
  built 
  up 
  of 
  units. 
  Allied 
  forms 
  

   are 
  separated 
  from 
  each 
  other 
  without 
  interme- 
  

   diates. 
  Transitions 
  are 
  wholly 
  wanting, 
  al- 
  

   though 
  fallaciously 
  apparent 
  in 
  some 
  instances 
  

   owing 
  to 
  the 
  wide 
  range 
  of 
  fluctuating 
  variabil- 
  

   ity 
  of 
  the 
  forms 
  concerned, 
  or 
  to 
  the 
  occurrence 
  

   of 
  hybrids 
  and 
  subvarieties. 
  

  

  These 
  physiologic 
  units, 
  which 
  in 
  the 
  end 
  

   must 
  be 
  the 
  basis 
  for 
  the 
  distinction 
  of 
  the 
  sys- 
  

   tematic 
  units, 
  may 
  best 
  be 
  designated 
  by 
  the 
  

   term 
  of 
  ' 
  ' 
  unit-characters. 
  ' 
  ' 
  Their 
  internal 
  na- 
  

   ture 
  is 
  as 
  yet 
  unknown 
  to 
  us, 
  and 
  we 
  will 
  not 
  

   now 
  look 
  into 
  the 
  theories, 
  which 
  have 
  been 
  pro- 
  

   pounded 
  as 
  to 
  the 
  probable 
  material 
  basis 
  un- 
  

   derlying 
  them. 
  For 
  our 
  present 
  purpose 
  the 
  

   empirical 
  evidence 
  of 
  the 
  general 
  occurrence 
  of 
  

  

  