﻿Double 
  Adaptations 
  445 
  

  

  cases, 
  we 
  may 
  resume 
  our 
  theoretical 
  discus- 
  

   sion 
  of 
  the 
  subject, 
  and 
  try 
  to 
  get 
  a 
  clearer 
  in- 
  

   sight 
  into 
  the 
  analogy 
  of 
  ever-sporting 
  varie- 
  

   ties 
  and 
  the 
  wild 
  species 
  quoted. 
  All 
  of 
  them 
  

   may 
  be 
  characterized 
  by 
  the 
  general 
  term 
  of 
  

   dimorphism. 
  Two 
  types 
  are 
  always 
  present, 
  

   though 
  not 
  in 
  the 
  same 
  individual 
  or 
  in 
  the 
  same 
  

   organ. 
  They 
  exclude 
  one 
  another, 
  and 
  dur- 
  

   ing 
  their 
  juvenile 
  stage 
  a 
  decision 
  is 
  taken 
  in 
  

   one 
  direction 
  or 
  in 
  the 
  other. 
  Now, 
  according 
  

   to 
  the 
  theory 
  of 
  natural 
  selection, 
  wild 
  species 
  

   can 
  only 
  retain 
  useful 
  or 
  at 
  least 
  innocuous 
  

   qualities, 
  since 
  all 
  mutations 
  in 
  a 
  wrong 
  direc- 
  

   tion 
  must 
  perish 
  sooner 
  or 
  later. 
  Cultivated 
  

   species 
  on 
  the 
  other 
  hand 
  are 
  known 
  to 
  be 
  

   largely 
  endowed 
  with 
  qualities, 
  which 
  would 
  

   be 
  detrimental 
  in 
  the 
  wild 
  condition. 
  Mon- 
  

   strosities 
  are 
  equally 
  injurious 
  and 
  could 
  not 
  

   hold 
  their 
  own 
  if 
  left 
  to 
  themselves. 
  

  

  These 
  same 
  principles 
  may 
  be 
  applied 
  to 
  

   ever-sporting 
  or 
  antagonistic 
  pairs 
  of 
  charac- 
  

   ters. 
  According 
  to 
  the 
  theory 
  of 
  mutations 
  

   such 
  pairs 
  may 
  be 
  either 
  useful 
  or 
  useless. 
  But 
  

   only 
  the 
  useful 
  will 
  stand 
  further 
  test, 
  and 
  if 
  

   they 
  find 
  suitable 
  conditions 
  will 
  become 
  spe- 
  

   cific 
  or 
  varietal 
  characters. 
  On 
  this 
  conclusion 
  

   it 
  becomes 
  at 
  once 
  clear, 
  why 
  natural 
  di- 
  

   morphism 
  is, 
  as 
  a 
  rule, 
  a 
  very 
  useful 
  quality, 
  

   while 
  the 
  cultivated 
  dimorphous 
  varieties 
  

  

  