﻿STRUCTURE 
  AND 
  CLASSIFICATION 
  OF 
  THE 
  ARACHNIDA. 
  207 
  

  

  conditions 
  upon 
  like 
  material 
  in 
  two 
  separate 
  lines 
  of 
  de- 
  

   scent.^ 
  

  

  So, 
  too, 
  degeneration 
  is 
  not 
  to 
  be 
  lightly 
  assumed 
  as 
  the 
  

   explanation 
  of 
  a 
  simplicity 
  of 
  structure. 
  There 
  is 
  a 
  very 
  

   definite 
  criterion 
  of 
  the 
  simplicity 
  due 
  to 
  degeneration, 
  which 
  

   can 
  in 
  most 
  cases 
  be 
  applied. 
  Degenerative 
  simplicity 
  is 
  

   never 
  uniformly 
  distributed 
  over 
  all 
  the 
  structures 
  of 
  the 
  

   organism. 
  It 
  affects 
  many 
  or 
  nearly 
  all 
  the 
  structures 
  of 
  the 
  

   body, 
  but 
  leaves 
  some 
  — 
  it 
  may 
  be 
  only 
  one 
  — 
  at 
  a 
  high 
  level 
  

   of 
  elaboration 
  and 
  complexity. 
  Ancestral 
  simplicity 
  is 
  more 
  

   uniform, 
  and 
  does 
  not 
  co-exist 
  with 
  specialisation 
  and 
  elabora- 
  

   tion 
  of 
  a 
  single 
  organ. 
  Further, 
  degeneration 
  cannot 
  be 
  

   inferred 
  safely 
  by 
  the 
  examination 
  of 
  an 
  isolated 
  case 
  : 
  

   usually 
  we 
  obtain 
  a 
  series 
  of 
  forms 
  indicating 
  the 
  steps 
  of 
  a 
  

   change 
  in 
  structure 
  ; 
  and 
  what 
  we 
  have 
  to 
  decide 
  is 
  whether 
  

   the 
  movement 
  has 
  been 
  from 
  the 
  simple 
  to 
  the 
  more 
  complex, 
  

   or 
  from 
  the 
  more 
  complex 
  to 
  the 
  simple. 
  The 
  feathers 
  of 
  a 
  

   peacock 
  afford 
  a 
  convenient 
  example 
  of 
  primitive 
  and 
  degene- 
  

   rative 
  simplicity. 
  The 
  highest 
  point 
  of 
  elaboration 
  in 
  colour, 
  

   pattern, 
  and 
  form 
  is 
  shown 
  by 
  the 
  great 
  eye-painted 
  tail 
  

   feathers. 
  From 
  these 
  we 
  can 
  pass 
  by 
  gradual 
  transitions 
  in 
  

   two 
  directions^ 
  viz. 
  either 
  to 
  the 
  simple 
  lateral 
  tail 
  feathers, 
  

   with 
  a 
  few 
  rami 
  only, 
  developed 
  only 
  on 
  one 
  side 
  of 
  the 
  

   shaft 
  and 
  of 
  uniform 
  metallic 
  coloration 
  — 
  or 
  to 
  the 
  simple 
  

   contour 
  feathers 
  of 
  small 
  size, 
  with 
  the 
  usual 
  symmetrical 
  

   series 
  of 
  numerous 
  rami 
  right 
  and 
  left 
  of 
  the 
  shaft 
  and 
  no 
  

   remarkable 
  colouring. 
  The 
  one-sided 
  specialisation 
  and 
  the 
  

   peculiar 
  metallic 
  colouring 
  of 
  the 
  lateral 
  tail 
  feathers 
  mark 
  

   thorn 
  as 
  the 
  extreme 
  terms 
  of 
  a 
  degenerative 
  series; 
  whilst 
  

  

  ' 
  A 
  great 
  deal 
  of 
  superfluous 
  hypothesis 
  has 
  lately 
  been 
  put 
  forward 
  iu 
  the 
  

   name 
  of 
  "the 
  principle 
  of 
  convergence 
  of 
  characters 
  " 
  by 
  a 
  certain 
  school 
  of 
  

   palaeontologists. 
  The 
  horse 
  is 
  supposed 
  by 
  these 
  writers 
  to 
  have 
  originated 
  

   by 
  separate 
  lines 
  of 
  descent 
  in 
  the 
  Old 
  World 
  and 
  the 
  New, 
  from 
  five-toed 
  

   ancestors 
  ! 
  And 
  the 
  important 
  consequences 
  following 
  from 
  the 
  demonstration 
  

   of 
  the 
  identity 
  in 
  structure 
  of 
  Liraulus 
  and 
  Scorpio 
  are 
  evaded 
  by 
  arbitrary 
  

   and 
  even 
  fantastic 
  invocations 
  of 
  a 
  mysterious 
  transcendental 
  force 
  which 
  

   brings 
  about 
  "convergence" 
  irrespective 
  of 
  heredity 
  and 
  selection, 
  Mor- 
  

   pliology 
  becomes 
  a 
  farce 
  when 
  such 
  assumptions 
  are 
  made. 
  

  

  